Rick's Free Auto Repair Advice

Yellow gunk under oil cap

What is the yellow gunk under oil cap?

Yellow gunk in the oil cap does NOT AUTOMATICALLY mean a failed head gasket

Let’s get this straight from the get-go; Yellow gunk inside the oil filler cap is

Yellow gunk under oil cap

Yellow gunk under oil cap

NORMAL in winter if the vehicle is driven short distances. By itself, it is NOT a sign of a head gasket failure.

What to check if you see yellow gunk under the oil cap

A head gasket can fail in several ways; 1) Breach between the cylinder and a cooling passage 2) Breach between a cylinder and oil passage, or 3) Breach between a cooling and oil passage.

When a head gasket fails between the cooling jacket and an oil port, coolant mixes with motor oil to form a chocolate shake, chocolate milk or chocolate pudding-like formation in the crankcase. That’s not a good sign and you should NOT drive the vehicle. Continuing to drive the car in that condition can cause complete destruction of your engine.

If you see yellow gunk in the oil fill cap, check the dipstick

First, check the dipstick. If the oil on the dipstick matches the yellow gunk in the oil fill cap, then you can start to worry about a failed head gasket. If the oil on the dipstick is normal, then you the yellow gunk on the oil filler cap is NOT caused by a head gasket breach between an oil and

headgasket failure, yellow gunk on dipstick

Normal oil on dipstick and dipstick with oil and coolant

However, if the oil on your dipstick looks like normal golden brown or dark brown oil and it’s at the proper level, then you don’t have coolant leaking into the crankcase.

Next, shine a flashlight into the valve cover and examine the condition of the valve springs and rocker arms. If they appear clean, you do not have a sludge problem.

So what causes the yellow gunk?

Engines need a rich air/fuel mixture to start. When the air/fuel mixture ignites, some of the gas and exhaust seeps past the piston rings and enters the crankcase. This is called blow-by and every engine has some amount of blow-by, even new ones. The exhaust contains carbon monoxide, oxygen, soot, and water. So there’s fuel vapor, exhaust gases and oil vapor in the crankcase on all cold starts. As you drive and heat up the engine, the heat evaporates the water and fuel from the oil.

But if you take short trips where the engine doesn’t heat up enough to evaporate off all those vapors, they’ll all rise to the top of the engine when you shut it off. Since the oil filler cap is located at the top of the engine and is in contact with the cold outside air, the vapors condense inside the cap. So the yellow gunk is actually congealed combination of oil vapor, fuel vapor, water vapor, and exhaust gases.

Yellow gunk in oil cap is caused by short trip driving

As long as you’ve checked the dipstick as described above and the quick check of the inside of the valve cover comes up clean, you can eliminate an engine problem. The yellow gunk is normal and is caused by short trip driving. Your goal is to get the vehicle out for longer trips OR, change your oil more often. Short trips are very hard on oil because it can’t drive the water out of the oil.

©, 2018 Rick Muscoplat


Posted on by Rick Muscoplat

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