Why does my oil filler cap have a creamy substance on it?

A creamy, mayonnaise-like liquid on the oil filler cap of a vehicle can be a sign that coolant has mixed with the oil, which is a major sign of a blown head gasket.

How does the creamy liquid relate to a blown head gasket?

The head gasket is a component within the engine which acts as a seal between the oil circulatory channels and the coolant circulatory channels, keeping the two systems separate. When damage occurs within the head gasket, the seal is broken and water can enter the oil channels.

When engine oil and water mix, they coagulate into a creamy white substance which is often discoverable underneath the oil filler cap, and is a telltale sign of a blown head gasket.

Why should I fix a blown head gasket?

As the head gasket is the main seal dividing the oil and coolant channels, the primary reason for the presence of water in the engine oil is a damaged head gasket, which should be fixed as soon as possible for a multitude of reasons you can read about here [link to Should I Fix My Blown Head Gasket?]

Coolant loss

Coolant moving from the cooling system to the oil circulation system will inevitably deplete the amount of coolant inside the cooling system. A vehicle needs a certain amount of coolant to regulate the internal running temperatures of the engine and it can be difficult and costly to maintain this when the coolant is being pushed into the oil circulatory system instead of around the cooling system.


Without as much or even any coolant to cool the engine and maintain a consistent, healthy operating temperature, the engine is likely to overheat. This is dangerous and can cause further damage to its components.

Further engine damage

An overheating engine is a fire hazard and an enemy to itself, with excessive temperatures capable of warping, seizing and breaking other internal components. For example, it is common when replacing a blown head gasket to skim the cylinder head too as it sustains damage when operating without a healthy head gasket to work with.

My oil filler cap has a creamy substance on it: What should I do?

If you have noticed a creamy white liquid or substance sitting on the underside of your oil filler cap, take your vehicle to a mechanic to diagnose the problem, and research your repair options.

How much does it cost to fix a blown head gasket?