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For a few years, I've been using a Bluetooth <-> OBDII adapter with the Android Torque app. One of the cool things about this is that it's taught me things about a car's operation that I may not have learned about had it not been for that data. One of those things is Short Term and Long Term Fuel Trim.

My understanding of these concepts goes like this: Assuming some baseline level of fuel injection which is measured in the pulse width of the injector (i.e. the amount of time the injector stays open), the trims are increased or decreased to add or remove fuel from the mixture in order to achieve a more ideal fuel mixture for maximum fuel economy and minimal wasted fuel dumped into the exhaust system.

I know that the fuel trims are based off of readings from the oxygen sensors and as such are not used until the car has "warmed up" enough to operate in a closed loop mode (using the oxygen sensor values).

But my question is what are the inputs used to provide that base fuel calculation?. The only inputs I know of are Mass Air Flow (How much air is being pulled into the engine), Intake Air Temperature (If cold air is coming in, supply more fuel) and Engine Coolant Temperature (If the car is cold, supply more fuel). The last one also includes open/closed loop, and I thought that once it hit a certain threshold, it switched to a lower fuel input, but from my examination of the data from my vehicle, the fuel flow slowly decreases as a function of temperature, whether in closed or open loop mode.

  • There is no calculation. There is a very big look up table. The table incorporates RPM, mass air flow, engine coolant temperature and air temperature. This is open loop operation. When the vehicle goes closed loop it verifies what is coming out the tail pipe and then adds or subtracts fuel based on that. – vini_i Jan 4 '17 at 16:48
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This is for the 80's-90's Dodge ECU. A bit out of date, and for a MAP (not MAF) system, but has some good info. http://thedodgegarage.com/turbo_pfi.html

It will vary by manufacturer, but typically the ECU will compare the input from the MAF against a look up table to determine the base injector pulse width.

The other sensors are used for adjustments. Your main ones here are:
Air Temperature Sensor - more or less fuel based on incoming air
Coolant Temperature Sensor - Add more fuel when the engine is cold
Throttle Position Sensor - give an extra bit of fuel when the throttle is opened quickly
Oxygen Sensor - Add or remove fuel based on readings

Are you sure the fuel trims are not used when the engine is cold? As long as all of the sensors are reading, I don't see why the engine wouldn't use them. Heated oxygen sensors should be reading within a minute of starting the vehicle.

  • You are correct, heated oxygen sensors should start reading quickly, but on my 1998 Buick Century I can correlate the oxygen sensor readings/fuel trims to a temperature level (Short Term Fuel Trim starts changing at 71.6 F engine coolant temperature) – StephenH Jan 4 '17 at 16:55
  • Ok, I was thinking by cold you meant not fully warmed up, like less than 150* F. 71.6* F seems a bit more reasonable. – rpmerf Jan 4 '17 at 19:36
  • Also the ignition timing is controlled as well – Solar Mike Jan 8 '17 at 18:32

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