Rick's Free Auto Repair Advice

Yellow gunk under oil cap

What is the yellow gunk under oil cap?

Should you be worried if you find yellow gunk under oil cap?

There’s a pretty common misconception in

Yellow gunk under oil cap

Yellow gunk under oil cap

the car world that yellow gunk under oil cap always means you’ve got serious engine problems like a blown head gasket or sludge buildup. Yellow gunk under the cap CAN mean that if you have other corroborating signs. But yellow gunk under oil cap, by itself, is NOT an indication of a blown head gasket or sludge.

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

First, check the dipstick for signs of a blown head gasket. 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.

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 gets past the piston rings and enters the crankcase. This is called blowby and every engine has some amount of blowby, even new ones. The exhaust contains carbon monoxide, oxygen and water. When the water gets whipped into the crankcase oil, it turns into a pudding-like consistency. As you drive and heat up the engine, the heat evaporates the water from the oil.

But if you take short trips where the engine doesn’t heat up to normal operating temperatures, the heated water vapor and oil vapor rises to the top of the engine. Since the oil filler cap is at the top of the cylinder head, the oil/water settles in the oil cap, forming a yellow gunk.

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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