M104 oil leak from oil filter area
 

I have quite a bit of the left side of my engine apart chasing down an oil leak that is coming from the oil filter area. The leak is rather bad, leaving puddles wherever I park and lots of drips from the underside of the car. Though I don't know the exact source of leaking, judging from the pattern I suspect it's either the oil pressure sending unit or the base of the oil filter housing where it bolts onto the block.

Has anybody seen an appreciable number of M104 engines to give an opinion as to the common source for leaks in that area? The head gasket was replaced at 59K, and it doesn't appear to be leaking from there. It's too dry at the head gasket.

Could the plastic cap be loose? Could the O ring that seals the cap to the housing be defective? Could the cap be overtightened, thus squeezing the O ring out?

Steve

This leak has persisted over a number of oil changes over the past couple of years, getting worse with time, so I doubt it's my oil changing technique. Just to be thorough, I'll closely inspect the cap for cracking or other defects.

I had the exact symptoms you describe on my 95 E320. Turned out to be a loose oil filter cap.

J. M. van Swaay

I inspected the cap at 20X magnification - no cracks, wear, or other defects.

Just to verify, does the cap have two gaskets?... a square gasket in the square groove above the threads, plus the replaceable o-ring just under the lip?

I think you just found your problem. I believe you have inadvertently left the original gasket, square profile, in place and then added the new O ring, round, on top of that. This would prevent a proper seal. I am certain that it takes only one O ring.

Interested to hear the result.

Steve

Again, to verify, I've attached pictures.

The first picture shows how I've been sealing. This is the way I bought the car. I simply put things back the way I find them.

The second picture shows how you say I should be sealing the cap.

I hope the fix is this easy. I checked a parts diagram (http://www.detali.ru/cat/oem_mb2.asp?TP=1&F=124066&M=104%2E992&GA=722%2E369503&GM=717%2E450&CT=M&cat=520&SID=18&SGR=045&SGN=0 2), which verifies there's only one gasket. I wonder where the square seal came from?

I wonder where the square seal came from?

You know, when I recently changed the thermostat on my S320, the installed O ring had the square cross-section. I'm betting square was OE, but replacements are normal circular cross-section.

Steve

I'll bet the square seal was actually the original round o-ring that has flattened over time. Heat and exposure to hydrocarbons in the oil do bad things to rubber over time.

The seal didn't look like an o-ring, so nobody thought to remove it. The second picture is the way everything should look - your leaks should be a thing of the past.

It makes sense. The square seal is kinda hard, and the leak got progressively worse.

My only regrets are spending the time and frustration of taking everything apart. Oh well, maybe I'll replace the wiring around the oil pressure sender and in the ETA to head off any future worries. There's not much else to do "while you're there".

Remember the saying, "If it ain't broke don't fix it."

I like the explanation of the original assuming a square shape over years of heat. Makes a lot more sense than my lame explanation.

Steve