I have a stubborn oil leak behind/beneath the SLS pump in my W124 wagon. The leak is engine oil, not SLS hydraulic fluid.



The SLS pump is mounted to an aluminum plate, and the plate is mounted to the cylinder head. The aluminum plate seals to the cylinder head with a large thick o-ring. Based on where the oil is dripping from, it appears to be leaking from this o-ring.



However, I replaced the o-ring today (along with every other seal inside the SLS pump) and the leak remains . It's a significant enough leak that I can't just ignore it.

The o-ring sits in a groove so it'd be hard to install it improperly. I pulled it and checked again to be sure it wasn't seated incorrectly or something, but it looks fine.

Has anyone had this happen before? I'm not sure what to do about it... A couple of extra pictures and diagrams are attached in case it helps.

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1989 250TD Wagon 5-speed, 160,000mi ::: Dark gray metallic / black cloth
1984 190D-2.2 5-speed, 287,000mi ::: Silver-blue metallic / black MB-tex ::: SOLD

You sure it isn't coming from the valve cover gasket and weeping down around the pump boss to the lowest point?

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Thanks for the reply - that's definitely a solid possibility. I'm glad you suggested it.

I replaced the valve cover gasket about a month ago and that didn't fix the problem. In hindsight the leak may have actually gotten a little bit worse after doing that (but I've also done a ton of driving in the past month so it's hard to say for sure).

The top half of the seal (o-ring to valve cover gasket) is visible and I can see that it isn't leaking. The bottom half of the seal (o-ring to cylinder head) is hidden from view but the design seems fairly idiot-proof.

However, I am suspicious about how well everything seals at the point where the valve cover gasket, aluminum plate/o-ring, and cylinder head all meet. If the valve cover gasket doesn't seal perfectly at the corners there, it would leak exactly as you suggested.

Is there a trick to getting it sealed up in a leak-free manner? Or should I try replacing the valve cover gasket again? I'm a bit stumped on this one...

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__________________

1989 250TD Wagon 5-speed, 160,000mi ::: Dark gray metallic / black cloth
1984 190D-2.2 5-speed, 287,000mi ::: Silver-blue metallic / black MB-tex ::: SOLD

I'd pull the valve cover and make sure that the gasket is well seated all the way around. It's really easy to have it sitting slightly cocked, then when you tighten it down it'll leak. Be aware that the VC bolt tension is only 10nm, if you have it too tight you'll have leaks. How's your blowby situation?

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

No blowby to speak of on this engine - but that's another good idea of something to check. Excess pressure could have been forcing oil out.

I pulled the cover again and put a tiny dot of RTV at the two points where the cylinder head, valve cover gasket, and SLS pump o-ring all meet (photo attached).

Then I reinstalled the cover, with the bolts just a little more than hand-tight with a screwdriver-style nut driver (I ordered a proper torque wrench that'll do 10Nm accurately but it is stuck in the mail).

It's been 3 days with zero leaks, so hopefully that's the end of it. Thanks again for the tips and suggestions!

Attached Thumbnails

 

__________________

1989 250TD Wagon 5-speed, 160,000mi ::: Dark gray metallic / black cloth
1984 190D-2.2 5-speed, 287,000mi ::: Silver-blue metallic / black MB-tex ::: SOLD