What causes high positive short-term fuel trim? (2023)

What causes high positive short term fuel trim?

If the LTFT fuel trim readings are trending up (POSITIVE), it means the fuel feedback control system is compensating for an air/fuel mixture that is becoming progressively leaner over time. The most likely cause would be dirty fuel injectors.

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.

Rich. Running rich means that there is too much gas in the air-fuel mixture. This can happen for a variety of reasons, but in a normally functioning system, the ECU will tell it to cut back on the fuel flow to compensate.

When you increase engine RPM, three things happen: airflow greatly increases, manifold vacuum drops, and the amount of fuel leaking through the diaphragm becomes much less significant. This would cause total fuel trim numbers to work their way back to normal ranges as the RPM increases.

A failing evap purge solenoid - as a special case of vacuum leak - is able to cause both rich and lean codes: when stuck open after refueling massive fuel vapors will be sucked into the intake and can trigger a P0172/P0175.

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.

Positive fuel trim values

If the displayed fuel trim value is a positive number, it means that the ECU is increasing the injector pulse width to add fuel to the air/fuel mixture to enrich the mixture because the input data it is receiving tells it that the mixture is too lean.

As the exhaust valve leak increases, the short-term fuel trims are driven negative enough to force the long-term fuel trims to go negative as well. Remember, even with a small leak, a cylinder can still be ignited. An example of this is an engine that misfires at idle only.

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.

How do I fix my engine running too rich?

How to Fix P0172 Engine Code in 2 Minutes [2 DIY Methods / Only $8.77]

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.