This may seem like such a simple, non-technical item, but I never noticed until after the engine was cleaned to better pinpoint oil leaks.

The oil filler cap ! Oil seeping out around the cap and cruding up the engine again.
The gasket and the tensioner had weakened.
(Afterall, it's the original metal cap, now 18 years old).

The new cap, made of plastic, went on much tighter. No more leaks, at least around the valve cover area.

Now I'm wondering if the leaky cap was causing other small problems (from air leak).

M-B used to sell just the gasket so that you could replace that only. Now, as you say, they sell you a new cap - though it is fairly inexpensive.

As I understand it, maintenance schedules that dealers use call for replacing this thing periodically (don't know the interval). My practice is to replace it as soon as I spot it leaking - this is easy because I check the oil every time I put diesel in the tank.

Good luck-

Terry

you can take the rubber gasket out of the new cap (which is plastic) and put it in your old cap (if it is metal) and retain the look--syd

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syd 81.300SD.196k
72.300SEL 4.5
mileage.Only god knows!

Very common to see the filler caps leaking after a few years of use. If never run across one though that leaked enough to cause any significant air leak into the crankcase to upset the air/fuel mixture.

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ASE Master Technician, Lead Technician for Deutsch American Inc. Over 6 years of Import experience-specializing in German Automobiles.