Oil in my overflow tank

I would like to find out why I have oil in my puke tank

I would like to find out what the year, make, and model of your car is, as well as how many miles it has on it and whether it has a turbo, And if you’ve been having other operating problems.

The only way a typical engine would be able to get oil in the coolant is through a headgasket breech between the water jacket and an oil channel. Typicallly a breech like that would definitely also involve the combustion chamber.

If it has an automatic transmission what you might be seeing is transmission fluid. The transmission fluid cooler is in the radiator.


Good point, Tester.

@mountainbike Agree, it’s likely to be transmission fluid. It is hard for oil to get into a prrssurized cooling system from an unpressurised crankcase.

Docnick. oil is pumped from the block to the head at 40+ psi. It goes through a hole in the head gasket. If the head gasket were to breach around this hole, 40 psi from the oil pump would trump the 15 psi of the cooling system.

But I would surely want to check that transmission too.

@keith Thanks, but I discounted the low probability. However, a good mechanic would look at all possibilities.

If it were a headgasket, the vacuum from the intake stroke and the blast from the combustion would cause things to mix anyway…and oil return lines come through the headgasket too…unpressurized oil.

But anyway, I think Tester is probbaly right. It’s more likely tranny fluid.

For the OP’s reference, I’lll point out that it’s common for cars to use a two-part radiator, the top part dissipating heat from the coolant and the bottom part dissipating heat from the tranny fluid. In these cases, it isn’t unheard of for the radiator to leak between the two and mix the coolant and the tranny fluid.