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Help! I have coolant in my oil, and oil in my radiator. would this be just one of the gaskets or is it likely the block is cracked?

Head Gasket !

With oil in the coolant, it's not likely to be the UIM or LIM gaskets. The choices are pretty limited to a head gasket or internal radiator failure. Neither is particularly common. A compression test would be needed to figure out if it's a head gasket.

Cool. Thank you. Pretty expensive repair I imagine, right?

I haven't heard much about head gasket repair as its pretty uncommon as mentioned. I would think it would be pretty expensive, particularly if a shop did the work.

I think it would almost be easier/cheaper to do an engine swap. You can usually find a decent used engine for $500 - $700 with decent mileage.

I think the radiator is more likely than the head gaskets. I would get a compression test done.

I agree definitely get a compression test done. I am going to say not a head gasket though. The oil doesn't run through any passages in the head gasket, so for the head gasket to be bad enough to allow oil to get into the coolant passages the engine itself would be completely flooded with coolant and even then I seriously doubt much if any oil would get into the coolant path to the radiator.

Just how much are we talking about here?

If there really is a problem, I would put my money on a failed oil cooler inside the radiator. When the starts up the oil pressure will always be higher then the pressure in the cooling system, this will allow oil to bleed into the radiator. As soon as the engine is shut off, oil pressure is gone but the cooling system is still under pressure until it cools, so then coolant can be forced into the oil cooler lines.

I have never seen either issue on a Bonneville, but I do have a friend with a 2005 CTS that the transmission cooler in his radiator failed filling his cooling system with transmission fluid and his transmission with coolant. It killed his transmission.

Good point. Oil goes up the pushrods to the heads, then it's no longer under pressure and simply drains to the lifter valley.

We're putting the wagons before the horses here. You *could* potentially get oil in your coolant from a bad LIM. Not really sure how, but it's happened in the past.

I too second a compression test, and a pressure test of your radiator. A cracked block wouldn't let oil in your coolant, so I doubt it's that.

How's your transmission fluid look?

Another question I have is, has the motor ever been overheated? Or ran hot?

It has overheated a little...but not bad. slightly over 200. However, the tranny was acting funny this last weekend, and it was HOT. Hot enough to burn me when I pulled the dipstick out. The fluid looks thin....my biggest problem is im completely indigent at the moment. guess I need to let it sit for a while, which complicates things. Thanks for all your help, though..at least I have an idea where to start.

Are you still running Dex-cool and are you aware of the sludge it makes? I want to make sure you are not thinking the dex sludge is the oil in your coolant. Can you take a picture of the oil you are seeing?


A little over 200 is normal, 260 is too hot. I agree with those above and have a compression and leak down check done. You could also pull the LIM and see what the gaskets look like. Even if the block is cracked, they would have to come off

What are possibilities of trans fluid in the coolant, guys? Havnt heard of it happening on Bonnevilles but have seen it in other makes/models.

Ok, to take a step back...............the 2k+ does not have the oil cooler in the radiator. The only point for coolant and oil to mix would be internally in the engine.

If there is coolant in the oil (verify by draining the oil into a clear container and see if the oil floats or if it has the chocolate milkshake appearance) this suggests LIM gasket failure.

If there is truely oil in the coolant, it has to be getting there from the engine internally.

I suggest checking the oil to verify the coolant is in the oil. Then get a compression / leak down check done. Verify the "oil" in the coolant is not from the dex.

myfirstbonnie wrote:Ok, to take a step back...............the 2k+ does not have the oil cooler in the radiator. The only point for coolant and oil to mix would be internally in the engine.

If there is coolant in the oil (verify by draining the oil into a clear container and see if the oil floats or if it has the chocolate milkshake appearance) this suggests LIM gasket failure.

If there is truely oil in the coolant, it has to be getting there from the engine internally.

I suggest checking the oil to verify the coolant is in the oil. Then get a compression / leak down check done. Verify the "oil" in the coolant is not from the dex.

I was just seeing if maybe he is mis-identifying engine oil for trans fluid. Both obviously having an oily look in water.

Hence why I said "How does your transmission fluid look?". If it's thin and smells sweet, that means your trans cooler in your radiator is leaking, and the source of an oil in your coolant, and vice-versa.