no doubt

Hmmm... my 88 S4 automatic will start up, but the idle is low, fluctuating, and rough. Very rough.

If I pump the gas pedal, the rpms will jump, then fall.

If I hold the gas pedal steady, the rpms just keep falling until they reach the idle range...where the idle fluctuates and runs very rough.

The exhaust is threatening to pop, as if a backfire was imminent, but she hasn't quite backfired yet.

Any tests that I can do to isolate the problem? Vaccum hose or relay or MAF??

EDIT: Resolution on page 3: https://rennlist.com/forums/6106757-post39.html

soontobered84

I would say vacuum leak, but what did you do to the car just prior to it running like this? That is where I would start and work from there.

no doubt

Drove it this morning. Ran fine. School. Starbucks. Office.

Went to go each lunch this afternoon and that's when it started misbehaving.

Mongo

Is it running on all 8 cylinders?

no doubt

Hard to say. It's just barely running...rough sounds coming out of the exhaust...idle fluctuating and well below the 1,000 rpms that it "normally" idles smoothly at...and the best that it will do is rev up briefly before falling.

It won't hold the rpms steady at any level. Not 1,000...not 2,000 rpms...not 3,000 rpms, etc. You can rev it up like a ***** to 4,000 or so...maybe more but I didn't try...but it will only go there briefly before falling back into the 600 to 900 rpm idle fluctuation range no matter where you hold the gas pedal (or for how long).

ZEUS+

Possible loss of fuel pressure or defective maf.

no doubt

While it was idling rough just now, I sprayed starter fluid into the air intake, and the motor ran smoothly as long as I sprayed in the starter fluid...so this should be a good clue.


...but, how do I test and isolate the problem?

ZEUS+

Do you have a fuel pressure gauge ? Pressure should be about 48 lbs psi at idle and 55 with vacuum hose to regulator removed. How old is the fuel filter ?

no doubt

No fuel pressure gauge and the fuel filter is years old. How do I isolate the fuel problem to either vacuum leak, computer, or fuel (filter +) lines?

Mongo

Wait. Did you say it ran fine before?

Possible LH Failure????


I know they just don't go out but that could be a possibility..


Another thing...

Was it running fine on a cold start, then crappy on a warm start?

no doubt

Ran fine cold.
Ran fine hot.

Made multiple stops and starts this morning. No problems.


This afternoon: problem.



Any suggestions for tests that I can perform to help isolate the fuel flow problem to a vacuum leak, compture/relay, or fuel delivery system line/component would be much appreciated.


After spraying starter fluid in to the intake while it was idling roughly, and having the starter fluid make the motor run smooth as silk, I'm reasonably convinced that this is a fuel issue in one of those vaccum/ecu/fuel-line areas.

But how to narrow it down to which specific one?

SharkSkin

Sounds like fuel delivery -- If not for your starter fluid test and the sudden onset, I'd say use the vacuum gauge method to check for plugged exhaust. Zeus may be right about the MAF. Unplug it. Does it run any different?

no doubt

I will unplug the MAF and test. Thanks!


*I actually looked in the tailpipes for a banana...the car *felt* as though the exhaust was plugged (cue Axel Foley theme song to Beverly Hills Cop).

Stromius

Would it be worth unplugging the TempII also to see if it's defective? What about the trying some WD40 to lube a possibly stuck ISV?

FYI Just sent my 88's MAF to Injections Labs (Kevin). It tested 40 mV off which is borderline. My symptoms were rough running but would hold idle and yet had coughs and flat spots driving around. FYI getting it checked was complimentary and rebuild is $175 + shipping.

no doubt

OK, I unplugged the electrical connector to the MAF and started the shark. Idled at 900 rpms steady. Smelled like fuel everywhere, but would rev if I didn't hammer the throttle too hard. Would hold revs.

Plugged the ecltrical connector back into the MAF (left the air filter off this time) and restarted...loud hissing/vaccum/air intake leak sort of sucking sound in time with the massive idle fluctuations up and down. The 928 would eventually (30 seconds) die. No fuel smell with the electrical connector plugged back in to the MAF, though.

Thoughts?!