How to find an air leak using fuel trims
One two-minute experiment halves a dozen possible causes — and costs nothing.
Warm the engine up
Everything below holds only on a warm engine: when cold, the unit deliberately runs rich and the trims mean nothing. Wait for operating temperature, usually 85–95 °C.
Open the live data and find the fuel trims
You need two numbers: short term and long term trim. Read them TOGETHER — minus ten with plus fifteen is plus five in total, which is nearly normal.
Note the total at idle
Car standing, air conditioning off, idle steady. Plus five or less is normal. Plus ten is a conversation. Plus twenty-five means the unit is at the end of its range and the lamp is coming.
Hold 2500 rpm and note it again
Hold the revs steady for half a minute so the unit can adjust. Note the same total.
Compare the two numbers
A large plus at idle and almost nothing at revs means an AIR LEAK: at idle the manifold is under vacuum and extra air gets in easily, while at higher revs its share becomes negligible. About the same plus everywhere points at the supply instead: the air meter, the pump, the filter.
Find the place
If the experiment pointed at a leak, look at the intake boot after the air filter for cracks, check the hoses are seated, the throttle gasket, the crankcase ventilation hoses and the vacuum lines. Looking costs nothing and usually settles it.
Check the result
After the repair, repeat the same experiment. The trims should return to zero — not at once but over several trips: the accumulated long term value comes down gradually.