For the past few months I noticed a decrease in my coolant level, so I had to top it off every 1,000km or so. Out of curiosity (and fearing the worst) I sent an oil sample to Blackstone Labs, who called today and said they found traces of coolant in the oil. Not high enough to break it down, but present nonetheless.

I just bought this car so I never changed the oil. Judging by the service interval, this oil has been in the engine for 9000km.

How do I locate this leak?

In addition, the plastic coolant container (located near air filter) had a leak in the past, because it corroded the body right underneath it. I think that's not an issue anymore, because the cooling system is under pressure. At least that's what I think because whenever I open the cap there's a "hiss".

Any suggestions would be highly appreciated. I'm kind of worried.

The engine is OM606, a turbo-diesel.

When running hot, there's no smoke, unless I floor it really hard.
3 GPs are burnt, so when I first start it there's smoke for 30 seconds, after which it runs very smooth.
City gas mileage is 10 l/100km... not sure what that is in miles/gallon. Sorry - old European habits.

Thanks for reading and for your feedback!!!

when is the last time you changed the overflow tank cap?

I never changed the overflow tank cap... I haven't owned it for too long.

Could the cap be the cause of this problem?

Oh, I should add - the oil looks black. I see absolutely no signs of foaming/mocha-looking stuff - nothing.

Headgasket is going, they are fairly easy to do. Spend 2-3 hours researching the diy and do it over a weekend. Be careful with the timing chain details.

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1976 Mercedes 240D (Sold)
1985 Mercedes 300D Turbo Diesel: 500,000KM
1986 Mercedes 300E (Sold)
1988 Mercedes 300E (Sold)
2002 Mercedes C240 (Sold)
2008 Mercedes C350 4matic

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Industrial Rubber

If you have an external Oil Cooler the only source I can think of for Coolant in the Oil is the Head Gasket.

If your Oil Cooler is cooled by the Engine Coolant you could have a leak in the Cooler or the Head Gasket.

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84 300D, 82 Volvo 244Gl Diesel

How do I find out if my oil cooler is external or cooled by the engine coolant?

If your model had oil cooler hoses just follow them. I suspect your car would have an isolated oil cooler. Just not positive.

Thanks Barry, Diesel911,

I looked up my model on EverythingBenz - Mercedes-Benz Forum and Web Search Using Google and it looks like it's an isolated oil cooler.

I guess that leads to a bad head gasket. Time to plan for a repair.
Oh well, I guess things could be worse.

If you decide it is the head gasket, before you tear it apart, you might try some radiator sealant. About two years ago I found the slow coolant leak on my '97 E300D. It looked like the block behind the exhaust manifolds were coated with bird droppings where the coolant leaking from the head gasket had splashed around and evaporated. I removed about a quart of cold coolant, mixed in about 3 to 4 oz. of plain old BarsLeaks, shook it up and let it settle a couple of minutes, and poured 3/4 of the mix back in the reservoir. The heavy particles settle out quickly and I left them in the remaining unused coolant. I drove the car for about 1/2 hour to heat the engine up and let the coolant pressure seal the leak. The head gasket leak is gone and I have stop leak circulating, in case it is tempted to misbehave again.

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'97 E 300 D

Note; Radiator Sealant is different from Block Sealant.

Radiator Sealant often has particles of stuff that circulate through the Coolant system to block off holes.

Block Sealant often uses something called generically Water Glass (don't remember the real name.)
You need to read the makers instructions but typically with Block Sealant you get all the Coolant out of the Engine and Flush several Times with Water. You add the Block Sealant according to the Instructions and run it according to the Instructions. After that the Water is drained out and you let the Engine sit to dry out something like overnight before adding the Coolant/Water mix.
On the Internet I have read to run the Engine for a longer amount of time with the Block Sealer in it; the claim is that that works better.
Most Block Sealers have a money back guarantee.
I doubt if Mercedes wants people using them.

I have read some articles that say to keep the Block Sealant out of the Heater Core when you use it.

The Radiator Stop Leak often as stuff they bill as Water Pump Lube and particles of stuff to block the holes. You mostly just dump them into the Coolant.

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84 300D, 82 Volvo 244Gl Diesel

If it were my car I might do the 603 test for cracks in the head. Even though it is not a 603.That is next morning after use and sitting for eight hours check the rad hose for still being pressurized. If it was I might again consider a waterglass treatment.

It will also coat the internal passages of the water jackets. The reason I might try this approach as I see it as a nothing to loose scenario..If it seals the head crack or cracks you may be okay.

If not you will need another head as they are basically unrepairable. I have always been concerned about any loss of heat transfer as the stuff seems to coat all the waterjacket passages as well on the engines. At least on the ones I have taken apart that have had it done at some point in their lives.

Then again it is hard to say how much waterglass was used. I wanted to deal with this area eons ago for all the cracked 603 heads that. were occurring. What stopped me was that it was a matter of judgement .

I did not want to see people pouring it into their systems when they should not be. Part of my concern was the possibility of reduction of thermal transfer. The cooling systems on these cars do not seem to have a lot of reserve capacity.

Waterglass evolved a long time ago as a leaky boiler repair substance that I suspect is silica based. The high heat of combustion seems to really fuse the stuff in cracks. You could even order the stuff through your friendly drug store and it may still be able to.. Was cheaper that way to get a usable quantity.

Thanks for all the suggestions guys.
I finally got my report back from Blackstone. There's signs of coolant in the oil, but what puzzles me is that there's no water, and I am definitely losing both. For the past few months I've been adding just water to make up for the loss of coolant, but none of it is in the oil. So where is it going? There's no dripping, as far as I can tell, so I'm confused to say the least.
I considered that in the past there was a leak that contaminated the oil and the previous owner fixed it but didn't change oil (or didn't do it properly and there's still contaminants in the oil). That would explain why there's no water in the oil. I'm only taking short trips, so there's no time for the water to evaporate from the oil, and I have no foaming on the oil cap.
Anyway, I'll change the oil and will take it from there.
Thanks and have a great day,
Tudor

The water component of the coolant just boils off in the hot oil. The oil temperature exceeds the boiling point of water. So I would not expect to have any. The lab had detected the actual anti freeze component though.

If the leak is small what Barry said can be true.

But, some leaks do not open up until the Engine is up to temp or even under a load. This can apply to cracks or if the Head is warping when it get up to temp and causing a small head gasket leak.
When that happens the water in the Combustion chamber wants to turn to steam and the Coolant Chemical would want to separate from the Water. Most of the stuff is going to go out with the Exhaust.

Evidently come of the Coolant Chemical has made it into the Engine Oil.

If you think it Coolant contamination from a previous job; doing a Oil change and paying for another test On the Oil would tell you.

However, you said you Coolant was disappearing. Without an external leak that leaves where else would the Coolant go?
External leaks can also only show up when the Engine is Hot.

If you disconnect the Hoses to the Radiator and the Heater Core so they do not get over pressurized an pressure test on the block might show up an external leak.

Would a pressure test of the block cause water to get into you Cylinders??? Maybe.

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84 300D, 82 Volvo 244Gl Diesel

I just want to make clear that I was not referring to the water glass treatment in my earlier post. I used BarrsLeaks radiator stop leak. It is simply a ground up organic, that makes fuzz balls of various sizes. They are carried to the leak site by circulation and thru the leak until some big enough to wedge in the leak,come along. Then they build up like a dam. The only downside I am aware of is that some particals are too large to continue circulating, and will settle in your block and radiator. To minimize this I mix the sealant in some coolant and let the large particles settle. I then pour the upper portion with the small particles, into the car.

I have worked with similar stop leaks in a lab some 30 years ago. At the time Zerox was adding it to their premium antifreeze as sort of a value added thing. My company was producing ethylene glycol and was thinking about selling retail. That was back when the latest thing in oil technology was Arco Graphite. I bet no one on here remembers that oil.

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'97 E 300 D