Do '90-'92 300sl have the (engine) wiring harness & head gasket probs..
 

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The 92-95 M104 series were plagued with the dreaded harness problem.

Mine started freaking out just over 100K miles.

Though the head gasket issue is a well-told tale, not EVERY M104 engine has the problem. Yes, between 100-150K is the typical failure age, but my W124 is hovering at 152K and the gasket is still holding.

The leak starts at the rear of the cylinder head and just gets worse. Won't overheat like the M103 engine, and it can leak forever without adverse effects.

Replacement depends on how much oil leakage you are willing to tolerate before you feel that a correction is imperative.

90 to 93 300SL do not have engine wiring harness problems. They were CIS engine management rather than the HFM in 1994. The 104 head gasket problem is not terribly predictable. I've heard of the problem as early as 20k miles. I had a car that needed new head gaskets at 35k and 68k. Some cars don't have the problem at all. It's really tough to predict although 104s seem to be a little more prone to it than the 103s.