Oil in the coolant but no coolant in the oil?

[Jordan]

Vehicle is a ’04 Buick Rendezvous (GM 3400). 68k miles.

Had a shop replace the thermostat. They said that when they bled the coolant, it had oil in it, indicating a possible head gasket/intake gasket leak.

Engine oil looks good, no sign of contamination. No smoke from the exhaust. It’s not using any oil or coolant. No ticks or knocks. No overheating.

Given these facts, I have a hard time believing that there’s a head gasket leak. I would really expect to see coolant in the oil if this was the case. What do you guys think?

Topic

[popoften]

Yes, oil can leak into the cooling system but that does not mean coolant will necessarily simultaneously leak into the oil. For example, if you have an oil cooler inside your radiator and that starts to leak, you will find oil in coolant but not necessarily any coolant in oil. This happened to me recently on my ’94 Suburban.

There might be no other symptoms at first. This warrants further investigation, because if oil is fgetting into your cooliung system, it needs to be fixed and pronto.

[Jordan]

Thanks for the reply, I know that there is an issue to be fixed but I was wondering about the head gasket

[popoften]

A head gasket leak does not always result in coolant in the oil. I had a head gasket leak in my Cadillac that resulted in exhaust gases in the cooling system; but no coolant whatsoever in the oil. It was only confirmed by means of a block test. Suffice it to say that if you have oil in the coolant and assuming no one inadvertently poured motor oil into the radiaor/reservoir, you have a leak somewhere; so the appropriate tests need to be conducted to locate the leak(s). Fortunaely, Eric has videos on how to do these tests yourself if you so desire.

[Mike]

Consider yourself lucky your oil is still ok and replace the intake gaskets. They are guaranteed to eventually leak on every 3400 and 3100. They can leak oil into the coolant, coolant into the oil, one or both into the intake or externally, or any combination thereof.

[Bill]

It’s very common for the intake manifold gaskets to leak on that engine. I have replaced at least a dozen of them from the early 3.1 to the 3.5lt. When the engine cools and the cooling system goes into a vacuum, small amounts of oil are drawn into the cooling system from around the gaskets. When the engine is restarted the cylinder heads and intake manifold expand faster than the cooling system pressurises so no coolant leaks into the oil.

If neglected long enough it will either leak coolant into the oil or will leak externally.

Head gaskets on a 3.4 have no pressurised oil running through them and don’t allow oil into the coolant. The oiling is done through the pushrods.

[Author]

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