JustAnotherPorsche

I have a 99 C2 that has always run well. Recently (3 - 4 months) my idle has been rough, although the car run's fne. I hooked up an OBD2 scanner and see that O2 sensors are fine and that both after Cat sensors are balanced. I have a vacuum of 15 at idle. NO DTC's or CEL.

The scanner shows an unbalance in fuel trim between bank 1 and bank 2. Bank 1 is consistantly 7 - 10 percentage points more negative than Bank 2. For instance: Bank 1's long term fuel trim reads -15% and Bank 2's reads -5. I suspect that I might have a small vacuum leak effecting bank 1 causing the bank to run rich, but not rich enough to cause the O2 sensors to have to go out of range to adjust.

Is there any connection, hose, etc, on Bank one that could cause a vacuum leak that would only effect that bank? I am just guessing, so any other ideas are very welcome.

PTEC

If you still have the scanner check your maf voltage and sensed airflow. You should have about 1.4-1.5 Vs at the MAF sensor and at idle with no consumers, about 14 grams per second of air (this from a known good car at an altitude of 1500 feet, so i dont know how that will affect you.

JustAnotherPorsche

MAF reads just under 1 on a warm engine. This is the same reading I have always had. Wouldn't a bad MAF effect both banks anyway?

PTEC

1v seems pretty low to me as all the cars I've tested have around 1.4 when operating properly.

Anyways its wierd to see mixture adaption refered to in short term and long term as Porsche doesnt use that system or those terms. Im not sure if your generic (I'm assuming, unless you popped 4gs for a PST2) scanner is relabeling TRA's and FRA's as STFT and LTFT.

Anyways, the way I read those numbers, one bank being more negative than the other, is that the o2 sensor in that bank is seeing a richer exhuast mixture, therefore your DME is subtracting fuel on that bank to keep the mixture where it had it. A vacuum leak would mean more air in the exhuast (lean mixture) so the DME would be adding fuel in that case. If that were the case, never having seen the car or the readouts from the DME, you might have a leaking intake distributor on that bank. I think in this case though (judging from your numbers, one side being more negative than the other) you might have a problem with a fuel injector. Thats the only thing I can think of that would affect only one bank in the manner you describe.

JustAnotherPorsche

Thanks for your help, but what is a intake distributor? And yes my scanner reads both the STFT and LTFT on both banks and bank 1 is adjusted more negative than bank 2. Ihave switched the injectors from bank 1 with bank 2 and the difference stayed the same, so I am guessing it is nit a fuel injector

Thanks.

PTEC

Intake distributor/ intake manifold

How long did you drive the car for with the injectors switched? Did you switch all the injectors from bank one over to bank two?

I imagine it would take a while for the numbers to reflect a change, especially considering the fact that the adaption isnt far enough out of range to set a DTC. Perhaps something is only slightly out of spec and it wouldnt immediately cause the other side to start taking away fuel.

JustAnotherPorsche

I never put the injectors back to whrere they were, so it's been at least 2 months and 2500 miles and no change. I appreciate you thinking about it. I can reset the computer and within 30 minutes if driving, the LTFT is back tp those number mentioned above.

PTEC

Hmm. Have you tried swapping the o2 sensors from bank to bank and seeing if the mixture adaption changes?

Desribe your rough idle, is it just like a shake or does the idle bounce up and down. What RPM are you idling at. IIRC, you should be at around 700 rpm on a warm engine and 820 with the AC switched on.

Im kind of grasping at straws right now but its kind of fun, heh.