Is it my head gasket?

Milky oil under the oil cap: what coolant in the oil really means

It is the symptom everybody has heard of and the one a field technician sees least. Here is how to check for it, what the dipstick adds, and the one reading that changes the whole conversation.

The ten second check

Remove the oil fill cap and look at the underside. If there is coolant or water in the oil, the cap has a milky coffee look, sometimes described as a coffee milkshake. If it just looks like oil, you do not have coolant in the oil. That is the whole test.

Clean oil does not clear your engine

Two of the most repeated lines a technician hears are "there is no smoke" and "the oil is not milky, so it cannot be the head gasket." Neither one rules it out. In Dave Whitmer's field experience, across more than 1,100 repairs, coolant mixes visibly into the oil in at most 1 in 10 blown head gaskets. Most never show it. Coolant loss is the constant; where the coolant goes is what varies, and most of the time it goes out the tail pipe as steam.

So if your oil looks fine but you are adding coolant, or there is white smoke, keep going. See the four signs a technician checks first.

Milky oil is not a disqualifier

If the cap is milky, it is not the end of the road. Coolant in the oil is another symptom of a blown head gasket, and by itself it is no big deal: you change the oil after the gasket is repaired. What matters next is how much is in there.

Now read the dipstick

• At the full mark. A small amount of coolant has mixed in. Repair the head gasket, then change the oil.
• An inch or two over the full mark. More coolant in the crankcase, but you should still be alright. Repair, then change the oil.
• Five inches or more over the full mark. That much coolant in the oil points to other problems, possibly a cracked block. Do not pour anything in yet. Get tech support on the phone and go through it with them.

A quick word on false alarms

A thin milky film on the cap alone, on a car that only makes short trips in cold weather, can be condensation collecting in the top of the engine. If the dipstick oil looks normal, you are not adding coolant, and there is no steam from the tail pipe, the cap film is not enough on its own. A block test settles it for about $10: How to do a block test.

What to do next

1. Note the dipstick reading. Full, an inch or two over, or five inches or more.
2. Check whether you are adding coolant and whether there is white smoke from the tail pipe.
3. Drive it as little as possible. Pressure from the pistons erodes the breach every mile, and an engine that will no longer start cannot be repaired chemically.
4. Run the qualifier below. If the dipstick is five inches over, call tech support instead: (844) 796-7968.