Fuel Trim Issues

[Pierre's Porsche]

MY 2000 Boxster base, which has been desnorkled and muffler modified to reduce backflow, no CEL, engine running good but... obseving high Long Term (LTFT) and Short Term (STFT) Fuel Trim numbers using the record data function of my Actron diagnostic tool.

Here is my test procedure:

I disconnected the battery for 30 minutes, then reconnected the battery and turned key on for 1 minute to perform DME adaption. Then I took the car out for about 30 minutes mostly highway driving, and returned home to hook up the Actron device. Attached is a table of values I recorded.

I'm trying to maximize my car's performance and based on what I've learned from this forum, my MAF numbers look low (min should be 12ish) and my LTFT and STFT are way high.

Please advise: am I correct in my interpretation of the data? And second, data suggest MAF issues unless the exhaust modification I performed is causing the rise if fuel trim numbers...? Third does anything else present itself in these data?

Thanks for your help!

Edited June 1, 2012 by Loren
made content viewable here

[logray]

AFAIK and what I've obsereved is that the data that comes out of the generic OBD2 tables does not always agree or match up with the actual data in the DME.

Translation, LTFT and STFT might not match up with TRA and FRA which is what Porsche uses to record fuel trims.

For example, an "in spec" "long term" FRA trim would be around 1.0 +/- 0.1 and an in spec "idle range" fuel trim would be in the range 0.00 +/- .2 ish.

So unless someone has a lot of experience cross-referencing and making sense of the different values, I think you would need a Durametric, PST2, or PIWIS to make sense of the actual numbers, otherwise you might just be chasing your tail or looking for a problem that might not exist.

That being said, generically speaking if you increaes the amount of air going in the engine is going to compensate and add more fuel to reach the correct stotiometric ratio.

[wvicary]

The fuel trim values will quickly be altered by driving conditions, if you apply full throttle during the road test it skew readings.... the readings are very dynamic and also take in account your altitude.

If it's running well and no faults codes then I would say everything is operating correctly.

[logray]

+1 to above and more info...

I've noticed the STFT and LTFT readings from generic OBD change MUCH more quickly than the TRA and FRA values that can be obtained via Durametric/PST2/PIWIS.

[Pierre's Porsche]

Logray, I was struggling with the appropriateness to correlate FRA and TRA with LTFT and STFT. I thought i may be comparing apples to crab apples (look alike but certainly not the same). (Also, thanks for pulling my data table into the message. I played with it for about 30 mins and could not get it into the message field without loosing formatting).

WVICARY, you also answered a question I had, that being effect of elevation on the numbers. A local independent that saw my car once (and only will once) told me elevation has no effect on the numbers. That didn't seem right to me especially with the "lower" MAF flows (the air is certainly less dense in Colorado Springs, CO than sea level).

Final note, my fuel economy is about 20 mpg combined city/hiway when I keep her running near 3000 rpm cruising engine speed. When at the track (PPIR), red-lining the heck out of her, she got about 8 mpg. So I guess all is good in Pierre's Porsche world..!

Thank you guys very much.

[logray]

Sounds like a healthy Porsche.