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Old 11-24-2007, 07:50 AM
pmckechnie pmckechnie is offline
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Join Date: Jan 2003
Location: Matthews, NC
Posts: 1,356
I am just going to relate the times I have had this problem with my 500SEL.
1. Found someone (Maybe me) used a coil wire that had resistance in it.
Probably just something lying around.
2. Found a oil fouled plug. Not completely, just some.
3. Used Bosch plat plugs with resistance. Did this on a bet that it would
cause a missfire at idle. I won the bet.

I have replaced the coil wire with a new correct one. Replaced the valve guide seals.
Installed NGK BP5ES non-resistor plugs. Now I have to keep checking the tach when at a light to make sure the engine is still running.

The above things didn't come all at once but one at a time over the last 5 years. I know vacumn leaks can also cause this problem but seems you have ruled out that. Just make sure you are using NON-resistor copper core plugs, Solid core wires with the MB resistor ends called for, and you should be able to get rid of this problem. Good luck.
84 500 SEL (307,xxx miles)
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