Description of the Stock MFI System
To start things off
The following content was taken from a 1995 shop manual and personal experience. Almost all of it should apply to the 1996-1999 cars too. The Multi-port Fuel Injection system [MFI] consists of sensors which detect the engine conditions, the Powertrain Control Module [PCM] which controls the system based on signals from these sensors, and actuators which operate under the control of the PCM. The PCM carries out activities such as fuel injection control, idle air control and ignition timing control. The PCM is often used synonymously with the Electronic Control Unit [ECU].
The fuel injector drive times and injector timing are controlled so that the optimum air/fuel mixture is supplied to the engine to correspond to the continually changing engine operation conditions. A single injector is mounted at the intake port of each cylinder. Fuel is sent under pressure from the fuel tank by the fuel pump, with the pressure being regulated by the fuel pressure regulator. The fuel thus regulated is distributed to each of the injectors. Fuel injection is normally carried out once for each cylinder for every two rotations of the crankshaft. Remember that the piston makes 4 strokes for every 2 rotations of the crankshaft [intake, compression, power, and exhaust stroke]. The fuel is generally injected during the intake stroke. The firing order is 1-3-4-2. The 4th cylinder is closest to the driver. This is called multiport fuel injection. The PCM provides a richer air/fuel mixture by carrying out "open loop" control when the engine is cold or operating under high load conditions [approximately 80% throttle and above] in order to maintain engine performance. In addition, when the engine is warm or operating under normal conditions, the PCM controls the air/fuel mixture by using the heated oxygen sensor signal to carry out "closed loop" control in order to obtain the theoretical air/fuel mixture ratio that provided the maximum cleaning performance from the three way catalyst [catalytic converter].
The sensors that the ECU uses
MAP or Manifold Absolute Pressure sensor: Semiconductor type sensor that read the absolute pressure of the air in the intake manifold. It outputs a voltage from 0-5 volts that varies with pressure. This helps the ECU figure out how much air the engine is injesting.
IAT or Intake Air Temperature sensor: Thermistor type sensor that reads the temperature of the air in the intake manifold. It outputs a resistance from about 0.6 to 11 kilo-ohms that varies with temperature. This helps the ECU figure out how much air the engine is injesting.
ECT or Engine Coolant Temperature sensor: Thermistor type sensor that reads the temperature of the coolant in the engine cooling system. It outputs a resistance from about 0.6 to 11 kilo-ohms that varies with temperature. This lets the ECU know then the car is warmed up.
O2 or Oxygen sensor: A zirconia type sensor with an integral heater. It reads the oxygen content of the exhaust gas. It outputs a voltage from 0-1 volts that varies with oxygen content. This helps the ECU figure out how much fuel the engine really needs.
TPS or Throttle Position Sensor: A variable resistor type sensor that reads how far open the throttle plate is. It outputs a resistance that varies with throttle plate angle. The ECU uses this to determine the load being placed on the engine.
Vehicle Speed sensor: This sensor reads the actual speed of the vehicle. Not much to say here; its runs the speedometer and the ECU uses it for some other purposes.
CMP or Camshaft Position sensor: Hall-effect type element. It reads what position the camshaft is in at any moment. The ECU uses this sensor to decide when to fire the spark plugs and fuel injectors.
CKP or Crankshaft Position sensor: Hall-effect type element. It reads what position the crankhaft is in at any moment. The ECU uses this sensor to decide when to fire the spark plugs and fuel injectors.
Knock sensor: Its a piezoelectric type element. The ECU uses this sensor to listen for the sound of knock. The ECU can retard ignition timing if it hears knock.
The actuators that the ECU uses
Fuel Injector: Electromagnetic actuator. When a current is supplied to it, the valve opens and fuel is shot out.
EGR or Exhaust Gas Recirculation transducer solenoid: On/Off type solenoid valve; its either open or closed. This lets exhaust gas enter the intake manifold to be reburnt.
EVAP or Evaporative emission purge solenoid: Duty cycle type solenoid valve; it remains open to variable amounts of time like a fuel injector. This vents gases that build up in the gas tank.
IAC or Idle Air Control motor: This is a stepper type motor. It opens a valve that can allow some air to bypass the throttle plate. The ECU uses this to control idle speed.
And now a word on fuel trim
This only applies under "closed loop" conditions [warm engine, less than ~80% throttle]. In case you didn't know already, the ECU has basic fuel maps permanently in its memory. When you reset the ECU, those are the maps it uses first. It takes in data from IAT, MAP, TPS, etc and looks up in its memory [fuel map] how much fuel it should squirt into the cylinder. Since every car is a little different, chances are that the basic fuel maps the factory created won't provide perfect combustion. This is where "fuel trim" comes in. The ECU uses the oxygen sensor mounted in the exhaust to see if the engine is running rich or lean. If the oxygen sensor says that you're running lean, the ECU will put a little more fuel in to get perfect combustion. How much extra fuel the ECU puts in is the "fuel trim". There are two types of fuel trim...short term and long term. Short term fuel trim is the ECU's quick reaction to providing the proper amount of fuel. If the ECU keeps having to add or subtract fuel with the short term fuel trim, it will start to build a new fuel map to incorporate this change. It uses long term fuel trim to do this. Long term fuel trim changes slowly; it might take several seconds to change by a few percent.
At a certain operating condition, the basic fuel map says you should put in X amount of fuel. The ECU squirts in X amount of fuel and looks at the exhaust to see if its guess was correct. The exhaust gas looks 15% lean. On the next try it will squirt in 1.15*X amount of fuel [short term fuel trim = 15%]. The ECU's guess was correct and it keeps squirting in 1.15*X amount of fuel at that operating condition. The long term fuel trim will slowly change from 0% to 15%. Eventually the long term fuel trim will be 15% and the short term fuel trim will be 0% to squirt in 1.15*X amount of fuel at this certain operating condition.
A datalogger can read fuel trim and help you tune
The short term and long term fuel trim both max out at 25%. If you max out both of your fuel trims, the ECU can do nothing to adjust the fuel to get the car to run right. You might see knocking or stumbling. Its OK if your long term fuel trim is high, the ECU can still adjust fuel considerably with the short term trim. You want your short term trim to be low all the time (like 5-10%), so your ECU has plenty of room to make adjustments to keep the engine running well. This is good for driveability, emissions, and mileage. If you have an adjustable fuel pressure regulator and S-AFC, you should adjust them so that your short term fuel trim is low at all "closed loop" driving conditions. This includes idle, cruise, and normal acceleration [less than ~80% throttle].
Contributed by Corbin
Cars GenEd ECU Description of the Stock MFI System
Last modified on 2008-12-14 13:12:43