How to interpret fuel trim long/short %'s?

[subawang]

Did a little searching and did not find a straight-forward answer. How exactly do I interpret fuel trim long/short %'s that I datalogged from my AP-V2?

 

I am trying to diagnose a misfire on all four cylinders (P0301-P0304) along with running lean (P0171).

 

Thanks.

 

FYI, I see that my short trim is around 25% and the long is around 5%.

[rao]

The short term will bounce around, the long term of 5% is not bad. If the short term stayed at 25% it would move up the long term. I would clear the codes and see which one comes back first. The misfires are a classic sign of a bad injector or a bad coil.

[testes1010]

I am trying to diagnose a misfire on all four cylinders (P0301-P0304) along with running lean (P0171).

 

I see that my short trim is around 25% and the long is around 5%.

 

The P0171(lean) code is because of the +25% fuel trim you have. The ecu has added the maximum amount of fuel it can and it still not hitting map target AFR. Along with the misfire codes I'd suspect a relatively large post MAF/Preturbo intake leak.

[subawang]

testes,

 

Thanks for the input. So if I understand you correctly, the short trim is a "short term memory" adjustment to the fueling maps, where the long trim is more of a "long term memory" adjustment?

 

FYI, the misfire is only at idle. At part throttle there is no misfire and the a/f ratios are around stoich. This would lead me to believe there is no leak post-MAF/pre-turbo. Do you agree?

[testes1010]

FYI, the misfire is only at idle. At part throttle there is no misfire and the a/f ratios are around stoich. This would lead me to believe there is no leak post-MAF/pre-turbo. Do you agree?

 

Post-compressor vacuum is the greatest at idle, so a post-compressor leak could be the culprit since your afrs are ok at part throttle where the vacuum wouldn't nearly be as great at post-compressor.

[subawang]

testes,

 

If you mean post-compressor/pre-throttle, then I would kindly disagree. There should be no vacuum at this point in the system. The vacuum would be post-throttle, or rather in the manifold. You think there is a leak there? I'm doubtful, but won't rule anything out as this issue has eluded me (and the dealer) for over a year now.

[testes1010]

At idle there would still be vacuum at post-compressor/pre-TB, granted less than than the manifold since the orifice is the TB but there's still vacuum there. It would take a very small leak at idle to cause large fuel trims to surface because there is actually very little fuel being injected to idle. Once you go part throttle the vacuum post-compressor/pre-tb gets closer to '0' pressure making the 'leak' less intense while fueling is being increasing due to part throttle.

[mickeyd2005]

An exhaust leak near the front O2 sensor will cause the same thing.

 

At low MAF, the exhaust will suck air from outside and fool the ecu into thinking it's running too lean. It will then dump a lot of fuel into the engine when it's not needed which would cause the misfire.

[subawang]

Another piece of info, the misfire only occurs on a cold start before the engine has reached operating temp. Once fully warmed, the engine no longer misfires. So it could still be due to a leak, but it seems this leak somehow reseals itself after fully warm cause I do not misfire, and am also able to hold target boost.

 

I appreciate the help thus far. Thanks guys.

[mspt]

test1010 is spot on!