i'm stumped, car runs great but has a bad rough idle problem. HELP - Benzworld.org

ProV1x · 11-07-2006, 01:46 PM

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?)
5. vacuum/air leak on vacuum lines
6. clogged fuel filter
7. bad CPS sensor

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

Any ideas/suggestions??

Texas · 11-07-2006, 03:54 PM

Ok, here's my $.02. Not a mechanic by any means, but glad to add some insight. Let's start with some of the items on your list that I feel will trigger a CEL or malfunction light. As you said, you have no CEL light on, so these items I feel can be eliminated: MAM, O2 sensors, and CPS sensor. Bad sprak plugs, doesn't sound like this is your culprit given your symptoms. Fuel filter, not likely as well. A clogged fuel filter will exhibit poor exceleration and bad gas mileage. Again, just my opinions. Vacuum line(s) or part they control, could be. I had a bad Idle Control Valve in one of my previous cars. Carbon had built up on the Idle Control Valve that a vacuum line controlled, causing the line not to close fully, hence, rough idle (almost a hiccup effect). Engine mounts, possibly. I had my engine mounts replaced not too long ago. Had the same symptoms as you. Here a quick check to see if the mounts have collapsed (or about to). Next time you are at a stop, shift into Neutral, if the RPM's return to just about normal, and the vibration/shaking lessens, your mounts need replaced. Why Nuetral? It takes the strain off the engine and drive train that the engne mounts suppress. If the shaking comes back after you return to Drive, you got the culprit!! Hope this helps.

**Musikmann** · 11-07-2006, 04:41 PM

If this was happening to me, I'd take it to someone who has that Mercedes diagnostic box. It doesn't have to be a dealer, but it's not a bad idea to establish a working relationship with one. I see that you are in Ann Arbor; there is bound to be at least one good indy shop close that is capable of an accurate diagnosis. If you are a DIY-type, it sounds like Texas has "been there, done that". Good luck!

ProV1x · 11-08-2006, 07:15 AM

Texas, I agree with your thoughts.

ICV - 97 E420 does NOT have a ICV. But is there a way to clean the MAM?

Vacuum lines - i'm buying some carb cleaner and will spray on all the vacuum lines to see if there's a leak.

Engine mount - It doesn't feel like the engine mounts are bad, b/c the shaking is intermittent and I've heard that when the mounts go bad, you can REALLY feel it. The uneven RPM gets better in N or P, but I thought that was because the engine revs higher in N or P. The shaking only occurs after the rpm drops, and I'd think that if the mounts were bad, the shaking would happen all the time at idle.

Are the engine mounts easy to replace DIY? The parts aren't too expensive, if it is DIY-able I might just get em replaced.

GrantBenzE320 · 11-09-2006, 05:01 PM

I have a 98 E320 with 109K and am having a similar problem. I replaced the spark plugs and that didnt take care of it. Finally, I got my service guy to plug the computer up to it and it shows an EGR malfunction. He checked it out and that's what part I am replacing. I bought the part online for just over $100 and he said he can change it in about 45 minutes...hope that helps...

Texas · 11-10-2006, 07:58 AM

I don't believe the engine revs higher in N or P. Or it's not suppose to in my opinion. In Neutral, the engine is basically, at rest, still awake, but not under as much stress as it would, of course, in Drive. Thus, the engine mounts do not have as much work to do to suppress the load on the engine and drive train. Hope this comes across in an understanding way. I am not good at putting thoughts into words. As far as checking the mounts and replacing on your own, it can be done. There is a well documented procedure on how to do this from one of our members. Believe it is included in the DIY sticky. I was set to do it myself, but my wife advised that I just let a pro handle it. BUT, it can be done relatively simply. I just didn't do it, so my wife would feel more at ease that a pro did it. Cost for my local Indy to do it (including new tranny mount), I believe was $238 for parts, and $316 for labor. You can always go to the local Indy and have them check out if the mounts are leaking or collapsed. Will stil cost you, but you can then get a proper dignosis first, then do the work yourself. Like I said, check the sticky for procedures and tools needed to see what you may be getting yourself in to, and to see if you have the mechanical apptitude to do it.

ckster · 11-10-2006, 08:24 AM

My 97 E420 does the exact same thing, it's especially noticeable when first started. I haven't done much trouble shooting but I'm fairly sure it's not an engine mount. I think it's probably an EGR (Open/Closed loop) issue. I'm interested to see what causes it.

GrantBenzE320 · 11-11-2006, 06:43 PM

Replaced the part today (took him about 20 minutes) and its running as smooth as glass again! The diaphram was barely operational in the old part, and the new one works perfectly! It was an IMMEDIATE change. It also reset the CE code. Hope that helps you all some!

ckster · 11-20-2006, 10:01 AM

Thanks for the info! I'll be replacing mine soon.

will420 · 11-26-2006, 01:46 PM

Think I'll change egr valve in my 97 e-420. Does it matter where I get it from or should I order from mb dealer? If it doesn't matter where I get it from does anyone know where I can get the best deal?