Workshop calculator
Fuel trim calculator: vacuum leak, MAF or fuel supply?
Total fuel trim is the short-term plus the long-term correction. How it changes between idle and higher rpm tells you where to look: a vacuum leak matters most at idle, a MAF or fuel delivery problem shows everywhere.
- Total trim at idle
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- Total trim at 2,500 rpm
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How to use it in a diagnosis
- Warm the engine fully and turn off loads (AC, lights).
- Note short- and long-term fuel trims at idle.
- Hold 2,500 rpm steady for 30 seconds and note them again.
- Enter both pairs; compare banks on V engines.
Questions mechanics ask
What are normal fuel trims?
Total fuel trim (short-term plus long-term) within about ±10 % is normal. Beyond ±15–20 % most ECUs set a lean or rich code such as P0171 or P0172.
Why are my fuel trims high at idle but normal at 2,500 rpm?
That is the classic sign of a vacuum leak: at idle the unmetered air is a big share of the total airflow, at higher rpm it becomes small. Smoke-test the intake and check the PCV system.
What do positive and negative fuel trims mean?
Positive trims mean the ECU is adding fuel because the mixture reads lean. Negative trims mean it is removing fuel because the mixture reads rich.