I would like to know the step by step troubleshooting procedure for discovering the source of oil appearing in coolant. There are several different points of failure that can be the root cause for oil in coolant.

What are the various points in an engine that could be the source of oil appearing in coolant?

How do I rule out various sources of oil in coolant?

What kind of tools do I need to troubleshoot the source of oil in coolant?

How do I use those tools to determine the source of oil in coolant?

  • 1
    Good question. I'm having a real hard time trying to figure out how I could test between a cracked block/head and a blown head gasket without taking everything apart.
    – rpmerf
    Feb 22, 2016 at 12:07
  • If oil is getting into the coolant and not vice versa then the only guarantee is that the oil is coming from a high pressure source. This eliminates lower intake gaskets for example but that's it. Anywhere there is an oil galley next to a coolant passes is a potential problem spot. This includes cracks and gaskets. Experience can tell you that some engines have known problems but other than that a tare down is needed every time. (You could cat scan the engine but lay men can afford that and well funded race teams would just tare the engine down and not waste their time)
    – vini_i
    Feb 22, 2016 at 14:53
  • I'd agree with vini_i if it's not transmission fluid than the customer needs to be prepared for a tear down.
    – Ben
    Feb 22, 2016 at 23:00
  • Does anyone think that a change to the question to include transmission fluid or remove an aspect of the question would be better? Looking for input. Feb 25, 2016 at 1:03
  • 2
    I think transmission fluid/coolant mixing is a different question, since the only place they can mix is the radiator. Oil/coolant can be the head gasket, cracked block, or cracked head.
    – rpmerf
    Feb 25, 2016 at 12:53

2 Answers 2


Number one answer is always Head Gasket. The reason is that there are oil and water passages in close proximity, being separated by a gasket (which might just be flimsy paper or rubber, but is often steel or copper) sandwiched between two pieces of metal with (usually) different thermal expansion rates. This means that any time your engine gets far enough out of the proper heat range, the seal will fail and oil and water will begin to mix. Other problems will also result.

The number two answer is "oh, so you decked the block/shaved the head and replaced the head gasket... did you check the head for cracks?" Again, it comes down to the fact that the head is typically full of oil and water passages and the head will typically be the loser (structurally speaking) when something bad happens in terms of overheating. The block does have both oil and water passages but they're much farther apart (the block is primarily concerned with oiling the rotating assembly and the piston walls while the coolant passages are on the outsides of the top halves of the cylinders).

The third answer is a lot more car specific. Some cars (like the mazda bp) use a little housing that contains both oil and water flow to cool the oil with water. This almost never leaks, but it might be something worth checking out if you recently messed with it. Or maybe your car is equipped with a water cooled turbo... if the seals went bad, you might get oil flowing into the coolant that way.

There aren't special tools beyond those required to take apart the engine. External oil leaks can be spotted via dye and UV lights but that won't work here. Oil in the coolant is usually a prime diagnostic tool for head gasket woes. If it isn't your head gasket, you'll already have the head off, so you might as well drop it off at a head specialist to have them check it for cracks and warping. The most common causes of bad HG I've seen (other than overheating) were improper torque, improper orientation of HG, use of gasket maker when inappropriate, lack of gasket maker when required and severe uncorrected head or block warpage. If it is your head gasket, you should probably at least check that stuff with a straight edge (check straightness on either side of the cylinder line, across diagonally and across the top and bottom of the line). Also make sure you don't own a mk3 supra or similar car known for improper factory torque specs, etc.

One trick that might produce results is to remove the radiator cap and thread an air hose into the spark plug hole of each cylinder, one at a time (a compression tester will have the right threads), rotating the crank/cam assembly to close the valves for that cylinder and put it in TDC. Then dump a little oil and then 100 psi of air into the hole and see if it makes bubbles out the radiator hole. If it does, break out the wrenches and get to work on that head gasket. This might not find the problem though... you can get leakage between the oil and coolant passages without the leak intersecting the combustion chambers. You could put air pressure into the oil system, but you have a very high risk of blowing out seals on the engine by doing this. The combustion chamber is at least safe with high pressures (or should be, normally).

  • Nice comprehensive answer. As far as putting air in the oil galley, you'd not cause any issues with this as you'd only just blow the oil out of the orifices which it normally flows out of (ie: past the bearings). You might back flow the oil out through the pump pressure release valve as well. It wouldn't really tell you anything, though. There'd be no way to check this for anything significant. Also, if you are pressurizing the cylinders to check for a head gasket leak, any rise in the radiator fluid level when applying pressure means the same thing. It may take bubbles a while to escape. Feb 26, 2016 at 11:26
  • I was mainly worrying about the gaskets and seals around the oil pan. The front/rear main seals aren't normally under pressure either. On my car at least, blowing out those seals will mean pulling the engine and trans to replace them. It's a huge pain in the ass, much more so than a head swap.
    – Jim W
    Feb 26, 2016 at 16:09

Try this video,


Other than this my suggestion would be is to do a hydro carbon test on the car, I think you can get the kit for like 25 dollars. Open the radiator cap (only when the car is cool as opening it with a hot radiator can spray scalding liquid all over you and anyone around you), run the car for a while, until the temp gauge rises to centre. The radiator is the best place to look for oil. Smell the exhaust, it smells kinda sweet if the gasket is blown.

  • I believe you should run your vehicle up to temp with the radiator cap off. Do not open a hot radiator as you are suggesting ... you are just begging for injuries, not only to yourself, but to those around you. Mar 2, 2016 at 23:19
  • yeah, my dad burned his arm and face years ago doing that. Twas pretty brutal. Mar 2, 2016 at 23:35

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