'94 E320 W124 M104 with 168,000 miles. Daily driver. Still runs reliably, but this oil leak has me bewildered. Head Gasket replaced with new improved one in '07, as were the valve seals and timing cover seal.

History - Been plagued with a substantial oil leak last 6 months adding 1 quart every 2 weeks, from rear drivers side of engine. VERY difficult to inspect, even with a mirror. I now see faint stream while running, big oil puddle each night. Car leaves oil trail down driveway when leaving due to oil pooling in bottom open bell housing 'flywheel' inspection port (faces front). Originally thought/assumed it was rear main seal. Then as leak worsened able to trace running stream up to oil filter housing area, plus oil is all over on engine next to starter. Head Gasket seam is CLEAN. There is no oil in coolant, nor coolant in oil.

See these pictures and video of leak. (these are all before pictures, everything is clean now. Sorry for the rabbit trail )

With advice to do so, removed oil filter stand and replaced gasket. Removed oil filter stand and cleaned, replaced gasket and pressure sender seal. Polished up all gasket mating surfaces mirror smooth. Replaced oil filter and new cap gasket. All parts from MB dealer.

Filled with oil (10W40 Mobile Super), buttoned up and started, and LEAK IS STILL THERE AND FILLING UP TRANSMISSION BELL INSPECTION PORT. Only this time its new oil, not old.

Oil is running down drivers side of the bell housing - SAME PATH.

WHAT is on the rear drivers corner that could be doing this? Leak is a significant one. It's very difficult to inspect even with a mirror.

Since I've super cleaned EVERYTHING plan on getting back under while running to see. After reviewing the oil flow diagrams I really don't see head or other oil galleys in that back corner. And even if it was the head gasket, oil would run down to the oil pan while parked, but this oil continues to stream while running, but continues to drip/seep out all night (alot!). So either the oil filter's backflow check valve is keeping the oil filter unit pressurized, or its the rear oil pan... aggghhhhhh

My next suspicions are:
1. Could the POSITION SENDER UNIT-HFM (part no 003 153 01 28) leak oil?
2. check the oil filter housing CAP. It has a new (dealer) filter and gasket, but maybe its cracked.

 

did you check the oil pressure sending unit... its below the oil filter housing behind the firewall on the block... its a PITA to get to but it can leak oil if it has come loose or some catatrophic failiure to the case

 

Er, the cylinder head gasket! THE most common leak on an M104!! I have done many of them in my years as a Benz tech!! --Not a DIY job, unless you are very astute, mechanically. ...And have the right tools. Yes, the oil pressure sender could also have a leak, but if there is oil on passenger side of bell housing, it's the head gasket. They will also leak at the front passenger side corner, near the upper timing cover seal, which can make you think it's just the timing cover seal strip, but it usually isn't. ----Zach

 

Have not posted here in a while... the oil leak is coming from the rear radial seal. So I just have to get the courage up to remove the transmission and do it...

 

Its not so bad, but you are going to want two tools, a 17mm swivel socket (I think its 17) and like a 30"+ extension.

 

What's that for? Removing the tx?

 

The bell housing bolts, you have to reach from the back of the trans all the way up to the front. There is one bolt on the top that is can be really tricky.