Having chased the rough idle recently on my garage queen, I can't help but also offer up this thread: 300E Apparent Misfiring, Hesitation. She now idles very smoothly save for a motor mount which needs replacement and is only apparent at stoplights with tranny in drive. But no more misfiring, sputtering, and loss of power.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

Thanks for the tips gmerc. I had read your distributor problem thread before when chasing an issue with my "project" 1989 300E. Everything is fine in that area on the '90. I'll look into the Sears meter when time and weather permit. I'll probably check the duty cycle as is and then return the screw to it's pre-adjustment location just for reference, then set it for the 50% range that seems popular. At any rate and after looking at a lot of EHA related threads, I may try adjusting it if the problem persists.
Thanks once more.

Chuck.

Well, drove about 300 miles yesterday and today and the problem is as bad as ever. So, after arriving at home, I did the EHA adjustment (1/4 turn clockwise - very easy to do this job) and went for a short drive. It seemed markedly improved with a slight stumble during the drive and after getting back to the driveway and opening the hood, when I rev the car up in Park, I can still detect a miss. Might another tweak on the EHA line this out? The miss sounds almost ignition related. Got about a 100 mile trip tomorrow so will probably wait and see how it does before going back into the EHA. If that doesn't change things, I'll dive into the ignition. Have left this alone mainly because things are fairly new there and the problem started suddenly. Any more thoughts from anyone?
Thanks to all. Will update as able.
Chuck.

These cars are really sensitive to any secondary ignition component deficiencies. Double- and triple-check your plugs, wires, cap, and rotor. Do yourself a favor and pop the hood at night when it's pitch dark out, and see if the wires are arcing. It doesn't matter if they are 10K old, they may arc anyway.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

Hi Gmerc. No arcing at night. Added another 1/4 turn CW to the EHA (total of 1/2 now) and the stumbling at idle has improved but now I can detect a high speed miss on the highway. It actually seems better than before at low speed after warm but worse at higher speeds. Cold running is unchanged. Pretty spooky. I'm now starting to think that the ignition is the culprit and that I've been masking symptoms by tweaking the EHA (possible?). As soon as I have the time I'm going back in to the cap and rotor for a look and maybe the coil after that. It definitely has the age-old weak coil thing of missing at low rpm and high load and it also doesn't do anything until very warm. (Not overheat, high temp warm but post-highway running warm.) If it does indeed prove to be ignition related, should I go back to where I started on the EHA? Also, am I wise to just replace the coil anyway? 148K on the car and no idea if it's ever been done. What coil to use? It's my daily driver and needs to be dead-nuts reliable and I drive about 30K per year in it.
As always, many thanks.
Chuck.

Based on your results, I would return the EHA to its original position as it doesn't appear to be helping much.

In my experience coils usually go out completely, soon after they go bad. I personally wouldn't suspect the coil. I'd check plugs, maybe even replace them. Make sure your ignition wires are "clicked" into place properly on each and every plug - remove and reinstall them to be sure. Also make sure the connections at the cap are on properly.

Remove the cap and rotor and check the terminals, also look for oil fouling inside the cap which can interfere with proper operation. This can happen due to the infamous upper timing cover leak which occurs on the M103. Check the screw/bolt behind the rotor to make sure it is in there tight; they have been known to wiggle loose, allowing the rotor to go askew.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

Hello again and thanks. Have done most of what you mentioned, the exceptions being inside the distributor. I'll dive in there when time permits and in the meantime will reset the EHA and hopefully gain access to a proper meter and adjust the duty cycle.
Have a great today.
Chuck.

Well, Gmerc, I humbly bask in the glow of your genius and promise never to question you (even internally) again. With everything back to where it was the car started and ran better but still had the hot idle misfire. Figuring I had nothing to lose I took the distributor apart this evening and the problem was immediately apparent. First of all, the whole thing reeked like an old radio on fire and the terminals were burnt to charred lumps with bright green residue around them. The center of the cap itself was blackened and brittle and it looked like someone had taken a small nail and put about a grillion dimples in the recess where the rotor runs. The center tower of the rotor was crumbly and had the same dimpling appearance. Obviously much arcing going on here.
Here's where I need a question answered. How does the o-ring install around the cap? I had changed the cap and rotor (but not the base plate) 10K ago and put the new (red) o-ring around the base plate before fitting the rotor. Now upon disassembly I find the new red o-ring to be squashed and malformed and another, old, brittle o-ring in what appears to be the right location around the base plate. I put the dang thing together wrong in the first place and in doing so, didn't get the cap sealed properly and with the extra o-ring, the internal spacing on the cap and rotor was too large. After taking both o-rings off I attempted to put the base plate and the cap together to determine proper assembly and it seems they don't match up well. And they match up worse with either o-ring in place. I will be ordering a new cap and rotor and base plate with o-ring but need to know the proper fitment for the o-ring. Does it go around the lip on the base plate or squeezed into the annular groove in the cap outer edge? Maybe if I get a matched assembly (Bosch, I presume) it will all work since the base plate was oem and the rotor I used was aftermarket. Anybody know what I'm talking about?
Thanks for all the help and patience, especially to Gmercoleza. I felt like I was on "That 70's Show". "Son, the reason bad things always happen to you is because you're a dumba$$".

It has been a while since I had my cap off, but I recall the O-ring goes in a groove. Maybe someone else here can speak up since again I really don't remember. I do recall though it is a pretty straightforward install. If you buy OEM in a matching cap/rotor set you should be OK.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

OK, I got the cap, rotor and backplate for the cap. I also ordered a new o-ring. The new o-ring came separate (it's red) and there is a black o-ring already fitted on the base plate where I would think the new o-ring would go. Do I omit the new, red one and use the black, already in place one or do I put the red (it's a little thicker) one in place of the black one? I know I don't use them both, that's why we're in this boat in the first place.
Anxious to hear opinions since I'll be installling the stuff tonight or tomorrow night. Tired of driving the 2003 4WD Yukon XL for my daily commute. Many thanks.
Chuck.

My two cents in the misfire issue. I have the same problem. 300e 1993. I was looking into this problem and noticed the following. The misfire was much worse when the heater/ac motor was running. At idle, and no ac or heat, no misfire. When fan is on, misfire about 4 per minute. I wonder if there is arcing from a heater fan wire as the amps must be high? Anyone who is reading this thread try it with heat ac fan on and off and leave a comment.

cjlipps, replace the black O-ring with the new red O-ring.

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Fred Hoelzle

Talk about dragging up ancient threads!

Just kidding. The good news is that someone is searching before starting a new thread. A good thing.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

Annoy a Liberal, Read the Constitution

CJ have you checked the output from alternator?..I was reading ablout the same problem on the other forum.It may need a new alternator regulator. Check all earth points for bad contact clean and grease them up .Fuses as well at the same time , give them a clean.