I replaced the ignition system on the 1995 e420 this weekend (caps, rotors and plug wires) to resolve a little bit of a rough idle. Car has 88k on it so I figured it was time to nip it in the butt before the wiring problem got any worse. The problem for the most part was resolved and the motor doesn't even feel like it's on now when sitting at idle. (Smooth as can be)

BUT, here's the catch, in the morning when starting the car, the engine will go through its normal cycle where it holds a higher RPM to warm itself up and then drops down to a normal idle at park and that's when I can feel a little 'roughness' from the engine. It is ever so subtle but is definitely there. After running the car for just a couple of minutes everything is stabilized and the engine runs perfect.

Any ideas? It seems like the car is just dropping the idle a little too low following a 'cold' startup. Hmmmm .

Ps. Wiring harness hasn't been replaced but I'm hoping to get over 100k before having to rip into it.

Any feedback is always appreciated!

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

The idea is to nip it in the bud. Nobody likes to be nipped in the butt. Are you getting any fault codes?

I used to have an Old English Sheepdog that would nip you in the butt.

Butt I digress. Sounds too subtle to set any fault codes, though it's always worth a look. I would guess something like dirty-ish injectors. When the ECU begins the lean out the fuel mixture at the end of the warmup cycle, if the spray pattern is a bit compromised, the engine won't run quite a smoothly. Try using a good injector cleaner. Or possibly it is just old engine mounts which don't absorb vibration quite as readily as when they were fresh. Seems likely to be an interesting problem to troubleshoot. Most owners don't go after things this small.

- JimY

Motor mounts were replaced a month ago so I have that checked off the list. Injector cleaner on the way. I'll keep you posted on the outcome. Also, is it possible that the temp sensor could be bad? Cold start with in improper reading could equate to a fuel mixture that's too lean causing potential misses?

Fingers crossed that the injector cleaner does the trick.

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

Well, she started actually having a few misses yesterday. The missing is as random as it gets. When I'm sitting at idle (both in Park and in Drive) every 30 or so seconds I'll get a "hiccup" from the engine (A solid miss causing the car to shake a little) Also be advised that sometimes the car will go 5 minutes without having a hiccup at all. I'm planning on running Techron later tonight to see if it fixes the problem. Any other thoughts? Do bad sensors cause intermittent random missing? (ie; temp sensors?)

You've checked the fault codes?

Could be a loose spark plug connector, or maybe a connector that is starting to go. An ohm meter with long probes would be handy to have.

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It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D

Well, this weekend I found the source of the heavy engine missing. (Loose spark plug connector...wasn't seated in all the way ) Now comes the issue with how the engine performs under heavy acceleration. The engine lags and gives a jerking motion from missing. At idle it's dead quiet and when revved up in Neutral is perfect as well. It's only when the engine is under stress from the transmission. The only thing that I can think of is that the Techron that's currently running through the system is causing it. Has anyone had any similar symptoms? (I still have NGK plugs in as well)

Keep me posted!

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

I'm also considering widening the spark plug gap on the NGKs to around 1mm to help smooth out the idle. Is there a major caveat to this?

Found the culprit for the heavy missing this weekend. (Loose spark plugs (2)...gaskets weren't crushed all the way so two of them were loose..Lesson learned: Triple check everything when you're doing plug wires, caps and rotors because there's a lot of crap going on there and it's easy to space out on something. Kind of embarrassing if you ask me )

Now comes the question posed earlier about stabilizing the idle a little bit more. In the morning the car DOES still idle rough (sometimes it can even miss) and I'm considering widening the gap on the spark plugs. (0.9mm vs 0.8mm). Do the ignition coils on the 119 have enough juice to support this and would this resolve this issue? (Techron has been run through the system already so the injectors should be fairly cleaned out)

Keep me posted.

If you're spending that much with the pkugs...scrap them...get the OE Bosch Supers.

Plugs swapped out for the OE Bosch Supers. Car ran flawless for about 20 min. then dropped back into the missing cycle again. Now I'm dreading that something got disturbed with the wiring harness throughout the process of replacing the plugs and wires .

As of now, plan A is to double check all of the vacuum lines to ensure that nothing is screwy there. If that checks out then it's time to inspect the wiring harness in further detail. I'm planning on posting pictures of the areas of concern (wiring harness exposure areas in the engine compartment) to see if anyone can help to provide me with some insight to the problem. (I've read all of the posts so I know which areas to closely inspect)

With the car's history of roasting in the southern Texas heat for 13 years, I wouldn't be surprised if this was the cause of my problems.

Definitely keeping everyone posted...

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