Many of the 1996 cars with M104 engines did that from the start!

WHAT you are feeling is the de-cel shut-off system that turns injectors on/off so that the engine creates less smog!
ON many cars you can see the tach move 300-400 RPM as that takes place.
MB slightly modified the fuel computer so that shouldn't be felt!

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Check brake booster vacuum line. I had the same problem and that was the last thing I checked. Turns out the check valve had a crack in it.

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Mike

90 300D 511k
95 C280 133k
14 BMW 528i

Thank you everyone for your responses, I appreciate it. It looks like I need to spend some time tracking down a vaccum leak then.

MB Doc, if the issue you are suggesting is in fact what my car's problem is instead of a vaccum leak, then how would I go about resolving it? The car has never had this sort of an idle surge in the 5 years I've owned it. It is a 1994 model, not a 1996.

Thanks!

Jonathan

Terminator, can you tell me where the brake booster line and check valve is? Thanks. I didn't mention in my reply but the brake and some kind of fuel feed/cutoff connection that M.B. Doc mentioned sounded like it would describe the condition. Is there some "connection" (not literally a physical one) between the brakes and the fuel cutoff function?

On my 95 E320 (I don't think your car would be different) the line comes from the brake booster to the engine. The line looks more like a tube and in the middle (around half way from the booster to the engine) there is a check valve. Mine had a crack on the bottom side. But you should check the whole line just in case.

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Mike

90 300D 511k
95 C280 133k
14 BMW 528i