Negative fuel trims at idle

[Chris]

Eric,

I’ve got an 2003 Acura TL-S which exhibits the following:

At idle the fuel trims are around -10. When running at highway speeds the trims are around zero as should be expected. Front O2 sensors readings appear slightly negative (-2) and the long term fuel trims appear to be at zero. Coolant temp around 185F. It will throw a Random miss fire code or a miss fire core on cylinders 4 and 6 but this can be over a day or two or go for months.

Should this be a cause for concern? Any suggestion on things to check?

The car runs and starts with no problems and it has new plugs. I’ve cleaned the EGR valve. Timing belt was replaced recently. The air filter is new. EGR passages have been cleaned. Oil and filter are new Mobil.

Thanks Chris

Topic

[Rich]

What does your fuel pressure look like at idle and under load?

How does that compare with what your service manual says?

You should do the test both with the fuel pressure regulator connected and disconnected as per your service manual. What does that show?

[Richard Kirshy]

Total fuel trims ( long and short term) should be around 0 in a perfect world.
+ / – 10% is considered normal in real life

[Chris]

If I had a pressure tester that would be good advice.

My Bluedriver OBD2 does not have the ability to test for fuel pressure.

Thanks for the recommendation.

Chris

[Mike]

You never stated whether the -10% fuel trims are the total or if both STFT and LTFT are each -10% at idle.
-10% total fuel trim, while within the realm of normal is nearing the limits and could indicate an issue.

These 2 articles should help you sort it out.
Keep in mind that both leaking fuel injectors and vacuum leaks will have less effect at higher RPMs.
Note that vacuum leaks after the throttle body cause negative fuel trims at idle on Speed Density systems .

This first article I’ve started at page 3, just so you’re aware
http://www.autoserviceprofessional.com/article/94982/Fuel-trim-How-it-works-and-how-to-make-it-work-for-you?Page=3

Speed Density Systems: Performance Reliability

[Chris]

Thanks for the suggestions.

As far as negative fuel trims its only the short term ones the log term is around zero

[Author]

Replies