I have a '92 Audi 100, 2.8L 12V, automatic, fwd only. Recently, the thing has started 'stumbling' while driving during warm-up. Under part throttle ~2000 rpm the engine will 'die' and go to idle. It's quite annoying while trying to get out of other people's way...

Neither the engine or trans are showing any fault codes. I've checked for vacuum leaks, plugs, wires, O2 sensors, etc - nothing that seems significant.

Any ideas?

 

Make sure that you have the factory parts in it, like the correct NGK spark plugs and the OEM wires, etc. I've seen a lot of cars with major driveability issues and many times it boils down to a set of wiz-bang spark plugs, or super fancy look-a-like plug wires, etc.

Oxygens sensors don't come into play during the warm-up. They are only in control of the fuel metering process once engine coolant temp gets to the right value and the computer switches to "closed loop" function. So you can rule those out right off.

Think in terms of what does the car need to have during cold running, ie, fuel enrichment and the right amount of metered air. Fuel injector spray pattern will be important here, as will the amount of carbon in the intake runners and on the backs of the intake valves because carbon soaks up fuel like a sponge, thereby starving the engine. The coolant temp sensor for the ECU also needs to be sending the correct signal so the ECU knows to be lengthening fuel injection spray time, so that's something else to consider.

What you might do is clean the induction system with a product called BG44K. By the 3 can kit on ebay. One can goes in the fuel tank, one can sprays out your gummed up throttle body and one can you let the engine suck into intake and burn at a moderate rpm, sitting in your driveway.

The whole process might take you 30-45min and you'll definately notice the results. If you search the archives under "induction service" you'll find the write-up from when I did ours a few months ago.

 

Then I'd throw on a nice set of new plugs and plug wires...and a fuel filter for good measure.

What were the last 3 repairs you did and how long ago?

 

<ul><li><a href="https://forums.audiworld.com/a6100/msgs/73585.phtml">Useful write-up from Todd as usual</a></li></ul>

 

Thanks for the info.

First off, the car runs perfectly fine during start-up and initial warmup. The 'stumbling' behavoir lasts ~30 seconds, and then the car runs perfectly fine again.

It has been a while since I've done a throttle-body and EGR passage clean. I guess I have a new afternoon project.

Last few repairs over the last couple of weeks, all in an attempt to make this problem go away:

- Cleaned MAF
- New plugs (factory NGK's), no 'weirdness' on previous plugs
- Replaced various vacuum hoses
- Cleaned valve cover breather hoses/fittings

Thanks for the help,

 

once when I boughht it, and the last one after 50K.

Also it is good to clean the idle control valve.

 

I just did the induction cleaning and replaced all the parts in the system sans fuel pump. I haven't done the ICV so I would think that is more in line for the stumbling. I don't run correctly until the CEL comes on and it's running at WO (I think).