Rough Idle Solved, Has Returned !!!
 

OK, for those of you not familiar, here is the original thread.

http://www.peachparts.com/shopforum/showthread.php?t=123274&highlight=idle+bucking+cured

The long and short of it is that I was experiencing an extremely rough idle and stumbling/stalling when accelerating in my 1991 300E with 135K miles. I reduced the spark plug gap on all 6 plugs from about .040 to about .030 and the problem went away. The car idled and ran as smooth as silk for a week! Then just today at lunch it did it again. It only happened 3 or 4 times over the hour or so that I sat in my car eating my lunch. It did that same abrupt misfire which previously only got worse.

Does this mean I need a new coil? I'm thinking the spark had trouble jumping the .040 gap, and changing it to .030 reduced the resistance enough for it to jump.

Any thoughts? If I leave it like this, it'll probably be doing the rough idle and stumbling/stalling within a couple of weeks...

What do you guys think?

FWIW, if it was my car, I would have a scope-test done. ;)

One MB tech who posts here said that the std. .032 gap could cause a rough idle. He has always suggested a .040 gap.

You went the other way.

Cap and rotor?

Check those wires. You might have one that has rubbed through to the core and making an intermittent short.

Wires were purchased brand new back in September, only 15K miles ago. Same with the cap & rotor. I pulled the cap and rotor, and while it did have some pitting it looked fine so I cleaned and reinstalled. I also checked for tightness and proper installation of the rotor - everything checked out fine.

Should I switch to the Bosch H9DC0 plugs? I currently have NGK V-Power plugs in there; they are listed as acceptable replacements.

Or should I test and possibly replace the coil? I suspect that it is not putting out as powerful a spark as it should. I have done a search on testing the coil, but none of the results are very clear on exactly how you test (i.e. put the meter probes on which terminals) or exactly what the acceptable range of results would be.

I had a similar problem on my 560 SEL and replaced the coil and wires from advise in this forum and has been perfect since. Did you change the lead from the coil to the cap when you did the wires? When I did my wires I found that wire was difficult to get at and the previous owner had not chnaged it. It was stiff and I suspect it may have been the root cause of my troubles. Definitely an ignition problem IMHO.

Yes, I did replace the center coil wire, it was brand new along with the rest of the set. I am thinking of starting with the least expensive route and getting the correct Bosch H9DC0 spark plugs and setting the correct gap of .032. If the problem persists, I will order a new coil from FastLane.

Any other ideas?

My first thought would be that regapping the plugs did not fix the problem, but the PROCESS of doing that somehow affected the real issue. Analyse what you did - e.g., moved plug cables aside, unplugged/replugged connectors, R&R plugs, etc. You probably disturbed something in a way that temporarily allowed it to perform normally, but now it has shifted again.

Steve