FUEL TRIMS HIGH, LOW, MIDDLE (range of 60 to 140% for 91+ cars, 80 to 120% on 90's)

These are the "long term" memory fuel tables in the ECU. When you drive the car in closed loop (normal to low-boost driving, rpm<4500), the ECU is constantly updating these trim values. The ECU takes the signal from the MAS and calculates how much fuel to add to maintain an a/f ratio of 14.7:1, then compares the O2 readings against the value it calculated and adjusts the fuel trims (adding more fuel or taking away) to try and stay at a constant "stoichiometric" value (14.7:1 air/fuel ratio). A trim value of 100% is stoichiometric, anything greater than 100% and you are running lean so the ECU is adding fuel, anything less than 100% and you are running rich and the ECU is subtracting fuel. The ECU always tries to run at a stoichiometric 14.7:1 ratio under closed loop operation and the values will be reset to 100% when the ECU is cleared.

These values will always be changing up or down a little depending on the air conditions and many other variables. Low, middle, high refer to the amount of airflow that the ECU sees while in closed loop. When the ECU goes into open loop, none of the fuel trims are updated.

Remember the old days of a tune-up where you would have to play with things like carburetor jetting? Well, the ECU does that for you now. All cars will change a bit over their lifetimes. The sensor calibration will drift, the injectors will get a little clogged, etc, etc. Fuel trim is the ECU's attempt at tuning your car.

As you can see in the program, there are three levels of fuel trim - low, medium, and high. These three levels correspond to three levels of air flow. Low is for idling, medium is roughly up to 50 MPH (I'm guessing, will be different depending on upgrades to your engine) and high is even higher. The range of adjustment is 80 to 120% for 90's and 60 to 140% for 91' and newer. 100% is the default set and relates to a stoichiometeric air/fuel ratio (which is 14.7:1).

These trims are adjusted while the ECU has the engine in full feedback with the oxygen sensor. They only get adjusted if everything on the car has been relatively steady for at least 5-10 seconds. The ECU takes the amount of air entering the engine, multiplies by 14.7:1 ratio, and calculates the fuel needed. By using the O2 sensor, it can check to see if the final result of this calculation is correct. If the O2 reads lean, the ECU increases its gain term a bit. If it O2 reads rich, it decreases the gain term a bit. If the car is stable, it starts to bleed this gain term into the current fuel trim value. These fuel trim values are battery backed-up so that the ECU will be ready to apply them the next time the car starts.

You do NOT have to strive to keep the trims at 100%. You can actually run higher fuel trims and in my case I get better gas mileage. All three of my fuel trims were at 140% and the car ran fine and got better mileage because I had increased the airflow by gutting the MAF sensor which caused me to run leaner than stock (100%) so the ECU was adding as much fuel as it could (140%) to try and get the trim value down to stoichiometric again. I try and run my trims aroudn 110% now to get the best mileage and throttle response.

WARNING!!! You can run too lean at idle/part throttle and the car will not idle well and your drivability will suffer. EVERY MAF SENSOR IS DIFFERENT but most people have been able to run with the lower honeycomb removed, screw backed out all the way until it's flush and even trimming and filing the leading edges of the lower passage to a smooth "funnel" like flow surface and still run fine.