Hi, This involves my 1991 300 D 2.5 with 290k miles. I was hoping to get some opinions on what might cause a significant amount of diesel fuel to get into the engine oil. It amounts to about 1 quart of diesel in about 100 miles. Car otherwise runs well. I have changed both fuel filters and installed rebuilt injectors since the problem started. Car runs better with the new injectors, but no change to the diesel in oil problem. I have changed the oil and filter 3 times in the last 200 miles while trying to figure this out. My gut and other advice tell me it has to be an injection pump issue, but can anyone think of any other way to get that much fuel in the oil? Other work done to the car in the last 1,000 miles includes new timing chain and rails and new head gasket. Thanks in advance!

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Greg

Current
2005 E320 CDI (50k) W211 with OM648
1999 E300D Turbo (133k) W210 with OM606.962
1997 E300D NA (170k) W210 with OM606.912
1991 300D 2.5 Turbo (199k) W124 with OM602
1990 300D 2.5 Turbo (170k) W124 with OM602

Welcome to the forum.

I'm curious as to how you determined that fuel was contaminating the engine oil. Normally it's diluted and not easily visible.

Ways to spot Fuel Contamination in the Crankcase Oil:
If the Crankcase Oil smells like Diesel Fuel and the Oil Level keeps rising above the amount of Engine Oil added.
I have never had the newer Fuel Injection Pumps apart but these 2 items in the past used to cause leaks on Bosch and other types of Inline Fuel Injection Pumps with similar operating parts.
Worn Fuel Injection Pump Elements/Plungers and Barrels (Item #23 at the below parts site, you have 1 per cylinder). These are the parts that create the hight pressure for fue injection. I have read that WVO/SVO use can wear them out unless the Fuel is prepared carefully. If only Diesel Fuel is being used it can be a fuel filter or fuel contmination problem causing the wear of sometimes the IP is just plain worn out from long use.
If the Lift/Fuel Supply Pump Seals on the drive end of the pump (driven by the Fuel Injection Pump Camshaft) are no good Fuel pressurized in the Supply Pump can leak out of the seal.
The Lift Pump seals appear to be items #258 and #261 in below pic at the other site (I have not had one apart so I do not know exactly what seal is what).

http://www.detali.ru/cat/oem_mb2.asp?TP=1&F=124130&M=603%2E912&GA=722%2E415&GM=717%2E430&CT=M&cat=503&SID=07&SGR=045&SGN=01

I have also seen rare occasions when item #23 the Element/Plunger and Barrel assemblie; the Barrel portion is not seating well in the Fuel Injection pump Housing. There is no seal in this area. It seals Steel Barrel to Aluminum Fuel Injection Pump Housing metal to metal.

A source of the above problem could be if some one has change the Delivery Valve O-rings and managed to pull one of the Elements up a little. This could allow debre to get inbetween the sealing surfaces and interfere with the meta to metal contact.

The other stuff is farfetched; like loose Barrel,a split Barrel or rust erosion inside of the IP housing.

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84 300D, 82 Volvo 244Gl Diesel

The lift pump is lubed by engine oil....they don't fail often.

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)

I believe the OP'er had some trouble seating one of the DVs when he changed the seals (he posted this on the MBCA site)...I'd bet this was the cause...I didn't know it was possible to create such a leak this way but if so it is surely the source of the oil.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

If the DV leak, the fuel won't contaminate the oil.........it'll contaminate the driveway........

I guess we need some clarification from diesel911 as it seemed like he was implying that this could cause the leak of fuel into the crankcase somehow...

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

Fuel can get into the oil from poor combustion (past the piston rings), a severely worn injection pump (past the plungers) or a failed lift pump shaft seal. At 1qt/100mi you should be able to smell it in the oil and feel the thin viscosity when cold (It will feel like 5W30).

Seems to me that if the copper washer is damaged and fails to seal the fuel will reach the oil while the o-ring prevents an external leak.

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1987 W201 190D

It would only leak back into the fuel chamber that surrounds and feeds the plungers.

I can sure smell the diesel in the oil and it is way thinner than when I put the Mobil 1 in only 50 miles ago. Plus, the oil pressure gauge starts to indicate reduced pressure and drops a little below 3 bar, even at 40-50mph. There is no question that I am getting a lot of fuel in the oil, and quickly. The weirdest thing is how well the engine runs. It sounds as good as its twin which has only 77k miles. That is why I am so puzzled. Thanks for all of your thoughts. If I take it to the dealer they are going to tell me they have to replace the IP. What is that going to cost?

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Greg

Current
2005 E320 CDI (50k) W211 with OM648
1999 E300D Turbo (133k) W210 with OM606.962
1997 E300D NA (170k) W210 with OM606.912
1991 300D 2.5 Turbo (199k) W124 with OM602
1990 300D 2.5 Turbo (170k) W124 with OM602

My vote is for the lift pump. A seal failure could allow fuel to enter the crankcase but with everything else good, the engine would run well, as the OP reports.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

It is one thing to have a RUN AWAY ENGINE while the car is in park. But if the oil ignites and you have a RUN AWAY while in gear; VERY bad things are going to happen as a result of the car being OUT OF CONTROL and accelerating to well above the RED LINE.

Replace the pump. Do not drive that car with diesel fuel in the oil at the level you are experiencing. DO NOT DRIVE THAT CAR!!! If you've ever seen a RUN AWAY, you would know what I'm talking about. Until you have seen a RUN AWAY, you can't imagine what it is.

DO NOT DRIVE THAT CAR. You have already used up your LUCK!!!

BenzDiesel

Does Fast Lane or that other parts company up at the top of the page sell a lift pump kit?

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84 300D, 82 Volvo 244Gl Diesel

I agree.

If the IP was allowing that much fuel to pass........there would most certainly be some operational signs.