8 cylinder front engine iconic vehicle
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By jej3
Hey Everyone -

I have a 1990 S4 that was under Howard Wright's care and he did an awesome job with the engine and trans rebuild. Immediately before he had my car for 18 months, I did an intake refresh. So everything is new (including all ignition components, dampers/regulator, injectors, etc...)

Since I got it back, I've periodically had some strange re-starts when warm where the idle will hunt up and down. At one point, shortly after I got it back it stalled when cold starting routinely and continued that pattern.

Howard suggested I swap the MAFs with my 1988. Did that this weekend and I didn't experience any of the cold start issues and only one or two episodes of the hunting idle. I am starting to think that MAYBE I have an O2 sensor on it's way out and the better MAF helped compensate.

I replaced the 02 sensor about 30K miles/10 years ago. It was a generic one from 928Intl that you can essentially solder with the correct plug.

I'm inclined to replace the 02 sensor and go for a rebuilt MAF. I'm not throwing parts at it but I consider both wear parts and everything else is essentially new.

By worf
O2 sensors are surprisingly delicate given the conditions under which you’d expect them be robust.

That said, I would first send your Mass-Air Sensor out for testing; find out if it is indeed wildly out of spec.

You write “after warm start.” The oxygen sensor loop doesn’t kick-in instantly ever. So, it is important to determine if your instability is before or after the 02 sensor warms up and the LH starts paying attention to it. Even on a warm start the 02 sensor will spend a few seconds heating.

If before, then definitely have the MAS tested. If after, then either throw 02 sensor money at it or do more diagnostics.

If the latter, I would first check to make sure that the LH and EZK (at the ECU connectors) see a closed idle switch contact after “warm starts.”
By jej3
Thank you, Worf. All logical points.

I am going to sound absolutely illogical for saying this but I've just dropped big money on the motor and trans. I'm not MADE of money but since I'm sure the MAS is part of the problem, I'm tempted to just order the O2 sensor and the MAS at the same time.

I know it sounds stupid but I just don't have a ton of free time and while I'd love to troubleshoot - and may need to - I just want to eliminate one other "wear" item.

Now - it's possible I'll figure out a new damper failed OR I have a new leaky injector that throws things off when I do a hot restart.

But before my car went to Howard I had 100 hours (in my car and online) over 12 months trying to figure out why it was screwed up after I did the intake refresh. I was in such a bad place, I bought another 928. Turns out a washer fell into the head and wedged a valve open. This left me without any compression, was the beginning of the 18 month, mega $$$ journey to getting a new engine and trans.

If the O2 doesn't fix it, I'll be making my 2,000 mile checkup to Howard soon. But I just want to drive the car not need therapy ;)
By worf
Why are you ”sure” that the MAS is a problem?

You can’t “order” a MAS. Not since about 2002.

You can send yours in for rebuild. Everyone has a favorite rebuilder. I use Rich Andrade, but the ‘obvious culprits’ offer the service.

Possibly some of the rebuilders might send you a good one before they get yours. I would not hold my breath.

It takes 30 minutes, if you are drunk and have never used a multimeter, to check the Idle Switch at the ECUs. Step-by-step instructions are in my inspection guide to which my signature block provides a link.

Check the idle switch when the engine is at operating temperature. If it checks out at both LH and EZK pins then sure, go ahead and throw money at it.

It takes 30 minutes, if you are drunk and have never used a vacuum hand pump, to check the fuel pressure regulator and dampers.

Believe me, I understand the frustration. But frustration often leads to holes in the wall with tool handles sticking out…
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