89 190E 2.6 plug fouling

It's the very well known camshaft problem.

I did one repair myself once about 15 years ago.

Reg: Harri

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.

Do a compression test.