1995 E420 Rough Idle - M119

[dproudfo]

I'm hunting down an unstable idle on my '95 E420. I know that this question has been posted well over a million times on the forum but I'm pretty much in the dark in determining what to check from here. Here's a quick rundown:

The idle problem is too subtle to throw any codes and doesn't fluctuate the tach at all. It is at its worst in P or N. When the car is put into Drive the idle becomes almost like glass but every so often you'll get a little bit of the symptoms that you experience when the car is in P or N. The car just doesn't feel smooth and once every couple of minutes the engine will hiccup, almost like its adjusting the fuel mixture. If you rev the engine in Park up to 1500-2000 rpm and then let off the gas and let the engine drop, the engine's stumble becomes more noticeable as the tach settle out.

At this stage the car feels like it's running too lean or there is a vacuum leak somewhere. (or both for that matter) Aside from the idle problem, the car seems to run fine. No hesitation during acceleration or any larger symptoms. Here are the specs on the car and the parts replaced/checked.

-The car has 100,000 miles on it
-New wiring harness at 90k.
-New Caps, rotors and plug wires and plugs (all Bosch)
-Ran 2 tanks of Techron (can you purge the injectors under pressure?)
-Replaced all vacuum hoses in engine compartment (intake, egr, etc.)
-Sprayed down all breather and vacuum hoses with carb cleaner following their install to ensure there weren't any further leaks. Also sprayed down each injector to test for any seal leaks and didn't get any results.
-New O2 sensor installed last month.
-Cleaned out throttle body to ensure a clean throttle plate.
-Replaced boot on the MAS to ensure that there weren't any cracks causing a vacuum leak. Also disconnected MAS to see if the idle improved; no result.
-Checked vacuum hoses on Fuel Pressure Regulator to ensure there wasn't any fuel leaking out to indicate failure
-Listened to both fuel pumps to ensure both were running

Any tips on where to go from here would be most appreciated! Thanks again!

[deanyel]

Motor mount status? I had a Sachs-Boge motor mount fail within days and it took me the longest time to find the problem. It's sometimes hard to tell if the engine is stumbling or shaking a bit.

[EricSilver]

The hose clamp that connects the overhead intake pipe to the rubber throttle actuator boot -- is it tightened?

[Actros617]

check your transmission mount...

[dproudfo]

Transmission mount and motor mounts were replaced last August. (Sachs/Boge) I'm assuming that if either of the mounts failed that there would be noticeable fluid leaking?

Clamp was tightened after install of the MAS gasket.

[dproudfo]

Are there additional tests that I can do with my injectors? Do places like witchhunter.com work well?

Keep me posted.

[deanyel]

Originally Posted by dproudfo

I'm assuming that if either of the mounts failed that there would be noticeable fluid leaking?

Yes, fairly quickly I believe. Depending on where you were at the time you might not get much on your garage floor.

For injector cleaning I've good experience with http://www.cruzinperformance.com/

[dproudfo]

One note I'd like to get a little feedback on:

For testing out the plug wires, I sprayed down each one with water/Windex along the shaft to the plug ends and had some really noticeable arching between the spark plug end and the cylinder wall. I thought all of that should be insulated preventing this. Am I correct in this assumption and in diagnosing a potential problem with the plug wires? (I used dielectric grease for everything as well)

Keep me posted...

[macdrone]

How about the EHA/ETA? It has bad wiring in it also, you just cant see it because its inside the unit. It controls all fuel delivery as the system is fly by wire.

[pesuazo]

In your first post you mentioned:
-New Caps, rotors and plug wires and plugs (all Bosch)

Your Last post:
-New caps, rotors plugs and plug wires within the past year (8,9,10 months?, 5,10, 15, 20K Miles ago?)


This is what I would do:

1.- Check the codes (if not checked, or do it again) and post the results

2.- Remove distributor cap and rotor and inspect them (look for residue, cracking, splitting, pitting, dirt, etc...). Replace again if necessary.

3.- Remove Spark Plugs and inspect them

[dproudfo]

Pulled caps and rotors and they look ok; no green build-up or residue. That portion of the ignition system was replaced approx: 8 months/15k miles ago.
From what I've researched about a failed ETA is that they either work or they don't. (There isn't an "in between" where they'll cause slight problems)

One additional "fishy" thing that I tried to get feedback on in an earlier post is the arching between the spark plug and the plug wall when sprayed with water. Anyone have any insight into this method of testing ignition components?

Keep me posted...

[RAYMOND485]

2001 E320
REPLACE YOUR CAMSHAFT POSITION SENSOR $ 76.41 this SITE 1/2 HR DIY

[daddi]

What part number on the plugs???? Found this failure once it was wrong plugs.

[400Eric]

Originally Posted by dproudfo

Also disconnected MAS to see if the idle improved; no result.

It would seem to me that disconnecting that should have caused some kind of reaction, the fact that it didn't makes me think something's amiss with it.

"From what I've researched about a failed ETA is that they either work or they don't. (There isn't an "in between" where they'll cause slight problems)"
When the insulation on all those wires inside those things go bad, they sometimes cause problems, but sometimes they work fine, just like when the main harnesses go bad, stuff works sometimes, sometimes it doesn't. I would think this would be less of an issue on a non-ASR car though. Does your car have ASR?
Regards, Eric

[gcstein]

I have the exact same issue on my '94 e420, including the occasional stumble and, once in a while, very poor running which suddenly improves. Just wondering if you ever found the problem.

Thanks -- Greg