Why are the numbers low?
If LTFT or total fuel trim is more than 10% negative, the PCM thinks the air/fuel ratio is too rich, so it is leaning out the mixture calculation to return STFT control to the correct range. This presents three possibilities:
One thing that restricts airflow into the cylinders is a damaged catalytic converter that causes high exhaust back pressure. At idle this can drive fuel trims in opposite directions, producing a positive STFT and negative LTFT. You can see signs of high exhaust back pressure on a scan tool; calculated load will be low at wide-open throttle (WOT) and fuel trims will trend negative as engine speed increases.
The most obvious source of excess fuel is a leaking injector, especially at idle. In this case, STFT will be low but increase with engine speed as the extra fuel represents less of the total fuel requirement. Excessive crankcase vapors or a flooded EVAP canister can also mimic excess fuel, especially at idle. If the oil hasn’t been changed in a long time, especially in an older engine with a bit of blow-by, fuel in the oil can drive total fuel trim negative. Often simply changing the oil will demonstrate this condition by returning the fuel trim numbers to normal.
In a speed/density system, the PCM will substitute a pre-programmed value for a faulty barometric sensor. If the vehicle is more than a few hundred feet above sea level, this will show up as negative LTFT numbers.
Confirming the repair
There are two ways to use fuel trim to confirm a repair. One is to make sure total fuel trim is within 10% and then go for a test drive. Long-term fuel trim should almost immediately begin returning to normal.
It may take a few miles and/or cold-starts, but it’s a good way to watch the PCM learn the “new normal.”
A faster way is to clear the codes and, along with them, the PCM’s adaptive memory. When you start the engine with all the fuel trims at zero, watch the short-term fuel trim.
When the system goes into closed loop, STFT should stay within 10%, and as the engine warms up, total fuel trim will stay within 10% at all speeds and loads. If STFT quickly starts swinging into double digits, there’s still something wrong.
To learn more about how fuel trim is affected by various problems and conditions, connect your scan tool to a known-good vehicle and create some problems: introduce vacuum leaks, unplug an injector, disconnect a sensor, add propane: See how the PCM compensates with substitute values and fuel trim adjustments. Note the different reactions in MAF systems versus MAP systems.
On multi-bank engines, note how problems on one bank can affect fuel trim on the other, or not.
Like everything else in diagnostics, there’s nothing like first-hand experience with known-bad and known-good vehicles.
Once you have a feel for what fuel trim is supposed to look like under specific conditions, it will become one of the fastest and most useful diagnostic tools in your tool box.
Author’s note: Thanks to Snap-on Diagnostics for their assistance in preparing this article. ●
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