car: 1997 E420, M119 motor, 107k miles

Car starts, runs, and drives great. 25.8MPG on the highway. Smooth, fast acceleration, no abnormal noises. Spark plugs, connectors, serpentine belt replaced recently. No Check Engine or malfunction lights on.

Problem: When driving around and coming to a complete stop at a traffic light, the rpm would drop to just below 500rpm and engine will almost stall. Car will shake, but the computer will shoot the rpm back up to compensate. This will go on every 5-10 seconds until I start driving again. Basically the RPM will never stay the same but always move around at idle.

After searching the forums, it could be one of many reasons, such as

1. bad spark plugs
2. bad Mass Air Meter
3. bad engine mounts
4. bad oxygen sensors (which ones? all 4?)
5. vacuum/air leak on vacuum lines
6. clogged fuel filter
7. bad CPS sensor
8. bad fuel pressure regulator


Now the last thing I want to do is start blindly throwing new parts on her and keep spending $$. But because there is no CEL on, pinpointing where the problem is tough.

Any ideas/suggestions??

__________________
Current cars:
2001 MB E55 AMG
2003 Honda S2000
2000 Honda CRV

Gone but not forgotten:
2004 BMW 325i 5sp sport
1999 BMW 540i sport
1997 MB E420
1995 BMW M3 coupe

Get the fault codes read.

tweak the EHA as i have done on mine but this is on E300 24v 104 LH engine

ProV, have it checked for codes.
Could be anything, air mass, clogged fuel filter, clogged engine air filters.
You know you can get a generic code tester at most auto part stores that you can check codes yourself and then come here and tell us what you found.
Some of theese smart guys on here will steer you straight.
Some may give you some crazy suggestion about what they did to fix their Grandma's 66 Pontiac Catalina and how it may relate to your problem, but you may get one out of ten suggestions right.

Has this problem just started? Have you done anything under the hood that would coincide with this? My first inclination is to check all vacuum lines for splits or cracks, or if one or more have fallen off their fittings. This could easily be a vacuum problem. On my S600, there was one vacuum line in the rear of the engine compartment that slipped off by itself a couple times. RPM would drop at idle, and the engine would hunt and run rough. If not, then see if there are any stored codes.

http://v12uberalles.com/pulling_codes.htm

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ProV1, try scanning for codes. Eventhough you have no CEL still scan the system. Also check for vacuum leak. Replace hoses if necessary. Inspect and clean ETA butterfly. Inspect distributor, rotor, caps, and sparkplug wires. It may have alot of carbon build up. The ETA linkage must be free to move. Keep us posted.