Knowing what open and closed loop is, how it works, and what sensors it uses is the first step to understanding fuel trims. But how does closed and open loop use fuel trims? Let’s start with what both sets of fuel trims are first.
The short-term fuel trim (STFT) refers to immediate changes in fuel occurring several times per second. So, for instance, if you start going up a hill and need more fuel, if a vacuum line comes off and creates a lean condition, or if there is any airflow/fueling change in the moment, the STFT is there to assist. A negative fuel trim percentage indicates the PCM is removing fuel while a positive percentage indicates the PCM is adding fuel.
A normal STFT reading will generally fluctuate quite frequently, so don’t be surprised to see it as positive one second and negative the next. Usually, they'll stay within %5 +/- of zero. On an older engine, they may go up towards 10% at times.
The long-term fuel trims (LTFT) are driven by the short-term fuel trims. LTFT refers to changes in STFT but averaged over a longer period of time. A negative fuel trim percentage indicates a taking away of fuel while a positive percentage indicates an adding of fuel.
A normal long-term fuel trim reading will appear to stay the same, giving a long-term average of fuel added. It, too, should be close to zero, positive, or negative single digits under normal circumstances. It will fluctuate much slower, possibly appearing static.
The PCM will keep all of your LTFT data in the Keep Alive Memory (KAM). So, when you shut your car off, it can reference these LTFTs the next time it starts up. This is not the same for the STFT’s. When the car is shut down, all data is lost for STFT and learned again on the next start-up.
If you experience ST or LT fuel trims that are in the double digits, positive or negative, this would indicate an abnormal adding or lessening of fuel. This could be due to leaking fuel injectors, an unmetered air leak, or something similar. For example, if the o2 sensors are reading lean due to, say, a vacuum leak, the engine computer will compensate by adding fuel. Double-digit numbers can indicate a tune issue or a mechanical issue. That is our job to find out and know when to go what way.
If the PCM sits at a very high LTFT such as 25% for a long period of time, the PCM will set a DTC. Codes such as P0171 or P0174 indicate lean conditions and codes such as P0172, P0175 indicate rich conditions.