I am considering a 400E having done 210,000 km. Checked spark plugs I found that the color of the electrode was good, i.e. b but the threaded part had oil on it. Had a test drive on the highway the acceleration was ok. I wonder if the oily thing is the indication of piston rings replacement or just the valve stem seals. Hopefully it is the latter.

Can anyone tell me at what mileage should we think of a piston rings replacement?

mileage for piston ring replacement??? Ha!

You need to do some homework, but to answer your question. Wet threads is most likely leaking from the v/c's. Great cars and engines- but expensive on maintence parts.
These engines have some pastic engine parts which can fail and kill a motor- it would be money well spent to get a professional MB mechanic to look the car over and avoid a money-pit.

A compression test will validate if the engine has ring or valve related problems which should be done if the car looks worth-while.

M

What do you mean by v/c's? And what are the plastic parts of the M119 engine you mentioned. I will have the compression checked soon but I believe 210K kilometers have not killed the piston rings yet.

MB mechanic? That's luxurious here. Believe it or not, I couldn't get my 300E's KE Jetronic fixed by any mechanic until I fixed it myself.

Your comments were general, be more specific.

V/C = Valve cover.

Was this more than one of the spark plugs that had oil on it? Which in particular?

__________________
1989 300E 144K

In other words it's most likely that the spark plugs have oil on the threads because there was oil pooled up in the spark well when you took it out. This is caused by a leak in the rubber donut gasket that surrounds the spark plug well. Each valve cover gasket set, left and right, comes with the gasket for the outside perimeter as well as four rubber donuts, one for each of the spark plug wells. Under normal usage there's no reason to ever worry about rings or valve stem seals on that motor. Valve cover replacement is a relatively minor job.

Plastic parts..

Let's see #1 is the timing chain rails. They break and wedge in the chain- yuck!
@210 km I'd expect all 8 cylinders to run very even only varying a few psi. Mine ran 179-182 for the engine @130,000 miles.

Lower ends are pretty stout in these engines.
Motor mounts, p/s return lines, fan clutch($600), fan clutch support brg($500), airpump brg, ETA, wiring harnesses (upper and lower) are all pretty big ticket items.
Plus, brakes and normal suspension parts.

If you get the car consider upgrading the diff and it will be more fun!

M