This page describes the various components used in an EFI system. 

ECM or Electronic Control Module:

The ECM is the main brain of the system.  The ECM will read inputs from all the sensors on the Vehicle, process information, and calculate how to activate its various outputs.  The ECM may be simple enough to have just a couple of sensors and a single output, or it may have 20 or more inputs and outputs and control everything but the kitchen sink.  Regardless of how much it controls, it is still the brains of the operation.  It contains a miniature computer that has a program designed just for one task, controlling an engine.  Some ECMs are programmable and can be tuned to work with a many different engines.  Others such as in production automobiles have their settings perminantly set and cannot be changed.

The fuel injector is a fancy name for an electric solenoid valve.  It is a valve that when you apply electricity it will open and allow fuel to flow through.  The catch is that it is a very precision valve.  The injector must flow a precise amount at a specific pressure and be able to open and close very fast.  The ECM controls the injector turning it OFF and ON for different amounts of time depending on how much fuel the engine needs.  There are several different shapes of injectors for different applications but they still work the same way.  Some operate at different pressure ratings.  They range 12 to 20 PSI for most throttle body systems and 40 to 50 PSI for most port injection systems.  Injectors have different electrical resistances.  Throttle body injectors are usually 1.2 or 2.4 ohm and are considered Low impedance.  Port injectors can be either 2.4 or 12-16 ohm with the 12-16 ohm injectors being considered high impedance. High impedance injectors are general purpose and used in many production cars and are driven with what is called saturation drivers.  The Low impedance injectors are usually found in high performance injectors or injectors used on high RPM engines and require a Peak and Hold type of driver.  High impedance injectors can be used on either type of driver, but Low Impedance injectors cannot be used on a Saturation driver.  Injectors also have a defined “spray” pattern.  Most Throttle body injectors spray in a wide fan spray while a Port injector sprays a narrow pattern that is directed at the open valve.  They can be interchanged but some performance may be sacrificed.  The last point about injectors is their flow rate.  Different size injectors are needed for different engines.  It is kind of like changing jets in a carb.  Most production injectors are rated at 15 to 30 pounds of fuel per hour.  This means that if the injector is open full time that it will actually flow this many pounds per hour of fuel.  There are about 6 pounds of Gas per Gallon.  Higher flow injectors are available for racing up to 200 lb./hr for alcohol racing applications.  A rough rule of thumb is that you can get 2 HP for every lb./hr of fuel.  This means if you have 8 20lb injectors then you have the ability to generate a maximum of 320 HP.  If you would want more Horsepower than you would need larger injectors and the corresponding engine modifications.

The fuel pump is as critical as the injectors are for sizing.  Fuel pumps for EFI systems are always electric and must provide a continuous flow of fuel to the engine.  EFI pumps come in two pressure ranges, Low and High.  The Low-pressure pumps are for the 12-15 psi injectors and have a maximum pressure of 30 psi if blocked off.  The high-pressure pumps have ratings from 30 to 80 psi and may develop over 150 psi if the output is restricted.  This can potentially be very dangerous if a leak were to develop.  The pump is noticeably smaller than an electric pump for a carburetor.  This is because they run at a higher rpm and run the fuel through the pump to cool it.  Some mount inside the fuel tank and others mount outside on the frame.  If a pump is mounted outside the tank, then it must be kept close to the fuel tank and near the bottom of the tank to make it not have to work as hard.  They are a rotor type pump and providing they never run dry, will last for a very long time.  If one runs dry, then they may fail rapidly. 

EFI systems use a different type of fuel line.  Because of the higher pressures involved, you should never use low-pressure fuel line intended for carburetors.  It WILL rupture and under 50PSI of fuel, a blown hose can make a nasty flamethrower if the leak were to catch fire.  Most production automobiles use a plastic elastomer hose that is not effected by the environment and is harder to make leak.  Some models use a special rubber hose that is rated at 250 to 300 PSI and special screw clamps.  This hose is quite acceptable but should be checked often for deterioration and replaced if necessary.  You should never stick the rubber hose over a bare tube end without some type of retaining barb.  The high pressure and lubricating properties of the fuel will blow the hose right off the tubing.

Fuel filters for an EFI system are not much different from a carbureted automobile.  The biggest difference is that the filter element has a finer mesh and will filter smaller particles.  This is necessary since the injector is very sensitive to dirt.  It also means that they will plug up easier if you get dirty gas.  When the fuel pump in a car gets noisy or the engine cuts out at full throttle, then the fuel filter is plugged and must be changed.  They can get so plugged up that you cannot blow through them but the engine will still run.  The EFI filter is also made especially to handle the higher pressures of the EFI system.  To use a normal carburetor filter on an EFI system may cause the filter to rupture.  It is also necessary to have a filter of some type before the pump because small particles will damage the pump.  In tank pumps have filters on the pump but if the fuel pickup in the tank for an external pump does not have a filter, then an external prefilter needs to be used.

The fuel injectors require a regulated pressure to work properly.  On an EFI system a bypass type pressure regulator is used.  This means that any excess fuel is bypassed and returned to the fuel tank, which necessitates having a return line in the fuel tank.  This return line must not just dump into the top of the tank, but go all the way to the bottom of the tank to keep from stirring up bubbles.  The pressure regulator is mounted on the fuel rail somewhere after the pump and injectors.  Throttle body systems have the regulator mounted in the throttle body.  On some systems there is another device that looks like a pressure regulator mounted on the inlet side of the fuel rail, but it is a pressure damper.  It dampens out the pressure fluctuations caused by the opening and closing of the injectors.  Some pressure regulators can have their pressure adjusted but most cant.  All EFI pressure regulators need to be referenced to the intake manifold vacuum so that they may adjust the fuel pressure to keep it constant across the injector regardless of the Amount of vacuum or pressure in the intake manifold.

The MAP sensor reads the pressure in the intake manifold and sends this information to the ECM.  It connects with a small piece of hose and is mounted in the engine compartment.  The most common types output a voltage that is proportional to the amount of vacuum in the intake manifold.  It is called an Absolute sensor because it measures the pressure with respect to a true vacuum.  This gives it the ability to compensate for altitude.

The TPS sensor gives the ECM information on what the position of the throttle plates are.  That allows the ECM to compensate for idle conditions and acceleration.  Most current systems use a sensor that is merely a variable resistor like a stereo volume control.  It will output a voltage proportional to the position of the throttle plates.  Some older systems had sensors that had a series of switches that made contact as the throttle was moved.

The ECM requires knowledge of the engine temperature so that it may run properly when first started and is cold.  Carbureted cars had a choke with a temperature sensitive actuator that would release the choke as the car warmed up.  The EFI system does this by increasing the amount of fuel going into the engine.  The result is no noticeable loss of power with a cold engine.  The coolant sensor is a special type of resistor that changes resistance as temperature changes.

Air Temperature sensor: 

The Air temp sensor is similar to the coolant temp sensor except it is measuring the temperature of the air in the intake manifold.  Measuring the Air temp allows the ECM to compensate for the slight density variations of air and thus maximize fuel economy and minimize exhaust emissions.  This is most critical in blown or turbocharged cars where the intake temperatures will rise under boost.

The O2 sensor is a device that is placed in the exhaust to measure the amount of unburned fuel in the exhaust.  The more unburned fuel, the less Oxygen is present.  For the Catalytic converter to work properly, the fuel ration must be kept near 14.5 to 1 air to fuel ratio.  If it gets way out, then the Catalyst is not as efficient and may be damaged.  Running too rich results in that rotten egg smell that came from many 70’s cars.  The O2 sensor needs to be hot to work.  When heated it forms a thermic reaction and will generate a small voltage depending on the amount of air in the exhaust.  The ECM can then adjust the amount of fuel to correct the fuel ratio.  Since the O2 sensor needs to be hot, they are usually not used right after the engine is started although some types have built in electric heaters to make them heat up faster.  Often, an O2 sensor can help make an engine run well with poorly defined fuel maps, but the engine should not depend on the O2 sensor for its primary fuel calculations.  Production cars have a default map that controls the fuel mixture in the event the O2 sensor fails to function correctly.

On ECMs that also control the ignition timing, they often employ the use of a special microphone to listen for engine knock.  This microphone is mounted on some hard portion of the block and monitors noise of the knock transmitted through the block.  If the sensor hears knock, then it alerts the ECM and the ECM will retard the timing to prevent damage to the engine.  Controlling timing this way will allow the engine to run on a wide variety of fuels and will prevent damage if one gets a bad batch of gas.