Here's a strange one on a '96 E300D... I've had minor engine surging issues (+/- 100rpm) at low highway cruising speeds and low engine load for some time now, but as long as I stay out of the 'surge sweet spot' I'm OK. I've had the intake crossover pipe off and the intakes look pretty clean. The first resonance flap was pretty sticky because the vac line was off for presumably some time. I got it free and moving again. So I could live with all that.

Last weekend I drove 3 hours in heavy rain and the engine bucked and surged like a texas bronco. As soon as the engine load eased during ANY cruise speed, it would surge, flutter, run rough, however it can be described. Pressing the accelerator casued some hesitation but then it took off nice and smooth.

Monday morning, sunny and dry weather and the engine runs normal.

I had just recently replaced fuel & air filter in my attempts to track down the surge so I know they're clean.

Anyone have any suggestions???