Oil in coolant reservoir

[Gary Schroeder]

I have a 2005 Cadillac CTS HI Feature 3.6 liter. Engine oil is leaking from the coolant reservoir mixed with coolant. Is this a lower intake leak, head gasket leak or something else?

Topic

[Mike]

It could be an intake manifold gasket, a head gasket, or a crack anywhere in the water jacket in the heads or block.

Pull your spark plugs and see if any of them look wet or otherwise weird. If the plugs look OK, at least the damage is outside of the cylinders and combustion chambers.

How long has this been happening, and are any codes being registered?

EDIT: Does your car have a transmission cooler that’s built into the radiator? It may not be engine oil at all, but an internal leak between the transmission and engine coolant fluid systems.

[Mike]

The HF 3.6L engine does not share that lower intake leak thing that all the pushrod engine families, it had dual overhead cams and the intake gaskets have nothing to do with coolant or oil. The thing I’m thinking is from I did tons of timing cover reseal jobs because of oil leaks back when I deal with those engines under warranty. The timing cover seals coolant in addition to engine oil, as the water pump mounts on the outside of the timing cover. I never saw the timing cover gasket leak coolant into oil, but it could happen, and I was dealing with engines new enough to be in warranty. Resealing that cover pays 14 hours, and nobody would pay that to fix a oil leak except warranty, so it could be leaking since it was out of warranty so many years ago and gotten bad enough that it started leaking coolant from the water pump housing into the timing chain area. It may not be that, it could be a headgasket or radiator/oil cooler failure, but I was just kind of having a train of thought there.

[Gary Schroeder]

It just started leaking. I checked the oil and it was low. Very low. I checked the tranny fluid and it was OK. I was hoping for the lower intake. Looks like its going to be the timing cover or head gasket.

[Gary Schroeder]

Thanks for the info guys. I’ll do a block test on each cylinder to pinpoint or eliminate the head.

[Jim]

I’m still figuring out the cut and paste stuff with the text, but someone mentioned the trans cooler. I would suspect that as well. I’ve seen trans leak only to the radiator, or coolant only to the trans, and seen them go both ways. On that CTS, the trans cooler runs through the lower tank of the radiator, the fittings corrode often, sometimes break off, or leak one fluid to the other. I’m not sure what sorts of tools you have access to, but if you dye the engine oil, since it’s easy to get to, note with a black light what color the oil glows with the dye. Run it for a bit and see if that bright dye shows up in the coolant. If not, if it looks dull or not as bright as the engine oil, it’s probably trans fluid. Replace the radiator and you’ll be good. The most I’ve seen with the 3.6 is timing chains, but I think yours is old enough to have some pretty stout chains still. I’ve seen some oil consumption, carbon on the valves, water pumps, but not much else. Not to say it’s not possible, but not too likely. I’d look at that radiator really close, fitting on the passenger side seems to be the one that will break off or leak.

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