Car Clinic: When Oil and Water Do Mix

Car Clinic: When Oil and Water Do Mix


In the past two years I’ve had to change the oil cooler three times on my 2000 Mercedes C240 V6 A/T. The oil and water mix and fill the water bottle and radiator with a muddy liquid. Am I doing something wrong here? Please help.



Vincent, the dreadful sight of that yellowish brown soup in the water bottle is enough to strike terror into the heart of the strongest man. It means that either engine oil or automatic transmission fluid is getting into the cooling system somewhere.

If the car has an ATF cooler (standard practice on Mercedes automatics), this is one place where it can happen —but the transmission fluid used on your Mercedes is red in colour and would give a distinctive reddish tint to the broth in the bottle.

If in doubt, you should monitor the engine oil level as well as the transmission fluid level to see which one is dropping. From your description, it seems that engine oil is leaking into the coolant. I regard it as highly unlikely that three new Mercedes-Benz oil coolers would develop internal leaks in quick succession. Unless you have pressure-tested the oil coolers and confirmed that they are indeed leaking internally, one should look elsewhere. The Following possibilities come to mind:

• Seals: O-ring seals are used between the oil filter housing and the oil cooler attached to it. If they leak, oil under pressure can be forced into the coolant circulating through the oil cooler.

However, these O-rings are robust —workshops often find it unnecessary to replace them when installing a new oil cooler — and, if they should leak, the oil is more likely to appear on the outside, rather than be forced into the coolant. O-ring seals are also found between the timing chest and the engine block.

These have been known to start leaking on the V6 engines, but replacing them is a major operation. A contraindication is that when they leak, coolant normally shows up in the oil, rather than the other way around. Still, a workshop manager told me he had seen oil getting past those seals if they are bad enough; or

• Head gasket. If the cylinder-head gasket was damaged slightly at some stage, perhaps during an overheating incident, it may now be allowing oil to migrate slowly into a water channel.

In a running engine, the oil pressure is higher than coolant pressure, so the direction of the leak will be as stated. The oil contamination will only become evident when enough oil has accumulated in the coolant. I think the balance of probabilities favours the head-gasket scenario, but it’s by no means a certainty.

I strongly suggest that if this problem recurs, you get the car to a Mercedes dealership for further testing in order to obtain greater clarity before undertaking any repairs.

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