My girlfriends car kept dumping oil on the driveway after a long drive. I replaced the valve cover gasket but didn't see anything wrong with the old one.

Then I found a bunch of oil in the air cleaner housing. We replace the air filter but the oil is coming in from the vacuum pump hose.

I can think of two sources, the vacuum pump itself or the injection pump diaphragm.

I have a new pump diaphragm and rebuilding the vacuum pump probably is doable too.

What's the best way to tell where the oil is getting into the vacuum system?

what car?

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

My guess is that it is a 220D or 240D w115, because they mention the injection pump diaphragm on the pneumatically governed IPs. If my guess is correct, then perhaps they also have an oil bath air filter, which might explain at least some of the oil found in the air cleaner.

Regardless of my worthless hypotheses, they stated that the oil is coming from the vacuum pump hose which means that their vacuum pump diaphragm has ruptured and needs replacing. It is a relatively easy fix and there are many good writeups on it, and one mediocre one:
http://www.peachparts.com/shopforum/diesel-discussion/318203-vacuum-pump-diaphragm-replacement-beginners-perspective.html#post2938702

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).

'78 240D with paper air filter.

I'll read about replacing the diaphragm in the vacuum pump - I bet you're right that that's it.

Well, hopefully my diagnosis of your oil problem is better than my guess of your car.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).

Well it looks like the ball bearings should be replaced. I found Nachi #626 at VXB for cheap.
Are there two ball bearings in there?

Can I expect to do this in one day if I have the bearings and gaskets on hand?

When I did the vacuum pump diaphragm on my '77 240D I didn't replace any bearings. I only kept the car for about another year following, but I didn't have any more issues with the vacuum pump during that time. Changing the diaphragm once the pump is off the car is pretty easy. Getting the pump off and back on is a bit more work, but still not what I'd call hard.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.