oil in coolant

[unplugged]

My car overheated due to loss of coolant. When adding some I notice a thin film of oil floating on top of the coolant when I opened the radiator cap. Not much but some.

Compression test is all good. I changed oil today and oil in pan was all good. Car has maybe 2 thousands miles on it since rebuild.

Due to the higher pressure in coolant system I would have expected coolant in oil and not other way around.

Any thoughts on what/where I should look ?

Thanks

[bradartigue]

I would be inclined to think you have some crap in there from the rebuild. I know when I rebuild engines I end up oiling the interiors of things to keep them from rusting, etc....so that may be all it is. It takes very little oil to cause a thin film. Replace the coolant, the oil, and monitor.

[DieselSpider]

I would be inclined to monitor it for a while if its not making the engine oil or the coolant soapy or lather. A teaspoon or so is all it takes to give a small pond a sheen and was used at times for mosquito control to prevent their larva from getting air.

Now if you start getting high pressure in the radiator that is causing the cap to vent then that could be combustion forcing its way into the cooling jacket and that would be an indicator of a blown head gasket or cracked head and requires immediate action.

Did you get to the root cause of the overheating?

[vandor]

> Due to the higher pressure in coolant system ...

Cooling system has max 12-13 lbs of pressure, however the oil pressure can get up to 50 psi, or more, so oil pressure is often higher.

[unplugged]

Thanks for the help.
I'll take a wait and see attitude and cross my fingers.

The original overheating was caused by a loss of coolant. I have a non-standard overflow tank and the mounting bracket cracked. I think it may have lost coolant when it fell after cracking. Just a guess. If it happens again I'll run another compression test.