My "new to me" 1994 E320 wagon with 159K seems to run great but I have a question about how quickly it should snap back to idle. It revs very quickly and smoothly from idle, then it returns quickly to about 1200 rpm and then slowly (2-3 seconds) returns to an idle of around 700 rpm. I have no check engine light and it doesn't stumble at idle as though there was a vacuum leak. This occurs with or without the AC on. Does this seem normal or should I be concerned about an air leak in the intake or something else causing this condition?

 

Air leak in intake, vacuum leak, and bad IACV (or similar) are usually causes of slow return to idle. I've seen cars run almost perfect and still have a vacuum leak bad enough to cause "hanging" revs, or slow return to idle. Double check for a vac. leak and check your IACV or similar if the car has it. I'm not familiar with the W124.

 

My 1995 HFM M104 exhibits the same behavior that you describe. It is not uncommon in most cars these days as it reduces emissions and improves drivability. If the throttle is permitted to immediately close, the sudden reduction in air flow causes a short burst of high hydrocarbon emissions. It can also cause drive train back slash that can be felt. My car has ASR and the electronic throttle control so the behavior is implemented by the ECU firmware. Other cars rely on the IAC to admit extra air on throttle close (I have written firmware for EFI systems that do this). Still older cars relied on a pneumatic dashpot the slowly bleeds down on throttle close.

Keep in mind that excessive engine speed on throttle lift-off or a very long delay in reaching idle speed most likely indicate a fault such as a vacuum leak or faulty IAC/throttle body.

Hope this helps,

- FD

 

Thanks for the input. The way it responds to throttle and how cleanly it returns to "the wall" and then slowly drops to idle actually seemed like it was by design rather than malfunction. I looked it over a bit today and tried spraying a little WD-40 around the intake and I couldn't get it to stumble or rev irratically. I think it is running fine but I've just never noticed one of my fuel injected cars do this. On the other hand, it is something I have always monitored closely on my bikes and other cars and there is usually an air leak and a lean condition that needs to be corrected pronto. I think you have put my mind at ease that I'm not sucking unmetered air into the engine.