Hello all,

I have a 2012 c250 that just recently had the issue of oil in the coolant tank. From trouble shooting I came to conclusion that it could be 3 things.. transmission cooler, oil cooler or a bad head gasket… nothing is standing out to point in any direction. Transmission and engine oil look normal. Any ideas? I apologize if this was posted in the wrong area.

 

Hi,
I think it is engine related which can lead to, failed oil cooler matrix (under the oil filter housing), cracked cylinder head (not so likely).

What you have is pressurised oil entering the cooling system.
With a head gasket failure it is 95% of the time combustion entering the cooling system and overheating and displacing of the water.
There is not many places where the oil and coolant are both fully pressurised and close, cracked head usually lets water in to the crankcase non pressurised area.

Transmission cooler failure will let water enter the transmission when the engine is not running but the water is still pressurised.

Have you noticed on the engine dipstick oil level is lower than normal, as it looks like at least 1 litre of oil is in the cooling system.

Very hard to 100% diagnose but if it was me I would order a new engine oil cooler and filter housing and new gaskets.

 

Cylinder head gasket diagnostics:
- chocolate milk sludge under oil fill cap? Yes means failed gasket
- bubbles in coolant reservoir while engine is running? Yes means fail
- rough idle? Yes might mean fail
- steam or white smoke from tailpipe? Yes probably means fail
- slow loss of coolant? Yes probably means fail
- misfire or CEL? Yes might mean fail
- low compression on one ore more cylinders of the same bank? Yes means fail
- "washed" piston surface and/or "washed" spark plug and/or muddy spark plug? Yes means fail

If engine oil is clean it's not an engine oil cooler.

The last diagnostic step in the chain is the transmission oil, because of the hassle of checking it. Drain a bit and look at it in a cup. Smell the liquid in your coolant reservoir and try to discern if it's ATF or engine oil. Engine oil has a sulphur smell. ATF has a make-me-vomit smell.

 

The photo looks like the catch tank is full with foamed oil I am guessing at least 1 Litre of oil.
From a head gasket point of view, the head gasket would have to have failed between the high pressure oil supply passage way and the water jacket, this is possible but very unlikely as it is usually directly by a head stud for the head to warp that bad there would be the other usual symptoms first.

The above list is good, yes If the head gasket fails one symptom can be oily gunk under the radiator cap, but defiantly not the entire cooling system filled with oil and no other usual head gasket failure symptoms present.

In the past I have had experience with a cracked cylinder head below the camshaft letting coolant water in to the engine, the engine ran hot enough that the oil did not change colour or change level the coolant evaporated/boiled just leaving us with always slowly decreasing coolant level.

I have replaced failed radiator trans coolers before and each time the auto fluid was like a milkshake, from memory very little if any ATF oil went in to the radiator, the biggest job was flushing the transmission.

With cracked cylinder heads if it is badly cracked along the camshaft tunnel area enough coolant can enter the oil to make it change colour, goes more white with no oil entering the coolant system.

Really regardless of what the diagnosis is the big problem is longterm viability of the repair, the oil coating in the rubber coolant hoses does not magically go away post the repair with a few flushes.

 

Thank you all for the help y’all helped me understand what I was dealing with a lot better. Turns out it was the oil cooler as you suggested. I had the oil filter housing replaced by a shop in January due to an oil leak and on the C250 the oil filter housing and oil cooler are one unit. Either bad install or faulty part caused this but luckily the part was warrantied and the issue is being fixed.