Q: Rich idle only, negative STFT

asked by on March 07, 2018

I've been having an issue lately with rich idle. Additive trim (stft) dips into the negatives very quickly, long term trims are right around 1. CEL for P0172 keeps coming up. The car is only running rich at idle, leading to the ECU to pull fuel which is affecting my trims and causing the code. It can correct enough to where the idle AFR is 14.7, but engine load at idle is around 30%. Compression is good, 160-170psi across all 4 cylinders. No exhaust manifold leaks, just checked that. If I induce a vacuum leak at the intake manifold it will help with the fuel trims. O2 sensor reads spot on with my wideband AFR gauge, so I dont suspect the o2 sensor is going bad. Car is setup to be MAP based, not MAF. Any help would be appreciated!

My car has 160000 miles.
My car has a manual transmission.

Some common causes of too much fuel being delivered in a situation like that you describe are: a faulty coolant temperature sensor (the temperature signal is a part determinant of injector pulse width); leaking fuel injector(s) allowing too much fuel to enter the combustion chamber; a faulty fuel pressure regulator; and worn spark plugs. To resolve this, the recommended service to request is a check engine light diagnostic during which a certified mechanic from YourMechanic would test the appropriate individual vehicle components or sub-systems (often a sensor, circuit, and the like, as noted in the short initial list above) to pinpoint the fault that has caused P0172 to set. If you have further questions or concerns as you decide what to do, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results.

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