i'm stumped, car runs great but has a bad rough idle problem. help..
 

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??

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

Check Vacuum
 

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.

I've got the exact same symptoms as you right now. Though it does not occur every time I am stopped. I'll pull codes and see if anything comes up.

A few months ago, I cleaned the ETA and replaced the O2 sensor - which cleared up a rough/stumbling idle. This is different.

Link to ETA cleaning:

http://www.peachparts.com/shopforum/showthread.php?t=162045

I am not familiar with e420..
My 300e (mechanical fuel injection) i had a similar problem. Improved greatly with tune up. Still idled rough. Tweaked the idle mix and it is perfect.

Just checked codes & did not really get anything, but found a vacuum line with a hole in it. It was the clear one that runs over/beside the brake booster. Patched the hole & will report back tomorrow after driving some more. Preliminary results look good...

Looks like the vacuum leak i discovered cured my fluctuating idle. no more problems last night or today driving.

Prov1 - I hope this helps.