[s-cars] '92 S4 Stumble when Cold

[racingiron at comcast.net]

Now that Jeff's problem seems resolved, let's try another... This time on my bone-stock '92 S4.

Over the past few months the car has had a split-second ignition cut-out shortly after cold startup.  It happens within 100 yards of me pulling away from my driveway.  Usually just one quick cut, but sometimes twice with a few seconds between.  After that, the rest of the drive is completely uneventful.

Now, I've had POS problems in the past, so I know what a single cylinder failure feels like.  This is definitely NOT like that.  It's a pretty violent jolt if I'm giving a moderate amount of torque (still relatively small, since this is right after startup--nowhere near boost levels), but the power comes back immediately (which probably adds to the jolt feeling).  I've been able to glimpse the tach on a couple of occasions, and it doesn't bounce at all.  I pulled codes a while back but got nothing.  I'll check again to be sure.

Here's the weird part.  I confirmed this is definitely temp related during the polar vortex two weeks ago.  Even down here in TN it got pretty cold, with a couple of days staying in the single digits.  On those days, the stumble didn't happen right after startup, it only appeared after several miles of driving.  Then, it remained for much of the drive, happening randomly several times a minute.  It's as if some component starts to fail at a very specific temperature, but is fine above or below that.

Even more strange, after that cold snap left, the morning stumble seems much less frequent, happening less than 50% of mornings rather than the 80% before.  So, anybody got ideas on what could cause a temp-dependent momentary complete ignition failure?  My '93 is a running parts car now, so I can swap most parts pretty easily, with the only know-bad component being the FPR.

TIA,
Eric R.
'86 urq
'92 urS
'93 urS