Oil leak

[Mark280SL]

While not noticed at rest or even when just starting cold I have observed an oil leak when the engine warms up.

I don't mean a couple of drops I mean a steady stream, not sure how to describe the flow, maybe an ounce per minute when the engine warms. Kind of alarming to see in real time.

At first it appeared to be flowing out from where the oil filter housing meets the engine so my first thought was to tighten the bolt that attaches the oil filter housing to the engine, no luck.

Upon closer examination the oil appears to be coming from the bolt #197 in the attached diagram which is identified as "screw plug - by pass valve" on the MB EPC

I can't get a wrench on this bolt/plug without removing an oil line or maybe more involved disassembly so before I start taking things apart my question is..... does this make sense as the real site of the leak and if so will simply trying to tighten that bolt-plug likely solve the problem, or maybe just replacing the O-ring for the plug or am I in for a more complicated problem? I've never thought about or had to do anything in this area and was wondering if anyone has come across this problem before...... Oil pressure seems to be fine by the way.

And by the way.... what is the exact purpose of that by pass valve? how does it work?

I tried doing a search of the forum for this but did not seem to find exactly what I was looking for relative to this problem.

[abe280SL]

Just a wild guess...it has a spring so must be pressure related.  Soooo, maybe its not protect the filter if too much pressure or the oposite...if the filter is clogged then it allows oil to bypass the filter and still allow oil circulation into the engine.
Just a guess, because I have no idea.
abe

[Benz Dr.]

Our engines have two or three by pass valves. All engines have one at the front of the main oil galley and it's there to limit oil pressure. I think they're set for around 100 PSI but could be a bit lower. Any more pressure than this and gaskets or seals could get blown out of the system. A good oil pump can produce far more than this and some of the late pumps have an additional valve on the bottom for extra protection.

The oil filter on our cars is the full flow type. This means that all of the oil running through the engine goes through the filter before it's sent to the bearings and cam shaft. If the oil is too thick when it's cold it won't be able to pass through the filter properly. If the filter gets plugged with dirt it will also restrict oil flow. The vital parts of the engine would get starved for oil so a lower pressure relief valve is positioned to allow oil to by pass the oil filter and maintain constant oil flow. Aside from a severely clogged oil filter this valve hardly ever opens and under normal running conditions even if it does it presents little risk to the engine. If anything, without it the engine would be at great risk.

There's a small aluminium seal under the head of the valve spring retainer or cap on the oil filter housing. Under that is a long spring and the spring pushes on a plastic valve that looks a bit like a mushroom.
 This is similar to the main oil pressure relief valve except it's made of metal because it has to withstand far more oil flow and stress. The by pass valve rarely ever opens but it's VERY important that it does if needed.

Remove the cap or retainer and replace the metal seal and tighten cartefully. You shouldn't have to remove the housing but don't be afraid to do so if needed.

[Mark280SL]

Thanks for your wisdom and great info Dan.