OM602 Engine Oil in Coolant Causes?
 

I have a 1991 300d 2.5 turbo. About 2 years ago, my wife was driving it and gave me a call saying that it started to overheat (I was out of town). The coolant was low. I had her dump in some distilled water from a nearby supermarket and try to continue, but it overheated again in a mile or so. She pulled over and got AAA to tow it back to our house. After I got home, I saw that there was a huge amount of what HAD to be black engine oil in the coolant reservoir. There is no sign of coolant in the transmission fluid, and the oil is completely black so I am really pretty sure the transmission heat exchanger is not the culprit.

I couldn't face a head removal at the time, and so it has sat in my driveway ever since. Last week I decided to bite the bullet and take the head off, figuring the head gasket must have blown. I couldn't imagine that this much oil (it must be a quart) could possibly get through a tiny crack in the head, although I guess it is a possibility.

Anyway, I removed the head without too much drama (I did drop a bolt into the timing chain area, but got it out with a magnet on a wire). There is no sign of a crack that is externally visible. I guess there could be one internal to the head somewhere. The old head gasket looked pretty good, which is puzzling. It was not obvious that there is any open passage between the oil galleries and the coolant. It was pretty oily at the cam chain sprocket end, so the problem could be there.

Is there anything else besides the head and the transmission heat exchanger that can cause a large amount of oil to wind up in the coolant? I'm going to put a new head gasket on it and put it back together, but it would be nice to know if there is another possibility.

Thanks in advance for any info..

Max Behensky

I'd have the head tested for cracks by a competent machine shop.

There may be another heat exchanger between coolant and engine oil, on the oil pan. I don't remember which side of the pan for sure, I think the driver's side. However, a leak there would normally go the other way - coolant into the engine oil.

If the front of the gasket looked like it was breached at the oil passage between #1 cylinder and the timing chain case, that is probably your culprit and certainly something that needed to be fixed with a new gasket.

Proper pressure testing the coolant passages requires a special plate bolted to the head, and the head immersed in a hot tank (80 deg C), and then the coolant passages are pressurized while you watch for bubbles. I made a poor man's version by only pressurizing the exhaust manifold, using a 55 gallon drum cut in half as the hot tank.

Do you know the location of your Oil Cooler?

As an example my 84 300D has a Coolant Radiator that has the Transmission Cooler built into it but has a separate Oil Cooler Radiator.

A lot of vehicles have the Transmission Cooler and the Oil Cooler built into the Coolant Radiator.

So if your Oil Cooler Hoses go into the Coolant Radiator that a likely source of the Oil getting into the Coolant as the Oil pressure is higher then the Coolant Pressure.

They don't.

An unlikely possibility is a cracked block. It's just a blown head gasket.

Sixto
83 300SD
98 E320 wagon

The oil cooler for a 602/603 is air-to-oil, it's located behind the left side of the bumper. It's plumbed into the oil filter housing. Since it's not a water-to-oil cooler, there's no possibility that this is the source of oil. I think the head gasket or a damaged casting are the only possibilities.

Hi, all, and thanks for the input. It is probably the head gasket. The car is old and tired enough that it isn't worth spending the money on a new head if the head is cracked. I think as soon as the rain lets up I'm going to finish cleaning the mating surfaces on the head and block and torque down the head with the new gasket. Before I do all the work of putting it back together I'm going to hook up all the coolant lines and pressure test the cooling system. Hopefully it will hold pressure; if not, I can investigate further.

Max Behensky

if you can check for flatness on the head at least that would give you something to go by.