89 190E 2.6 plug fouling

Does anyone know if there is a common cause for plug fouling in this engine? The offending cylinder is #4 and the plug looks to be fouled by a carbon residue. It is not oil soaked, but maybe the valve seals
are weak. I can take the plug out and clean it which cures the problem for about 10 minutes worth of idling. I would appreciate the advice.
Will
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Will wrote:

It's the very well known camshaft problem.
I did one repair myself once about 15 years ago.
Reg: Harri
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I don't know of a common cause but these are the possibilities (in order of likelihood and, luckily, increasing cost):
1. Bad ignition wire which can be checked with an ohm meter. 2. Dirty fuel injector that's now throwing a stream instead of a spray; this would cause #4's over fueling, especially at idle. 3. A leaking valve stem seal, but I'd discount that because only one cylinder is involved. 4. A burned valve that's causing low compression; a compression test will confirm, or deny, this possibility.
I believe you'll find #1 or #2 are the cause of your motor's problem.
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Do a compression test.
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