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?

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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

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

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2007 C 230 Sport.

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.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Cap and rotor?

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89 300te 222,222
92 300e 190,000

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.

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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

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

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7