1999Porsche911

If you have a constant vacuum leak for more than a single drive around town, your LTFT will take up the slack and your STFT will look normal. The STFT will always try to find neutral and reflects immediate need for more or less fuel. So, unless you have an ever changing vacuum leak, LTFT will adjust up to it's maximum.

A little reading on the subject would help you understand the relationship.

SpDfReAk980

All of your statement is true, if the engine had a vacuum leak that the DME can compensate for, aka, adjusting the fuel trim, if the DME were able to compensate for it yes, you FRA's (LTFT) would go positive and eventually your RKAT's (STFT) would steady around 0, but there would be no fault codes thrown, therefore would never see a problem or notice one because the dme would compensate for it, however, we are talking about P1128 <-- a fault code that the DME put out, idle range adaption fault code, bank 1 i think, his numbers were NEGATIVE on both banks, therefore no vacuum leak, AIR MASS METER at fault

1999Porsche911

Well, then you agree with me when I suspect the MAF as I indicated long ago in this thread. However, the question was posed whether the fuel trim readings provided ruled out a vacuum leak....which I explained why....it does not.

Again, his negative STFT readings can not be used to determine whether there is a vacuum leak or not nor do they give any indication of what the overall fuel adjustment is. It is his LTFT that tells you fuel is being added to a greater degree than normal.

signal

So where is a good post or website that explains basic ODBII MAF/oxygen/fuel trim info and how to read it? This is good stuff..........

1999Porsche911

http://www.ross-tech.com/vag-com/cars/fuel-trim.html


http://www.stealth316.com/misc/wells...el%20trims.pdf

SpDfReAk980

"Again, his negative STFT readings can not be used to determine whether there is a vacuum leak or not nor do they give any indication of what the overall fuel adjustment is." <----- THEN WHY ARE THEY THERE, just for good looks. Let's start with what is happening in the engine right now, Oxygen sensing values, instant <--- (I don't know if you can read those with code readers, I doubt it,) those will adapt the RKAT's (STFT), then those will adapt your FRA's (LTFT), or with the new car's (9X7) Oxygen sensing values adapt your RKAT's, RKAT's will adapt your FRAU's, FRAU's will adapt your FRAO's, you have your LTFT and STFT backwards, any questions???

SpDfReAk980

If you have a vacuum leak, the DME will see the oxygen sensing number start to climb (adding fuel), which will push up your RKAT's, which will drive up your FRA's eventually, but wait, the RKAT's were not positive they were negetive, taking away fuel, oh no, NO VACUUM LEAK !!! Good night

1999Porsche911

Ahh, a sure sign of a lost arguement....slowly changing the topic to fit more with your side of the debate. We are talking about a constant vacuum leak here and nothing else. Obviously the STFT will change and it changes constantly and continuously whenever the engine is running. A 3% change in STFT one way or the other is an indication of nothing except that the system is working properly. If all of a sudden you popped a vacuum leak, then the STFT WOULD turn positive, but only for a short while until the LTFT took up the slack, then the STFT would return to normal.

The same result would occur with a bad MAF. If the MAF is indicating less air than is actually getting to the engine, then the STFT would temporarily add fuel until the LTFT adjusted for the increase.

You have made no mention of the tremendous increase in the LTFT indicated. I guess you think this is because the is too MUCH fuel? If you understood the system, you would easily see the failure in your arguement.

SpDfReAk980

A constant vacuum leak, small enough for the DME to compensate for, oxygen sensing will eventually steady out around 1.00, RKAT will be slightly positive, FRA's probably will not change anymore positive than the RKAT's <----------RKAT's will never zero out, will always be positive, why, because there will always be a vacuum leak, small or not, it will always have to compensate for a vacuum leak by adding fuel, ONCE AGAIN LIKE I SAID BEFORE RKAT's are idle range, FRA's are UPPER rpm and HIGHER LOAD range, IF YOU knew the PORSCHE system and not a Ford or Chevy system with old STFT and LTFT, you might understand the real meaning of RKAT's and FRA's, and new car's RKAT, FRAU, and FRAO, there is a difference

A constant vacuum leak, large enough that the DME cannot compensate for, oxygen sensing numbers, 1.25, RKAT full positive, FRA, full positive, check engine light on, <---- this right hear, RKAT will never zero out in this situation, always will stay full positive, the DME cannot add anymore fuel to compensate for the enormous vacuum leak

viper501

FWIW, and not to get into an obviously heated debate, I had a similar problem with my 99 C2. I finally changed the oil/air separator bellows. No more CEL. The bellows was a mess, totally falling apart. And the CEL's got progressively more frequent. Noticed it most at idle or very very low load. It is an easy DIY and the part is cheap. I'd highly recommend examining it and replacing it. The bellows is rubber and is apparently notorious for cracking and creating vacuum leaks. Again, I'm just speaking from personal experience with my car and similar codes.