Hi,
I have an S5 convertible with the 200hp turbo engine. That runs really great, normally no problems.

But if I drive on rainy days, I seem to get a weird engine stumble. If I had to describe it, the engine completely looses power, then power comes back and the car jumps forward then repeats.
However, that only happens, when the gas pedal is only slightly pressed. If I step on it it accelrates fine. If I drive fast enough, that I have to keep the gas pedal pressed, it runs fine. Weirdly it even idles fine. But keeping a steady speed in town in 4th gear? Not possible.
Readout from the lambda says extremly lean while stumbeling.
I looked in the engine bay and inside, if I find any wet connectors but everythign seems fine. Letting the car dry over night "fixes" the problem.

My first guess would be TPS, but I see no water there. Does anyone have an idea, what else it could be or how I could test it?

*edit1* Checked error codes: 17 --> feedback system. Propably due to the broadband sensor taking to long to heat up. Related?

 

When was the last time you replaced your spark plug wires?

 

hmm, I would expect an issue with the ignition to affect idle as well.

On the other hand, those cables are some years old, so replacing them might be a good idea anyway.

 

Okay, cables have not arrived yet, but I was on tour last week and got the stumble on a day without any rain or wetness.

From trial and error it seems, it happens when a lot of electrical power is drawn, so if lights are on and whipers are working (like in rain).

I will close this thread and open another one, to avoid confusion.