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Fuel System Monitor — I/M Fuel System Monitor

Fuel System Monitor — How it works

The Fuel System Monitor is actually more of a Fuel System Correction program. The ECM is constantly monitoring how well it did in calculating fuel-to-air ratios. When it must make an adjustment from factory programming; either by adding fuel or subtracting fuel, it enters those corrections into either short term fuel corrections or long term fuel corrections. Those corrections are referred to as Fuel Trim

What is Fuel Trim?

Fuel Trim is a set of positive and negative adjustments to factory air/fuel programming to achieve the desired power and emissions goals. For example, if the oxygen sensor detects that a particular air/fuel mixture results in an oxygen stream that is too lean (too much air/not enough fuel) or rich (too much fuel/not enough air) air-fuel mixture, the Fuel Trim program adds or subtracts fuel to adjust, but only up to a certain percent (usually 25%).

Short term versus long term fuel trim

It’s normal to see the ECM adjust fuel trims as you drive. After all, you’re constantly moving the accelerator pedal to compensate for uphill/downhill conditions, wind, cornering and stopping. So it’s not uncommon to see wide variation is short term fuel trims.

However, if the ECM constantly has to add or subtract fuel to obtain the desired power and emissions, it will move the fuel adjustment out of the Short Term Fuel Trim (STFT) category and into the Long Term Fuel Trim (LTFT) category. From that point on, each additional engine start will adjust fuel based on the long term fuel adjustment.

When long term fuel trim stays too high

The ECM is programmed to detect a problem with fuel trims. If the fuel adjustment percentage is too high and stays high for too long a period of time (exceeds the time and percent allowed by the program), the ECM will light the check engine light and set a fuel related trouble code.

Generally speaking, LTFT less than 10% will not set a trouble code, while fuel trims approaching or exceeding 25% will set a trouble code

Fuel System Monitor is supported by both “spark ignition” vehicles and “compression ignition” (diesel) vehicles. The Fuel System Monitor may be a “One-Trip” or “Two-Trip” Monitor, depending on the severity of the problem.

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Posted on by Rick Muscoplat

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