Hello,

as you may well know I have replace my engine in my 300SD with a 617 out a an 84 300d with 252k. The issue AI am having is the coolant overflow tank looks like it has a thin film of oil onto of the coolant and it appreard to be oily, I have no water in the oil, but I did a copmpressionn test (cold) and it was 275 PSI across the board, My question is should I do a leakdown test, just run it, or put some additive in the coolant, I havent noticed the water level dropping... Neither has the oil, and by a thin layer i mean maybe 3/16" of an inch. I am not too concerned the engine was serviced my Phil Smart in seattle all it's life and runs excellent. Maybe i"m being too anal... Imagine that, I'm a machinist, so It runs with the trade

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Regards,
Ian White

1995 E300 Diesel w124 OM606
2014 E550 w212 M278 biturbo

2001 BMW 740i E38 M62 (past)
1981 300SD w126 OM617 (past)

I would flush the whole cooling system with whatever it takes to get it really clean. Do not neglect the heater core. Then use that as a benchmark to establish if there is really a problem or not.

I do not think the 617 is particularily notorious for this issue. It could have been residue from one of the last two engines you had in there for example hiding in the radiator or heater core. Or when you had the head off the last block a little of the oil may have gotten in the cooling system. Remember it would not have taken much.

I cannot see a leakdown test as being particularily helpful at this point. You might quickly check the head bolts for proper torque. If the benchmark test indicates there is oil leakage on this particular engine that is. Since they are the stretchable type basically. Just try to establish if any are under the normal pre final torque setting.

If one or two where found somewhat loose in the vicinity of the oil feed passage to the head. I would change them rather than retorque. Someone may have had the head off for example and not measured the bolts for pre stretch before using them again. Just what I might do and others may have better opinions.

You are due for a break. In my humble opinion the odds favor you a little in your senario. Although almost nothing is written in stone unfortunatly. For example requiring a head gasket is not particularily unusual at your milage either.