Oil in coolant reservoir

Hi,

Have you checked your transmission fluid for coolant? The only reason I ask, is these trucks has a trans oil cooler that has been known to crack. When it does the coolant and trans oil mix and create a milky sludge. Sometimes white in color, sometimes darker depending on the condition of the trans oil.

If not, here are the common causes for oil and coolant to mix. Even though you don't have overheating, it still could be a head gasket.

Have you done a compression check or checked for hydrocarbons in the coolant? Both will indicate a head gasket issue.

Here are a couple guides that will cover the basics. Read through this and let me know what you think and we can go from there.

https://www.2carpros.com/articles/coolantantifreeze-in-the-engine-oil

https://www.2carpros.com/articles/how-to-test-engine-compression

Unfortunately that sounds like transmission fluid. If you don't have dipstick that means you have an 8HP70 transmission. Here is a video and the written instruction on how to check the fluid.

If you notice in the video the color of the fluid he is using. It is a greenish color and feels very much like mineral oil. It is not traditional pink/red transmission fluid. If mixed with coolant it can darken in color.

I have seen a few of these where the cooler cracks and the trans and coolant mix. IF this is the case, it sounds like you caught it at the beginning stages.

If it were me, I would pull that plug and inspect the fluid in the trans just to be safe. Obviously if you do this after the leak down test, that would be fine because if you find a head gasket issue then this would confirm engine oil.

https://youtu.be/BPkCUfqi-gE

Let me know what you find. Thanks

That looks familiar. I agree that it looks like engine oil and it could be, but I have seen this a number of times and that white gooey residue around the side will eventually be throughout your cooling system. I would check both the cylinder leakage and the trans fluid just to be sure.