How a fuel injection system works

For the engine to run smoothly and efficiently it needs to be provided with the right quantity of fuel/air mixture according to its wide range of demands.

A fuel injection system

Petrol-engined cars use indirect fuel injection. A fuel pump sends the petrol to the engine bay, and it is then injected into the inlet manifold by an injector. There is either a separate injector for each cylinder or one or two injectors into the inlet manifold.

Traditionally, the fuel/air mixture is controlled by the carburettor, an instrument that is by no means perfect.
Its major disadvantage is that a single carburettor supplying a four-cylinder engine cannot give each cylinder precisely the same fuel/air mixture because some of the cylinders are further away from the carburettor than others.
One solution is to fit twin-carburettors, but these are difficult to tune correctly. Instead, many cars are now being fitted with fuel-injected engines where the fuel is delivered in precise bursts. Engines so equipped are usually more efficient and more powerful than carburetted ones, and they can also be more economical, as well as having less poisonous emissions.

Diesel fuel injection

The fuel injection system in petrolengined cars is always indirect, petrol being injected into the inlet manifold or inlet port rather than directly into the combustion chambers. This ensures that the fuel is well mixed with the air before it enters the chamber.
Many diesel engines, however, use direct injection in which the diesel is injected directly into the cylinder filled with compressed air. Others use indirect injection in which the diesel fuel is injected into the specially shaped pre-combustion chamber which has a narrow passage connecting it to the cylinder head.
Only air is drawn into the cylinder. It is heated so much by compression that atomized fuel injected at the end of the compression stroke self-ignites.

Basic injection

All modern petrol injection systems use indirect injection. A special pump sends the fuel under pressure from the fuel tank to the engine bay where, still under pressure, it is distributed individually to each cylinder.
Depending on the particular system, the fuel is fired into either the inlet manifold or the inlet port via an injector. This works much like the spray nozzle of a hose, ensuring that the fuel comes out as a fine mist. The fuel mixes with the air passing through the inlet manifold or port and the fuel/air mixture enters the combustion chamber.
Some cars have multi-point fuel injection where each cylinder is fed by its own injector. This is complex and can be expensive. It's more common to have single-point injection where a single injector feeds all the cylinders, or to have one injector to every two cylinders.

Injectors

The injectors through which the fuel is sprayed are screwed, nozzle-first, into either the inlet manifold or the cylinder head and are angled so that the spray of fuel is fired towards the inlet valve.
The injectors are one of two types, depending on the injection system. The first system uses continuous injection where the fuel is squirted into the inlet port all the time the engine is running. The injector simply acts as a spray nozzle to break up the fuel into a fine spray - it doesn't actually control the fuel flow. The amount of fuel sprayed is increased or decreased by a mechanical or electrical control unit - in other words, it is just like turning a tap on and off.
The other popular system is timed injection (pulsed injection) where the fuel is delivered in bursts to coincide with the induction stroke of the cylinder. As with continuous injection, timed injection can also be controlled either mechanically or electronically.
The earliest systems were mechanically controlled. They are often called petrol injection (PI for short) and the fuel flow is controlled by a mechanical regulator assembly. These systems suffer from the drawbacks of being mechanically complex and having poor response to backing off the throttle.
Mechanical systems have now been largely superseded by electronic fuel injection (known as EFi for short). This is thanks to the increasing reliability and decreasing costs of electronic control systems.

Mechanical fuel injection

Lucas mechanical fuel injection system

In the Lucas system, fuel from the tank is pumped at high pressure to a fuel accumulator. From there it passes into the fuel distributor, which sends a burst of fuel to each injector, from where it is fired into the inlet port.
The airflow is controlled by a flap valve which opens in response to the accelerator pedal. As the airflow increases, the fuel distributor automatically increases the flow of fuel to the injectors to keep the fuel/air mixture correctly balanced.
For cold starting, a choke on the dash or, on later models, a microprocessor control unit brings a special cold-start injector into operation, which injects extra fuel to create a richer mixture. Once the engine has warmed up to a certain temperature, a thermoswitch automatically cuts off the cold-start injector.

Mechanical fuel injection was used in the 1960s and 1970s by many manufacturers on their higher-performance sports cars and sports saloons. One type fitted to many British cars, including the Triumph TR6 PI and 2500 PI, was the Lucas PI system, which is a timed system.
A high-pressure electric fuel pump mounted near the fuel tank pumps fuel at a pressure of 100psi up to a fuel accumulator. This is basically a short-term reservoir that keeps the fuel-supply pressure constant and also irons out the pulses of fuel coming up from the pump.
From the accumulator, the fuel passes through a paper element filter and then feeds into the fuel-metering control unit, also known as the fuel distributor. This unit is driven from the camshaft and its job, as the name suggests, is to distribute the fuel to each cylinder, at the correct time and in the correct amounts.
The amount of fuel injected is controlled by a flap valve located in the engine's air intake. The flap sits beneath the control unit and rises and falls in response to airflow - as you open the throttle, the 'suck' from the cylinders increases the airflow and the flap rises. This alters the position of a shuttle valve within the metering control unit to allow more fuel to be squirted into the cylinders.
From the metering unit, the fuel is delivered to each of the injectors in turn. The fuel then squirts out into the inlet port in the cylinder head. Each injector contains a spring-loaded valve that is kept closed by its spring pressure. The valve only opens when the fuel is squirted in.
For cold starting, you cannot just block off part of the airflow to enrich the fuel/air mixture as you can with a carburettor. Instead a manual control on the dash (resembling a choke knob) or, on later models, a microprocessor alters the position of the shuttle valve within the metering unit. This activates an extra injector mounted in the manifold, causing it to squirt in extra fuel to enrich the mixture.

Electronic injection systems

Bosch electronic fuel injection

An electronic system is operated entirely by a microprocessor control unit. This measures factors such as the engine temperature, the throttle position and the engine speed to compute the fuel/air mixture required by the engine and its timing to the injectors.

The main difference between electronic injection and mechanical injection is that an electronic system is controlled by a complex microprocessor control unit (sometimes called an electronic control unit or ECU), which is basically a miniature computer.
This computer is fed with information from sensors mounted on the engine. These measure factors such as the air pressure and temperature in the air intake, the engine temperature, accelerator position and engine speed. All this information allows an electronic system to meter the fuel far more accurately than the simple mechanical system, which relies on sensing the airflow alone.
The computer compares the input signals from the sensors with information already programmed into it at the factory, and works out exactly how much fuel should be delivered to the engine. It then signals the on-off valve ig the injector to open and squirt fuel into the inlet port. All this happens in a fraction of a second, the control unit responding instantly to changes in accelerator position, temperature and air pressure.
As well as improved control over fuel flow, the electronic system also operates at lower pressure than a mechanical system - usually at around 25-30psi. This makes it run more quietly than a mechanical system does.
A typical system is the Bosch LJetronic, which is fitted to a wide range of European cars. In this system, fuel is drawn from the tank by an electric pump. It is then fed straight up pipes to the injectors. The system pumps more fuel than is needed for injection - a loop circuit returns the excess to the fuel tank via a pressure regulator which keeps the pressure in the pipes constant.
The injector valves are held closed by springs, and opened by solenoids (electromagnets) when signalled to do so by the control unit. The amount of fuel injected depends on how long the solenoid holds the injector open.

Q . I How to Test Fuel Injectors

The fuel injectors in your vehicle are designed to spray fuel into the cylinders of your engine where it is combined with air and compressed before being ignited by the spark plug to produce power. As a result, an issue with one of your fuel injectors can cause your engine to run poorly, or even fail to run at all. There are a number of issues that could cause your fuel injectors to fail. Although some may be beyond the expertise of most home mechanics, you can often diagnose a faulty fuel injector using common hand tools

Part 1
Listening for Bad Fuel Injectors

1
Put on the appropriate safety gear. Before beginning any automotive project, you need to take steps to protect yourself from injury. Eye protection like safety glasses or goggles will keep debris from falling or spraying into your eyes while you work. Choose eye protection that fits comfortably and won't interfere with your vision. Gloves are an optional addition to the safety gear required for this task.^[1]
- Gloves can protect your hands from sharp objects or pinches while working in the engine bay.
- Eye protection is required for this project.
2
Open the hood and locate your fuel injectors. The easiest way to locate the fuel injectors for your specific vehicle is to refer to the service manual for that vehicle. Most applications have one fuel injector for each cylinder. They are usually located on the intake manifold and are connected to one another with a fuel rail.^[2]
- The fuel rail is a cylindrical rail that will run along the top of the intake manifold, and each fuel injector will be between the fuel rail and the intake manifold.
- V style engines (V6, V8, V10) will have two fuel rails with half of the injectors on each side of the motor.
3
Find a long metal rod or screwdriver. Locate a thin piece of metal that’s at least a foot or so long. It should be made mostly of metal, but you could opt to use a screwdriver despite it having a plastic or rubber handle.^[3]
- Make sure the piece you choose is at least a foot long, but not more than two feet.
- A long screwdriver or thin piece of rebar will work fine.
4
Place the tip of the rod on a fuel injector. You will be using the metal rod to transmit sound from the fuel injector to your ear without having to bring your face too close to a running engine. Set one end of the rod or screwdriver on the injector itself while holding it up with one hand.^[4]
- Make sure to hold the screwdriver or metal rod at an angle that will allow you to bring your ear to it.
5
Bring your ear close to the rod and listen for clicking. Lean your ear close to the end of the metal rod or screwdriver that is opposite the injector. As the engine runs, listen for an audible clicking sound given off by the injector. This sound indicates the injector being activated.^[5]
- Be extremely careful leaning your head into the engine bay, and ensure you keep your eyes open as you listen to the rod to prevent accidentally getting injured.
- If you have long hair, tie it back tightly to prevent it from getting caught in any moving parts under the hood.
6
Repeat these steps for each injector. Use the same method to check each fuel injector in your vehicle. If you find one that is not clicking, there is an issue with the injector or the electronic control that is transmitting to the injector.^[6]
- If you have an OBDII scanner and your vehicle’s check engine light is on, you can check to see if there have been any errors in the vehicle’s computer regarding that cylinder or injector.
- Replacing this injector may solve the problem, but you may also need to have a diagnostic done of your vehicle’s electronic control unit and fuel system by a professional mechanic.

Part 2
Ensuring the Injectors Are Receiving Power

1
Turn the key to the “on” position without starting the engine. To conduct this test, the vehicle's electrical system must be active without the engine actually running. Insert the key and turn it until the electrical system activates, but stop before you engage the engine's starter. This should activate all of the vehicle’s electronics like interior lighting and the radio.^[7]
- If you accidentally start the vehicle, simply turn it off and try again.
- The vehicle’s battery is powering everything during this test, so you should turn off things like the headlights and stereo to converse power and ensure it has enough to start the vehicle again later.
2
Connect a test light to the negative terminal on the battery. A test light looks like a screwdriver with a finely pointed end and a wire hanging out of the handle. When the wire from the handle and the pointed end come into contact with a completed and powered circuit, a lightbulb lights up inside the handle of the test light. The wire extending from the handle will have an alligator clip at the end. Attach that alligator clip to the negative terminal of the vehicle’s battery.^[8]
- You can identify the negative terminal on the battery by looking for the negative symbol (-) or the letters NEG.
- Make sure the clip has a good metal on metal connection to make the test light work.
3
Locate the two wires going into each injector. Each fuel injector will have a metal clip plugged into it with two wires coming out of it. One of those two wires is a 12-volt constant that should be continuously receiving power from your vehicle’s electrical system. There should be a small portion of each wire exposed coming out of the plastic clip that connects to the injector.^[9]
- These wires are often grey and black, but can come in any number of colors.
- They will be the only wires coming from each injector.
4
Test each wire for voltage. Take the sharp end of the test light and press it firmly into the rubber coating around each wire until it penetrates into the metal wiring itself. One of the two wires should make the test light turn on when it comes into contact with the wire inside the protective coating. If the test light turns on with one wire, then the injector is receiving the necessary constant voltage.^[10]
- Make sure to wrap a piece of electric tape around any holes in the wiring’s protective coating that are big enough to see.
- If neither wire makes the light turn on, then there is an issue with the power reaching the fuel injector, which will result in in failing to fire.
- If the all of the wires that light up are a certain color, make note of which wires are the constants.
5
Repeat the process for each injector. Test each wire coming out of the fuel injectors in your vehicle. If you locate one injector with a power issue, that doesn’t mean others may not have the same problem. Once you identify an injector with a power issue, make a note of which one it was and continue to test the rest.^[11]
- Follow the wires on the injectors that fail to engage the test light to make sure there are no breaks in the wire that may prevent the electricity from reaching it.
- Let your mechanic know that you were able to identify the injector with a power issue. It may require replacing the vehicle’s electronic control unit.

Part 3
Checking the Trigger Circuit for the Injectors

1
Connect a test light to the positive terminal of the battery. Take the same test light that you used for the previous test, but this time connect the alligator clip to the positive terminal on the battery instead of the negative.^[12]
- You can identify the positive terminal by looking for the positive sign (+) on the battery or the letters POS.
- Make sure the alligator clip has secure, metal on metal contact or the test light will fail to function.

basic electronic fuel injection system

Q . II How Electronic Fuel Injection Works

Electronic fuel injection (EFI) replaced carburetors back in the mid-1980s as the preferred method for supplying air and fuel to engines. The basic difference is that a carburetor uses intake vacuum and a pressure drop in the venturi (the narrow part of the carburetor throat) to siphon fuel from the carburetor fuel bowl into the engine whereas fuel injection uses pressure to spray fuel directly into the engine

With a carburetor air and fuel are mixed together as air is pulled through the carburetor by the engine. The air/fuel mixture then travels through the intake manifold to the cylinders. One of the drawbacks of this approach is that the intake manifold is wet (contains droplets of liquid fuel) so fuel can puddle in the plenum area of the manifold when a cold engine is first started. The twists and turns of the intake runners can also cause the air and fuel mixture to separate as if flows to the cylinders, resulting in uneven fuel mixtures between cylinders. The center cylinders typically run slightly richer than the end cylinders, which makes tuning for peak fuel economy, performance and emissions more difficult with a carburetor

THROTTLE BODY INJECTION

With Throttle Body Injection (TBI), one or two injectors mounted in the throttle body spray fuel into the intake manifold. Fuel pressure is created by an electric fuel pump (usually mounted in or near the fuel tank), and the pressure is controlled by a regulator mounted on the throttle body. Fuel is sprayed into the engine when the engine computer energizes the injector(s), which occurs in a rapid series of short bursts rather than a continuous stream. This produces a buzzing noise from the injectors when the engine is running.

Because of this setup, the same fuel distribution issues that affect carburetors also affect TBI systems. However, TBI systems have better cold start characteristics than a carburetor because they provide better atomization and do not have a troublesome choke mechanism. A TBI system also makes it easier for an electronic engine control system to regulate the fuel mixture than an electronic feedback carburetor. Throttle Body Injection systems were only used briefly during the 1980s as US vehicle manufacturers transitioned from carburetors to fuel injection to meet emission regulations. By the late 1980s, most TBI systems were replaced with Multiport Injection (MPI) fuel injection systems.

MULTIPORT FUEL INJECTION

With MultiPort Injection systems, there is a separate fuel injector for each cylinder. The advantage of this approach is that fuel is sprayed directly into the cylinder head intake port. Since only air flows through the intake manifold, the intake manifold remains dry and there are no problems with fuel puddling when the engine is cold or fuel separation causing uneven fuel mixtures in the center and end cylinders. This allows the fuel mixture to be much more even in all of the cylinders for better fuel economy, emissions and performance.

Some early production multiport fuel injection systems were purely mechanical and date back to the 1950s (1957 Corvette with Rochester Fuel Injection , for example, and Bosch D-Jetronic and K-Jetronic systems with their mechanical fuel distributors and injectors). Later fuel injection systems such as the Bosch L-Jetronic systems of the late 1970s replaced mechanical injectors with electronic injectors. Today, all production EFI systems are fully electronic with computer controls and electronic injectors.

Most of the EFI systems that were offered in the late 1980s and early 1990s fire all of the injectors simultaneously, typically once every revolution of the crankshaft. The more sophisticated Sequential Fuel Injection (SFI) systems that came later fire each injector separately, usually just as the intake valve is opening. This allows much more precise fuel control for better fuel economy, performance and emissions.

GASOLINE DIRECT FUEL INJECTION

In the 2000s, some vehicle manufacturers began offering a new type of fuel injection system called Gasoline Direct Injection (GDI). With this setup, a separate injector is still used for each cylinder but the injectors are relocated on the engine to spray fuel directly into the combustion chamber rather than the intake port. This is similar to a diesel engine that sprays fuel directly into the cylinder. The advantage with this approach is a significant improvement (as much as 15 to 25 percent!) in fuel economy and power. However, it requires special high pressure fuel injectors and much higher operating pressures. Some current examples of direct fuel injection include VW TDI engines, Mazda direct injection engines, General Motors EcoTech engines and Ford EcoBoost engines.

FUEL INJECTOR PULSES

The relative richness or leanness of the fuel mixture in a fuel injected engine is determined by varying the duration of the injector pulses (called pulse width). The longer the pulse width, the greater the volume of fuel delivered and the richer the mixture.

Injector timing and duration is controlled by the engine computer. The computer uses input from its various engine sensors to regulate fuel metering and to change the air/fuel ratio in response to changing operating conditions. The primary sensor for fuel mixture control is the Oxygen sensor. The O2 sensor generates a RICH or LEAN signal that the engine computer uses to adjust the fuel mixture. For more information about feedback fuel control and fuel trim adjustments.

The computer is calibrated with a fuel delivery program that is best described as a three-dimensional map. The program directs the computer as to how long to make the injector pulses as engine speed and load change. During start-up, warm-up, acceleration and increased engine load, the map typically calls for a richer fuel mixture. When the engine is cruising under light load, the map allows for a leaner fuel mixture to improve fuel economy. And when the vehicle is decelerating and there is no load on the engine, the map may allow the computer to momentarily turn the injectors off altogether.

The programming that controls the EFI system is contained on a PROM (Program Read Only Memory) chip inside the engine computer. Replacing the PROM chip can change the calibration of the EFI system. This is sometimes necessary to update factory programming or to correct a drivability or emissions problem. The PROM chip on some vehicles can also be replaced with aftermarket performance chips to improve engine performance, too.

On many 1996 and newer vehicles, the programming is on an EEPROM (Electronically Ereasable Program Read Only Memory) chip in the computer. This allows the programming to be updated or changed by reflashing the computer. The new programming is downloaded into the computer through the OBD II Diagnostic Connector using a scan tool or J2534 reprogramming tool.

FUEL INJECTIN SENSOR INPUTS

Electronic fuel injection requires inputs from various engine sensors so the computer can determine engine speed, load and operating conditions. This allows the computer to adjust the fuel mixture as needed for optimum engine operation.

There are two basic types of EFI systems: Speed-Density systems and Mass Airflow systems. Speed density systems such as those found on many Chrysler engines and some GM engines do not actually measure airflow into the engine, but estimate airflow based on inputs from the Throttle Position Sensor (TPS), Manifold Absolute Pressure (MAP) sensor and engine RPM. The advantage with this approach is that the engine does not require an expensive airflow sensor, and the air/fuel mixture is less affected by small air leaks in the intake manifold, vacuum plumbing or throttle body.

A Ford mass airflow sensor also includes an Inlet Air Temperature (IAT) sensor inside.

With mass airflow systems, some type of airflow sensor is used to directly measure airflow into the engine. It may be a mechanical flap style airflow sensor, a hot wire airflow sensor or a vortex airflow sensor. The computer also uses inputs from all of its other sensors, but relies primarily on the airflow sensor to control the fuel injectors.

An EFI system will usually run without a signal from the MAP sensor, but it will run poorly because the computer has to rely on its other sensor inputs to estimate airflow. A common problem with MAF sensors is a buildup of dirt or varnish on the heated wire inside the sensor. Cleaning the MAF wire inside the sensor with electronics cleaner will often restore normal operation and cure a lean condition caused by a dirty airflow sensor.

On both types of systems (speed-density and mass airflow), input from the Heated Oxygen sensor (HO2) is also key for maintaining the optimum air/fuel ratio. The oxygen sensor (or Air/Fuel sensor on many newer vehicles) is mounted in the exhaust manifold and monitors unburned oxygen levels in the exhaust as an indicator of the relative richness or leanness of the fuel mixture. On V6 and V8 engines, there will be a separate oxygen sensor for each bank of cylinders, and on some straight six cylinder engines (BMW for example), there may be separate oxygen sensors for the first three cylinders and the last three cylinders. The feedback signal from the oxygen sensor or air/fuel sensor is used by the engine computer to constantly fine tune the fuel mixture to optimum fuel economy and emissions.

When the oxygen sensor tells the computer the engine is running lean (higher levels of unburned oxygen in the exhaust), the computer compensates by richening up the fuel mixture (increasing the pulse width of the injectors). If the engine is running rich (less oxygen in the exhaust), the computer shortens the pulse width of the injectors to lean the fuel mixture.

Input about the position of the throttle is provided by the Throttle Position Sensor (TPS). It is located on the side of the throttle body and uses a variable resistor that changes resistance as the throttle opens and closes.

Engine load is measured by the Manifold Absolute Pressure (MAP) sensor. It may be mounted on the intake manifold or attached to the intake manifold with a vacuum hose.

The temperature of the air entering the engine must also be monitored to compensate for changes in air density that occur (colder air is denser than hot air). This is monitored by an Inlet Ait Temperature (IAT) or Manifold Air Temperature (MAT) sensor, which may be built into the airflow sensor or mounted separately on the intake manifold.

Coolant temperature is monitored by the Coolant Temperature Sensor (CTS). This tells the computer when the engine is cold and when it is at normal operating temperature. The computer needs to know the temperature because a cold engine requires a richer fuel mixture when it is first started. When the coolant reaches a certain temperature, the engine goes into Closed Loop operation, which means it starts using inputs from the oxygen sensors to fine tune the fuel mixture. When it is operating in Open Loop (when cold or when there is no signal from the coolant sensor), the fuel mixture is fixed and does not change.

Faulty inputs from any of the engine's sensors may cause drivability, emissions or performance problems. Many sensor problems will set a Diagnostic Trouble Code (DTC) and turn on the Check Engine Light. Reading the code(s) with a scan tool will help you diagnose the problem.

EFI throttle body.

FUEL INJECTION IDLE SPEED CONTROL

Idle speed on fuel injected engines is computer controlled via an idle air bypass circuit on the throttle body. A small electric motor or solenoid is used to open and close the bypass orifice. The larger the opening, the greater the volume of air that can bypass the throttle plates and the faster the idle speed.

On newer vehicles with electronic throttle control, the computer also controls the opening of the throttle plate when the driver pushes down on the gas pedal. Position sensors in the gas pedal signal the computer how far to open the throttle plate.

Idle problems on EFI systems can be caused by varnish and dirt deposits in the throttle body idle control circuit. Cleaning the throttle body with throttle body cleaner can often solve idle problems (follow the directions on the product). Idle problems can also be caused by air leaks between the airflow sensor and throttle, the throttle body and intake manifold, and the intake manifold and cylinder head(s), or in the PCV or EGR systems, or vacuum hoses.

On most EFI systems, voltage is supplied directly to the injectors and the PCM energizes the injector by grounding the circuit.

INJECTORS

A fuel injector is nothing more than a spring-loaded solenoid pintle valve. When energized by the computer, the solenoid pulls the valve open. This allows fuel to spray out of the nozzle and into the engine. When the computer cuts the circuit that powers the injector, the valve inside the injector snaps shut and fuel delivery stops.

The total amount of fuel delivered is controlled by cycling the injector voltage on and off very rapidly. The longer the pulse width, the greater the volume of fuel delivered and the richer the fuel mixture. Decreasing the duration of the injector signal pulse reduces the volume of fuel delivered and leans out the mixture.

Dirty fuel injectors are a common problem. A buildup of fuel varnish deposits inside the tip of the injector spray nozzle can restrict fuel delivery and interfere with the creation of a good spray pattern. This can cause a lean fuel condition and misfiring. Cleaning the injectors with fuel injection cleaner, or removing the injectors and having them cleaned on a fuel injector cleaning machine can usually restore normal operation. Using a Top Tier gasoline that contains adequate levels of injector cleaner can also prevent varnish deposits from forming.

The fuel pressure regulator is usually mounted on the fuel rail that supplies the injectors.

FUEL PRESSURE CONTROL

Another important factor that helps determine how much fuel is delivered through an injector when it is pulsed, and that is the fuel pressure behind it. The higher the pressure behind the injector, the greater the volume of fuel that will spray out of the injector when it is opened.

Fuel pressure is generated by a high pressure electric fuel pump usually mounted inside or near the fuel tank. Pump output pressure may range anywhere from 8 to 80 lbs. depending on the application. The pump usually has an pressure valve to vent excess pressure and a check valve to maintain system pressure when the ignition is off.

In a multiport EFI system, the pressure differential between the fuel behind the injectors and the vacuum or pressure in the intake manifold is a constantly changing variable. Under light load or at idle, a relatively high vacuum exists in the intake manifold. This means less fuel pressure is needed to spray a given volume of fuel through the injector. Under heavy load, engine vacuum drops to near zero. Under these circumstances, more pressure is needed to deliver the same quantity of fuel through the injector. And in turbocharged engines, manifold vacuum can become 8 to 14 lbs. of positive pressure when turbo boost comes into play. Even more fuel pressure is required to force the same amount of fuel through the injector.

A means of regulating fuel pressure according to engine vacuum must be provided in a multiport EFI system to maintain the same relative pressure differential between the fuel system and intake manifold. This is done by the fuel pressure regulator. The regulator is mounted on the fuel rail that supplies the injectors. On returnless EFI systems, the regulator is part of the fuel pump assembly inside the fuel tank.

The fuel pressure regulator has a simple spring-controlled vacuum diaphragm with a vacuum connection to the intake manifold. The regulator decreases fuel pressure under light load and increases it under heavy load or boost conditions. The excess fuel pressure is shunted through a bypass port back to the fuel tank to maintain the desired pressure differential. Most systems are calibrated to maintain a pressure differential of somewhere between 40 and 55 psi.

On the older TBI systems, the regulator has an easier job because the injectors are mounted above the throttle plates. Since engine vacuum/boost has no effect on fuel delivery out of the injector on the TBI system, regulator only has to maintain an even pressure. On General Motors TBI applications, the pressure regulator is calibrated to maintain roughly 10 psi in the fuel system but most others run close to 40 psi.

Low fuel pressure will result in poor engine performance, possible misfiring and may prevent the engine from starting. Low fuel pressure can be caused by a weak fuel pump (a worn pump or low voltage to the pump that caused it to run slowly), restrictions in the fuel line, a plugged fuel filter, or a leaky fuel pressure regulator. Fuel pressure MUST be within specifications for the engine to run properly. Fuel pressure can be tested with a fuel pressure gauge connected to the service valve on the fuel rail, or teed into the fuel line.

Toyota fuel injection V6

HOW TOYOTA FUEL INJECTION MEASURES AIRFLOW

To regulate the air/fuel mixture, the engine computer needs to know how much air is being sucked into the engine. On the older Toyota EFI systems, air flow is measured mechanically with a flap-style airflow meter. A flap inside the meter rotates when incoming air pushes against it. Connected to the flap is a arm that rubs across a resistor grid (potentiometer). This changes the airflow meter's output voltage in proportion to airflow. The greater the air flow, the higher the resistance created by the potentiometer. So the meter's output voltage drops as airflow increases.

Older style Toyota flap-style merchanical airflow meter.

Over time, the potentiometer's contacts inside the airflow meter can wear causing erratic or inconsistent readings. Shorts or opens in the circuitry will also disrupt the voltage signal, depriving the engine computer of this vital bit of information. The result can be poor cold drivability, hesitation or poor performance.

The TCCS (Toyota Computer Control System) should set a code 2, 31 or 32 if the airflow meter signal is missing or out of range, but it may not always detect an intermittent problems. To find this kind of fault, an oscilloscope can help you analyze the airflow meter's output voltage as a waveform. If you don't see a nice linear change in the output voltage as the flap moves from idle to wide open throttle, it means the potentiometer is skipping and the airflow meter needs to be replaced.

Another way to check the operation of the airflow meter as well as the entire feedback circuit through the computer is to use a scope to compare injector dwell (on time) to the airflow signal. If you have a good airflow signal but injector dwell fails to increase as airflow goes up, there is a control problem in the computer.

The flap type air flow meters should also be inspected by pushing the flap with your finger. There should be no binding when the flap is pushed open, and spring pressure should return it to its closed position. A buildup of varnish or dirt may cause binding. Be sure to inspect the air filter if you find any dirt in the unit.

A temperature sensor located in the intake plumbing is used to measure air temperature so the computer can calculate how much air is actually entering the engine. Cold air is denser than warm air, and requires a slightly richer fuel mixture. The air temperature sensor changes resistance, so if the signal goes flat or disappears it too can upset the air/fuel mixture and cause drivability problems. Codes that would indicate a fault in the air temperature sensor circuit include 8, 23 and 24. You can use an ohmmeter to check the sensor's output. If the reading is out of specifications or fails to change as the temperature increases, the sensor is bad and needs to be replaced.

Newer style Toyota mass airflow sensor.

SECOND GENERATION TOYOTA AIRFLOW SENSOR

Starting in the mid-1990s, Toyota introduced a second-generation airflow sensor that combines the functions of the airflow meter and air temperature sensor into one unit. The new mass airflow sensor uses a hot wire to measure air mass rather than volume and has no moving parts. A reference voltage is applied to a thin wire inside the sensor that heats it to about 100 degrees C hotter than ambient air temperature. As air flows through the sensor and past the hot wire, it carries away heat and cools the wire. The electrical control circuit for the wire is designed to maintain a constant temperature differential, so the amount of extra voltage that's required to offset the cooling effect and keep the wire hot tells the control box how much air is entering the engine.

With both the early and late style airflow sensors, vacuum leaks can cause drivability problems by allowing unmetered air to enter the engine. Air leaks around the throttle body, injector O-rings, intake manifold gaskets or vacuum hose connections can cause the air/fuel ratio to go lean. So if you find a code 25 (lean air/fuel ratio), start looking for leaks.

Finding an air leak can be a time-consuming exercise in patience. One method is to use a propane bottle and hose to check out suspicious areas. When propane vapor is siphoned in through a leak, the idle will smooth out and the rpm will change. Another trick is to turn off the engine and lightly pressurize (no more than 5 PSI max) the intake manifold with compressed air. Then use a hand bottle to spray soapy water at possible leak points. Bubbles would indicate a leak. Another technique is to use a device that fills the intake manifold with smoke to reveal leaks.

Another often overlooked cause of air leakage is the EGR valve. If the valve sticks open, it will act much like a vacuum leak causing lean misfire at idle and hesitation problems.

TOYOTA FUEL INJECTION CIRCUIT

Fuel flows from a tank-mounted pump through the fuel line to an inline filter usually located in the engine compartment. It then goes to a common fuel rail (which Toyota calls the "fuel delivery pipe") on the engine to supply the injectors. The fuel injectors plug into the rail and are removed as an assembly with the rail. On V6 applications, there's a separate rail for each cylinder bank. Unfortunately, Toyota doesn't include a test valve on the fuel rail for checking fuel pressure. To perform a pressure check, you have to disconnect the cold start injector fuel fitting and attach a pressure gauge.

The pressure regulator is mounted on the end of the fuel rail, and maintains pressure at a constant level as engine load and intake vacuum change. A vacuum hose connected the regulator to the intake manifold so the diaphragm inside can react to changes in intake vacuum. A bypass valve inside the regulator routes excess fuel through a return line back to the fuel tank.

toyota fuel pressure regulator Toyota uses many different fuel pressure regulators so make sure you get the correct replacement.

System operating pressure varies depending on the application, but is typically from 30 to 37 PSI with the vacuum hose connected to the regulator, and 38 to 44 PSI with the hose disconnected and plugged.

NOTE: If you're replacing a regulator on a turbocharged engine, make sure you get the correct replacement because the regulator on these applications is calibrated differently from those on nonturbo motors.

Also, do not confuse the pressure regulator with a little round plastic gizmo that may be mounted on the end of the fuel rail. This is a pulse damper that helps dampen noise and resonance caused by the pulsing of the injectors.

Starting in 1996, some Toyota EFI systems switchd to a returnless EFI system. The regulator on the returnless EFI systems is located in the fuel tank with the pump.

TOYOTA FUEL INJECTION PRESSURE PROBLEMS

If fuel pressure reads low, or the engine seems to starve for fuel under load, don't overlook the fuel pickup filter inside the fuel tank as a possible cause. In many instances, the system may flow enough fuel at idle to develop normal pressure, but run out of fuel at higher speeds or loads. Rust, dirt and scum inside the tank may be blocking the flow of fuel into the pump. Likewise, accumulated dirt and debris may be clogging the inline filter.

Toyota says the best method for confirming a suspected fuel starvation problem is to road test the vehicle with a fuel pressure gauge safely installed on the engine. If the pressure reading drops when the engine is under load, it means the system isn't maintaining normal pressure. But is it the pump, filter or what?

You can rule out the pressure regulator if the system maintains normal pressure at idle, and the pressure rises when you disconnect the regulator's vacuum hose. No change in pressure would indicate a defective regulator or plugged vacuum line.

A good way to check out the pump, pickup filter and inline filter is to measure fuel delivery volume. Relieve system pressure, then disconnect the fuel supply line at the fuel filter or fuel rail, or disconnect the return hose from the rail. Place the open end of the fuel hose in a measuring cup or graduated cylinder. If you're disconnecting the return hose, you'll have to attach another piece of hose to the fuel rail and use that to route fuel into the container. With the engine off, use jumpers to bypass the pump relay. Energize the pump for 30 seconds and measure the volume of fuel delivered.

As a rule, a good pump should deliver about one quart of fuel in 30 seconds.

If a pump's output volume and/or pressure is low, the pump motor might be running slow due to internal wear. A typical fuel pump runs at 5,000 to 6,000 rpm and pulls about 3 to 6 amps. But as the armature brushes become worn and the brush springs weaken, increased resistance will reduce the pump's current draw and cause the motor to run slower causing it to deliver less fuel.

The pump motor can be checked using an ohmmeter to measure the motor's internal resistance. As a rule, most pumps should read 2 to 50 ohms if good. If the pump is open (reads infinity) or shows zero resistance (shorted), the motor is bad and the pump needs to be replaced.

Even if the pump motor is okay, fuel delivery problems can be caused by the pump's voltage supply. Low battery voltage, low system operating voltage, a poor ground connection or excessive resistance in the pump's wiring connectors or the relay can all have an adverse effect on the operating speed of the pump. The pump must have normal voltage to run at full speed, so always check the pump's wiring connectors and voltage supply when you encounter a pump with low pressure or volume output.

The pump's supply voltage should be within half a volt of normal battery voltage. If low, check the wiring connectors, relay and ground. A good connection should have less than a tenth of a volt drop (ideally no voltage drop) across it. A voltage drop of more than 0.4 volts can create enough resistance to cause a problem.

RESIDUAL FUEL PRESSURE

If an engine is hard to start when hot, fuel may be boiling in the rail because the system isn't holding residual pressure when the ignition is shut off. To prevent vapor lock and reduce the cranking time when restarting the engine, a check valve inside the fuel pump holds the pressure in the line. Toyota says pressure should remain above 21 psi for five minutes after the engine is turned off. If the system fails to hold pressure, either the check valve or pressure regulator is leaking, or an injector is leaking. Regulator leaks can be ruled out by pinching off the return line. Injector leaks can be checked by removing the fuel injector and rail assembly from the manifold, and pressurizing the rail. No fuel drips? Then it's the pump check valve.

TOYOTA FUEL INJECTORS

Four different types of injectors may be used in Toyota engines: pintle style, hole type (cone valve and ball valve), high resistance and low resistance. Bosch pintle style injectors are used on the older TCCS applications, while Nippondenso hole type injectors are used on newer engines. The hole type injectors spray fuel through holes drilled in a director plate at the injector tip. There are currently three different types including side-feed injectors used on the 3S-GTE and 2TZ-FE engines.

One of several different types of fuel injectors Toyota has used.
This one fits 1997 to 2000 Camry & Celica.

The valve design of the older pintle style injectors makes them more susceptible to deposit buildup than the hole type injectors. So if you're diagnosing a lean fuel condition on a Toyota with pintle style injectors, the injector may need to be cleaned.

Low resistance injectors are found on older Toyota up to about 1990, and measure 2 to 3 ohms at room temperature. The are used with an external resistor in a voltage-controlled driver circuit, or without an external resistor in a current-controlled driver circuit. High resistance injectors (13.8 ohms) are used on the newer applications, and do not require an external resistor.

When the ignition is turned on, voltage is supplied to the fuel injectors directly through the ignition circuit or through the EFI main relay depending on the application. The driver circuits in the computer then provides a ground to complete the connection and energize the injectors.

Toyota says never to apply battery voltage directly to a low resistance injector to test it because doing so can overheat and damage the windings in the solenoid. Use a resistor wire to protect the injector.

If an engine is misfiring and has a dead cylinder, and you've already ruled out ignition misfire or loss of compression as possible causes, use a stethoscope to listen to the injector. A steady buzz would tell you the injector is working and that the driver circuit is okay. No buzzing means a wiring or control problem. Check for voltage at the injector terminal when the key is on. No voltage? Check the EFI relay, fuse and wiring circuit. If there is voltage, use a logic probe or oscilloscope to see if the computer driver circuit is grounding the injector. No on-off signal would indicate a wiring problem or bad computer.

Injector resistance can be measured directly with an ohmmeter. An open, short or out-of-specification reading would tell you the injector has failed and needs to be replaced.

If the injector is buzzing but the cylinder is running lean or misfiring, the problem is likely a buildup of fuel varnish in the injector orifice or valve. Cleaning is the solution here, either on or off the vehicle. On-car cleaning saves time and can often restore the injectors to like-new performance. Off-car cleaning means you have to pull the injectors, but it gives you the opportunity to examine their spray pattern. There should be no solid streamers of liquid fuel, only a cone-shaped mist. If cleaning fails to restore the pattern, it's time for a new injector.

Something else that should be done if you're using off-car injection cleaning equipment is to compare the volume of fuel delivered by each injector. A difference of more than 10 percent can cause noticeable drivability and emission problems.

If injectors need to be replaced, always install new O-rings lightly lubricated with clean gasoline. Fuel rail banjo connections should also have new copper gaskets installed to prevent fuel leaks.

On 1991-94 1456cc Tercel engines, cylinders #1 and #3 use a different injector than #2 and #4, so be sure you install the correct injectors in each cylinder.

COLD START INJECTOR

Older Toyota applications use a cold start injector to squirt extra fuel into the manifold when a cold engine is first started. The "on time" of the injector is controlled by a start injector time switch and the computer. The number of seconds the cold start injector is energized (typically 2 to 8 seconds) is limited by a heater circuit inside the timer that has two coils. A bimetallic switch inside the timer is normally closed, so when the engine is started current flows through the cold start injector solenoid and both heater coils inside the timer. Within a few seconds, the heater coils trip the bimetallic switch causing it to open and turn off the cold start injector.

Toyota cold start fuel injector.

If the timer fails, the cold start injector will never come on and the engine may be hard to start when cold. The circuit can be checked by using a voltmeter to test for voltage at the cold start injector when the ignition is turned on. You should also check the resistance across the injector's terminals to check for an open or shorted solenoid. A good cold injector should read 2 to 4 ohms.

On most TCCS engines, an alternate ground may be supplied to the cold start injector by the computer at the STJ terminal. Using inputs from the engine's coolant temperature sensor, the computer can operate the cold start injector for up to three seconds regardless of the status of the timer switch. The maximum coolant temperature at which the computer will cycle on the cold start injector is 113 degrees F. Above that temperature the injector will not be energized by either the timer switch or computer.

Sometimes a cold start injector will hang open and leak fuel. The dribble may not seem like much but it may be enough to upset the air/fuel ratio and cause an increase in idle roughness and emissions. The cold start injector can be checked for leaks by removing it and pressurizing the fuel system.

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How a fuel injection system works