I recently replaced the oil cooler lines on my 85 300d, there is a small leak which appears to be coming from the bottom fitting on the oil cooler housing. (not the hose) The lines came off easy enough and the fittings did not turn. Is this a common leak? Is there a crush washer behind that fitting that needs to be replaced?

I'm sure many of the oil coolers have leaked at the fittings but I haven't seen one. There is a DYI section on this forum I'd suggest you research the issue. I'm sure it's happened before and the issue is covered there.

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84 300SD
85 380SE
83 528e
95 318ic

The rounded part of the male fitting presses against and seals against the sort of dished in end of the Oil Cooler Nipple. (If oil is leaking under the crimped collar the hose needs replacing.)

There has been 2 cases I have seen on the various forums where there there is a crack a crossed the aluminum nipple sealing area. I also supposes it is possible if that area got deeply scratched or the steel male end got scratched.

I think there has also read a couple of cases where the was some dimensional issue with the male end of the not being thick enough to make good contact with the nipple sealing area. But, that was on after market Oil Cooler hoses that people had bought.

I guess it is also possible for the male end to develop a crack under the nut. But, I have not read of that happening to anyone.

So what I am saying is simply see if you can find a reason why it would leak.
See posts #17 and post #22.
http://www.peachparts.com/shopforum/diesel-discussion/340694-911-oil-cooler-line-leak-cleveland-ohio-may-need-parts-2.html

If the Oil Cooler Nipple is cracked there is a repair for that but you have to cut the threaded area off drill and tap the Oil Cooler and there is a steel replacement nipple that goes in there (I have done this before).

The Oil that goes through the oil cooler is dirty oil and it goes back to the oil filter to be filtered. So any stuff you get inside of the Oil Cooler is not going to make it into your engine but I had some chips migrate to the oil Filter Housing where they later had to be carefully removed.

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84 300D, 82 Volvo 244Gl Diesel