So, sometimes when you open your oil tank to check the engine oil level or refill the oil, you may notice some creamy sludgy stuff under the oil cap. Sludge buildup under the oil cap can occur for a variety of reasons. 

What is the Sludge on Oil Cap?

An oil cap prevents dirt and debris from entering the oil tank while also preventing oil from splashing out of the engine. The tank’s oil, water, and moisture accumulation can cause milky sludge to form beneath the oil cap. 

Below, we’ll review possible causes of sludge accumulating under your oil cap.

Possible Causes of Sludge on the Oil Cap

Damaged Head Gasket

One of the causes of sludge buildup under the oil cap is a blown head gasket. The head gasket is in charge of sealing your car’s internal combustion engine so it can run with proper power. 

Head gaskets can be damaged for various reasons, including a breach between the oil passage and the cylinder or between the oil and cooling passages. A damaged head gasket can cause coolant to mix with the oil, resulting in sludge accumulation under the oil cap. It can even result in your car losing excessive oil.

Sludge accumulation under the oil cap does not always indicate that the head gasket has blown. Moisture in the engine could be to blame for the sludge buildup. Take a dipstick in the oil tank to see if the head gasket is damaged. Coolant has likely leaked into the engine oil if you see milky sludge on the dipstick. In this case, your head gasket is most likely damaged and needs to be repaired.

Moisture in the Tank

This is the most common cause of sludge accumulation under the oil cap. If you live in a cold climate, there is a chance that moisture will accumulate in your oil tank. This is because air vents allow air to enter the tank, which causes water vapors to form at the inside surface. This moisture will cause condensation in your oil tank, causing rust and water to accumulate and form sludge slowly.

Heat will remove moisture from your tank if you live in hot areas or drive for long distances. However, if you only drive a short distance. Oil is likely to mix with this vapor and form a sludge-like substance.

This is different than engine oil freezing which people are worried about living in cold climates.

Bad Cleaning Habits

Water can get into your engine parts when you use high-pressure pipes to clean your car. This can cause water to mix with oil, forming a milky, frothy substance under your car’s oil cap. This can also happen if your engine is not sealed correctly, as any gap in your engine can allow water to enter.

How to Fix Sludge on Oil Cap

  1. Take long drives in your car at least once in a while. This is due to the engine heating up and removing any moisture in the engine. You can also start the engine and let it idle for a few minutes to warm it up. You should do this more frequently if you live in a colder climate.
  2. Drain the old engine oil after opening the oil cap. Refrain from opening the cap when the engine is hot. Refill the tank using high-quality engine oil. This is because if you use cheap or local oil, its additives may cause moisture in your oil tank. As a result, sludge accumulated beneath the oil cap.
  3. Clean the car properly. If you’re using high-pressure pipes, close your car’s hood. If you suspect water is entering the oil tank, have a mechanic inspect the engine to see if it is properly sealed.
  4. Get your engine’s head gasket fixed if you notice coolant leaking into the engine oil. Repairing or replacing the head gasket is costly, but doing so is the only option.

FAQ’s on Sludge on Oil Cap

No, sludge buildup under the oil cap does not always indicate that the head gasket is damaged. Moisture might be mixed with engine oil for a variety of other reasons. The head gasket is most likely damaged if you notice coolant leaks, white smoke from the tailpipe, or overheating issues.

It is not recommended to drive the car if there is sludge buildup under the oil cap until the cause is determined. If moisture enters the oil tank, it will most likely disappear as you drive.

However, if coolant leaks into the engine oil, your engine may overheat and suffer from poor lubrication.


To summarize, sludge buildup under the oil cap is normal. It is essentially an emulsion of oil and moisture that occurs more frequently during the winter. It is therefore recommended that you drive your car for at least 5-10 minutes every day to remove any water vapors that may be present in the engine. 

But If you notice coolant leaking into the oil, the head gasket may have been damaged. Ensure that a mechanic repairs it.

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