What causes high long term fuel trim? (2023)

What can cause fuel trim to be high?

If the value is a positive number, it means that the ECU is adding more fuel to the mixture to enrich it because the information it's receiving indicates that the air-fuel mixture is too lean.

Long-term fuel trim values

Ideally, long-term fuel trims should be at, or close to 0% when the engine is running at a steady speed. However, while changes to the engine speed will (and must) produce changes in the long-term fuel trim value, this value should return to a point close to 0% when the engine speed steadies.

Both LTFT and STFT can be positive (more fuel) or negative (less fuel) percentages. LTFT and STFT are added together to produce the total fuel trim, and neither should exceed ± 10% in normal operating conditions. Fuel trims (LTFT, STFT, or LTFT+STFT) exceeding ±10% may indicate a problem in the engine or the sensors.

The oxygen sensors monitor the accuracy of the computer's fuel trim strategy. The oxygen sensor is located in the exhaust and monitors the amount of unburned oxygen in the exhaust. This is an indicator of the fuel mixture integrity.

This is done by disconnecting the battery, waiting for 10 minutes or so, and/or turning the ignition key to the starter position to discharge the system.

The voltage generated from the O2 sensor should be from 0.1V to 0.9V, with 0.9V on the rich side and 0.1V on the lean side. If your readings are within this range, the O2 sensor is functioning properly.

The ECM will adjust the fuel trim to compensate for continuous operation in cold or high altitudes. For example, a disconnected vacuum hose causes the fuel trim to trend lean (positive 10%). Conversely, a dripping fuel injector results in a rich air-fuel ratio, causing the fuel trim to trend rich (negative -10%).

What causes positive fuel trim at idle?

During idle when the throttle plate is closed, the vacuum in the intake manifold is high and very little air flow is entering the engine, so even a small amount of un-metered air will have an effect on fuel trim. This forces the PCM to react to the lean condition with positive valued fuel trim readings.

A properly functioning oxygen sensor will show a rapidly fluctuating output voltage between approximately 0.1 and 1.0 volts. The time taken for the voltage to change from 0.1 V to 1.0 V (referred to as the lean to rich response time) should be about 300 milliseconds.

A rich biased O2 sensor may cause inappropriately negative fuel trim on one bank, resulting in a lean misfire. There could actually be too much fuel going to Bank 1, but -35 percent fuel trim is still considered "in fuel control," so the engine should be running well. However, uneven fueling on Bank 1 could explain it.

What is the process for fuel trim diagnosis? Check the fuel trim at idle, at 1500 rpm and at 2500 rpm. After these no load checks, test drive the vehicle at various steady load conditions and look for any fuel trim variation.