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!

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1995 E420
1998 SL500 Sport
2009 E350 Sport
1995 S500 Coupe
1992 500 SEL

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.

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

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black

check your transmission mount...

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Current Garage
2008 Mercedes GL320 CDI 188k mi Repair/Work in progress
1994 S350 160k mi Garage Queen & prepping for repairs
2005 E320 CDI 203k mi Healthy & Daily Driver
1994 S350 357k mi Retried as parts car
1984 300TD 214k mi Blown OM617 Poss OM603 Swap??

Sold
1987 300SDL 200K+
1994 S320 181K mi
2008 E320 Bluetec 127k mi
1999 S420 130K mi
1980 240D 360k mi
15+ Others that has come, stay and gone

GONE, BUT NOT FORGOTTEN
1995 E320 SE 220k mi
1984 300SD 350k mi

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.

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

Keep me posted.

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/

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...

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1995 E420
1998 SL500 Sport
2009 E350 Sport
1995 S500 Coupe
1992 500 SEL

I wanted to post an update to see if this symptom may or may not shed any additional light on the miss at idle that I'm having:

The car sat for the last 3 days and upon starting it for the first time again, the car began missing A LOT (It was clearly not firing on all 8 cylinders) It ran poorly for about 30 or so seconds and then stabilized to its usual idle. This incident points me in the direction fuel delivery, possibly a leaking injector. Anyone else have any insights on the issue?

Yet another update on the rough idle problem...I sent my injectors out to Cruzin Performance to be cleaned and bench-tested. In addition, while the injectors were out, I pulled the throttle body and detailed the underside of it to ensure that 'build up' wasn't a factor in my idle. When the injectors were re-installed, the engine seemed to hunt/re-learn fuel/air mixture (a different post) but the end result was pretty much the same as it was before so I can now officially check injectors and dirty throttle plate off the list. Anyone have any ideas based on the previous posts on where I can go from here?

Thanks again!

This is a shot in the dark, but did you try to recalibrate the fuel adaption when you got it back together? The following post was from Arthur Dalton in post Cause of high fuel consumption E420

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

Tried the reset...no result. The car seems to run rougher in the morning if its sitting out overnight which leads me to believe that it's ignition related. (also runs rougher in the rain) I can't think of where to look at this stage. New caps, rotors plugs and plug wires within the past year. The only things left are the ignition coils and I'm not suspect of those because of the fact that my major problem is at idle which isn't very hard on the coils. ANY additional insights would be great!! Thanks again!

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.

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

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1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
609 Certified

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...