What is the yellow gunk under your oil cap

The yellow gunk under your oil cap does not automatically mean you have a bad head gasket

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

Yellow gunk under oil cap

NORMAL in winter if you drives are just short distances. By itself, it is NOT a sign of a head gasket failure.

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

Check the oil on dipstick. If it look like the brown or yellow gunk in the oil fill cap, then you can start to worry about

Normal oil on dipstick and dipstick with oil and coolant

a failed head gasket. However, if the oil on the dipstick is normal; 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 head gasket issue.

So what causes the yellow gunk?

Engines need a rich air/fuel mixture to start in cold weather. When the air/fuel mixture ignites, some of the gas and exhaust seeps past the piston rings and enters the crankcase. This is called crankcase blow-by and every engine, even brand new ones have blow-by.

Blow-by contains carbon monoxide, oxygen, unburned fuel, soot, and water. If you drive at least 5 miles in warm weather or 10 miles in cold weather, the engine heats up enough to evaporate off the water and fuel from the oil. That reduces the presence of water in the crankcase oil mist.

However, if you take short trips in cold weather, where the engine doesn’t heat up enough to evaporate off all the water you’ll get this yellow gunk in the oil fill cap.

Why is it found in the oil fill cap?

Simple. Hot air rises. Cold air comes in contact with the oil fill cap and the mixture condenses in the cap. 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. The yellow gunk is actually a 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.

You should not worry about gunk in the oil cap if there’s no matching color on the dipstick

A head gasket can fail in several ways;

1) A breach between the cylinder and a cooling passage can cause coolant to burn in the cylinder OR cause coolant to be forced into the oil pan. In that case the oil/coolant mixture looks like a chocolate milkshake. If you see that on the dipstick, stop driving and have it towed to a shop

2) A breach between a cylinder and oil passage can cause the engine to burn oil. You’ll notice that you’re always low on oil.

3) A breach between a cooling and oil passage will also cause a coolant/oil mixture. 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. In that case the oil/coolant mixture looks like a chocolate milkshake. If you see that on the dipstick, stop driving and have it towed to a shop

©, 2018 Rick Muscoplat