What’s normal, what isn’t?

When looking at fuel trim on a scan tool, it should be checked for at least 30 seconds at three different engine speeds: idle, 1,500 rpm and 2,500 rpm. If you make a scan tool recording while driving the vehicle, you can see how fuel trims change under changing loads.

When everything is stable and working correctly, fuel trim numbers should be no greater than 10%, and total fuel trim should be no more than 10% when the numbers are added together. For example, if LTFT is 4% and STFT is 4%, the total is 8%: that’s acceptable.

If LTFT is 12% and STFT is negative 6%, total fuel trim is 6%. This shows that the PCM has enough control to keep the catalyst working correctly, but the high LTFT number shows it’s compensating for something. On an older engine, LTFT is typically a bit higher as the PCM compensates for normal wear.

If fuel trim is significantly greater than 10% positive or negative, the PCM is compensating for more than just normal wear-and-tear. Whether it’s an older engine with a simple oxygen sensor or a newer model with a wide-band air/fuel ratio sensor, LTFT will continue to shift as needed to keep the STFT swings in the correct range.

LTFT can shift surprisingly far, but when it reaches plus or minus 25%, the MIL will be illuminated and a code will be set. Fault codes specific to fuel trim are:

P0170: fuel trim bank 1
P0171: system too lean (bank 1)
P0172: system too rich (bank 1)
P0173: fuel trim bank 2
P0174: system too lean (bank 2)
P0175: system too rich (bank 2)
By the time LTFT gets to 25%, there will be other codes, too.

But if LTFT is below that limit with or without other codes, you can still gain a lot of information from the scan tool before connecting additional test equipment to confirm your diagnosis.

Why are the fuel trim numbers high?

If LTFT or total fuel trim is greater than plus 10%, the PCM thinks the air/fuel ratio is too lean and it’s adding fuel to bring STFT control to the correct range. This presents three possibilities:

Unmeasured air is reaching the combustion chambers.
Less than the commanded amount of fuel is reaching the combustion chambers.
One or more sensors is reporting incorrectly.
When thinking about what would cause each of these conditions, the first thing to consider is how the PCM determines airflow. If the engine uses a mass airflow sensor (MAF), high fuel trims at idle are a classic symptom of a vacuum leak, especially if LTFT decreases at higher engine speeds.

Since the amount of air flowing through the vacuum leak doesn’t increase, the leak has less affect on air/fuel ratio at higher speeds and loads, so LTFT will come down as engine speed goes up. When you look for vacuum leaks, don’t forget the various “calibrated vacuum leaks” like crankcase ventilation, the evaporative emissions (EVAP) purge valve and, if equipped, air-shrouded injectors.

A dirty or faulty MAF sensor can also cause positive fuel trim numbers because it “under-reports” airflow, driving the base air/fuel calculation lean.

On engines that use a manifold absolute pressure (MAP) sensor to determine airflow, a vacuum leak does not affect fuel trim because the extra air (pressure) in the manifold is still measured by the MAP sensor.

Low fuel delivery will cause an increase in LTFT as the PCM tries to compensate for extra oxygen in the exhaust stream. Remember, the PCM doesn’t measure fuel flow; it only knows injector pulse width and assumes fuel delivery is correct as commanded. Is equivalence ratio changing, too? If you add propane and see LTFT and equivalence ratio numbers come down, there probably is a fuel delivery problem. If there’s no change, an oxygen sensor may be faulty or shorted to ground. Don’t forget to check this at different speeds and loads, because fuel flow problems often don’t show up at idle.

If the engine has two cylinder banks (even some four-cylinder engines are split into two banks), compare the readings to see if the problem affects both banks.

The PCM consults all oxygen sensors in the system when calculating fuel trim. Normally the rear (post catalyst) sensor voltage will be fairly stable near the middle of its range, but LTFT will likely increase if that sensor reading is low (remember, low is lean).

A catalyst code will influence both short- and long-term fuel trims. An exhaust leak after the catalyst will probably only affect the rear oxygen sensor.