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Basic Operation of EFI controlled Vehicles

This will describe the basic operation of EFI controlled vehicles.  I may have left some thing out because I wrote this on a road trip.

As the air comes out of the air filter it passes over the MAF (Mass air flow) sensor.  This sensor has a thin wire that is heated to a specific temperature.  As air passes by the wire cools and requires more voltage to maintain the preset temperature.  This thin wire is very delicate and should never be touched.  It can be cleaned by gently spraying it with electrical parts cleaner or MAF cleaner.  This tells the ECU (engine control unit, also know as the ECM engine control module, PCM power control module and other names as well) how much fuel to add for the specified air fuel ratio.  Some cars use MAP (manifold absolute pressure) in addition to, or instead of the MAF, for fueling calculations.  Generally MAP based systems are not as good as MAF based systems because they cannot react to barometric and temperature variations as well as MAF based systems. Some cars like the 03-04 cobra use both sensors.    After the air passes by the maf, it heads toward the throttle body (TB).  The throttle body uses a mechanical valve to control the air flow allowed into the motor by the throttle input cable or signal on ETC (electronic throttle control) car.  There is a small valve on the side of the TB called the IAC (intake air controller).  The IAC controls the air flow around the throttle plate during idle and part throttle operations.  Its primary function is to control idle speed by adjusting the amount of air that goes around the closed throttle plate.  Some symptoms of a faulty IAC valve can be stalling after stopping or clutch in, rough idle, and high idle.  Sometimes a high idle can be caused by a throttle plate that is set open a little too much.  This allows too much air to enter the motor even with the IAC valve fully closed.  Most of the time the throttle plate can be adjusted by backing off the set screw.  More closed is usually better than more open.  The air then flows through the intake manifold, past the intake valves, into the combustion chamber as the piston comes down.  At the same time the fuel injectors pulse at the pulse width determined by the ECU from the MAF input to maintain the air fuel ratio specified in the ECU.   As the piston comes up the spark plug fires at the specific timing specified from the ECU for the given conditions based on ECT (engine coolant temperature) and IAT (intake air temperature, or ACT air charge temp), rpm, and load.   If the spark plug fires too early the may be detonation resulting from the increased burn time and compression.  Please read more about detonation here.  As the piston is pushed down on the power stoke, most of the power is transferred from the burning mixture to the crankshaft producing hp and tq.  As the piston returns to TDC (top dead center) the exhaust valves open and let out the spent gasses.  If the timing values are too low the mixture will still be burning on the way out causing excessive valve temperatures and high EGTs (exhaust gas temperatures).  Please read more about EGTs here.    There is a port on the headers that directs some of the exhaust back into the intake via the EGR valve (exhaust gas recirculation).  EGR is only used at part throttle operation and not at idle or WOT.  It cools the combustion chamber by slowing the burn times and subsequently requires more timing to get the slow mixture to burn.  It also increases fuel mileage slightly and reduces emissions.  Removal of the EGR system may be against the law in your state and usually requires a retune to reduce timing in these areas to avoid knock.  Generally EGR is good and there are no performance gains to be had with its removal.  Some people remove it to clean up the engine bay.  The exhaust then flows down the headers and past the upstream O2 sensor.  These are narrowband sensors that do a great job of measuring the air fuel ratio near the stoichiometric point of 14.7:1.  This is 14.7 pounds of air for 1 pound of fuel.  You can read more about narrowband and wideband sensors here and why they cannot do the others job EVER!  Please do not use narrowband sensors for measuring the WOT AF because they will not be accurate.  The same applies for using the simulated narrowband output of a wideband for narrowband data.  They are just not optimized to do the others job.  The O2 sensor uses a ceramic bulb with platinum and zirconium to measure the left over oxygen in the exhaust gasses.  It relays this information to the ECU to constantly recalibrate  the MAF signal to maintain the AF ratio.  This is called closed loop operation because the computer is reacting on input from the O2 sensors in a "loop" to control the fueling based on how it just reacted.  This recalibration of the MAF signal can be logged as the STFT or LTFT Short or Long Term Fuel Trims.  Ideally it will be between + 5%, but there is no real danger if its + 10%.  If it is more than 10% the car should be retuned.  It pegs at 25% and can no longer adjust the air fuel.  This situation needs immediate attention.  At a certain throttle position the car will go to open loop operation and use the MAF signal to calculate fueling with no input from the O2 sensors.  During cold startup the car will be in open loop as well to give the O2 sensors time to heat up and work properly.   The exhaust then flows past the catalytic converters (cats).  The cats use a catalyst to burn up the unburnt fuel to reduce emissions.  Removal of the cats is generally illegal in most states and can come with stiff penalties in some cases.  Just behind the cats is the rear O2 sensor or downstream sensor.  This sensor is not used to control fueling like the upstream O2 sensor and its purpose is to monitor the efficiency of the cats and alert the driver with a check engine light if the cats are failing.  These sensors can be turned off in many cases with no effect on power or drivability.   This covers the general basic operation of EFI based systems.  If you have any questions or comments please email joe@tricktuners.com

Fuel Systems

Fuel systems come in 2 varieties, return systems and returnless systems.  Return systems are generally simpler and use a mechanical fuel pressure regulator to control the fuel pressure.  The fuel pressure is usually set to around 40psi + boost, or - vacuum.  For example at idle and 20" of vacuum (~-10psi) the fuel pressure at the rail (RAP rail absolute pressure) should be 30psi.  At 0 vacuum it should be 40psi, and at 10psi of boost it should be at 50psi.  This is why its important to have a vacuum/ boost signal on your fuel pressure regulator.  The regulator sends the unused fuel back to the fuel tank completing the return style system.  Returnless systems generally employ a sensor on the fuel rail that measures the fuel pressure as well as the vacuum/boost signal.  You can imagine it as a diaphragm with fuel pressure pushing on one side and vacuum/boost on the other side.  The diaphragm needs to be in the center at 40psi all the time, so if there is vacuum pulling it to the vacuum/boost side the computer will lower the fuel pressure to maintain the same pressure drop across the injectors (PDAI).  When there is boost pressing the diaphragm the other way, the computer will raise the fuel pressure to compensate and return the diaphragm to the center.  Returnless systems use a different type of fuel pump and cannot be used with pumps designed for retune style fuel systems.  The voltage to the fuel pump is constantly adjusted from input on the FRPS (fuel rail pressure sensor).  These sensors can sometimes be damaged on high boost vehicles resulting in hesitation, stumbling, etc.  Generally there will be fuel on the air side of the diaphragm when they blow, but not always.  There is a disk from Kenne Bell with a small hole in it that can be used to dampen pressure spikes common to returnless systems.  The pressure spikes because the fuel has momentum when the injectors close (on a shift).  This can cause a hesitation or nose over because the computer sees a high psi spike and tries to lower the fuel pressure, but it reacts too slow and the driver is back on the gas, but the fuel pressure is still to low causing a lean condition.  Most of the time this can be resolved by recalibrating the fuel pump settings in the tune.  Sometimes removal of the PPRV (positive pressure release valve) inside the fuel tank near the fuel pumps will help.  The PPRV was designed to hold fuel in the system to aid in quick startups with a 1 way valve.  It has a blow off around 67 psi to reduce the spike, but due to production variances the spike can sometimes be much higher.  Its removal allows this fuel pressure spike to flow back through the pumps reducing the pressure spike.  It generally requires slightly more cranking time after its removal, but is generally never a problem for anyone.

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Last updated: 03/07/10.