Auto News

“Butter” under the oil filler cap – what is it?

We understand what kind of emulsion and plaque is formed under the oil filler cap.

Recently, a frightened reader of the auto magazine Autokult turned to the editors. He said that some strange substance had appeared under the oil filler cap of his car, and asked to explain what it was. It was a yellow mass, similar to butter (Check, by the way, if your car has anything like that). Let’s figure out what it could be.

The reader sent a photo of the oil filler neck of the engine of his car to the editorial office of the magazine. It was there that he saw a yellow mass resembling fat in consistency. The motorist found the same plaque under the cork, and even on the oil dipstick. An incomprehensible emulsion was discovered by the car owner when checking and filling oil.

“Does this mean damage to the cylinder head gasket of the engine (cylinder head)?” the owner asked anxiously. Before the editors of the auto magazine came to a conclusion, they conducted a short interview with the motorist. It turned out that he drives his car mainly to work, and this distance is less than 10 km. As a rule, the engine warms up, but not completely, and we are not talking about a warm interior in winter. His way from home to work is so fast that the trip takes only a few minutes.

This was enough for the experts of the Polish magazine to make a preliminary diagnosis – the “patient” does not warm up properly, so a large amount of water accumulates in the oil. This is a natural process of water condensation on the metal parts of the engine. Water mixed with oil forms a “slurry”, which some call butter or emulsion, while others call mayonnaise or plaque.

By consistency, this formation resembles both products: butter and mayonnaise. Unfortunately, this is not a harmless sign. As a result, the formation of such a “butter” under the oil filler cap indicates premature engine wear due to insufficient warming up during vehicle operation. 

What should I do with the emulsion under the oil filler cap?

You definitely don’t have to panic. Before believing the mechanic’s statement that your car’s engine needs to be repaired immediately, it’s best to do what the experts in a Polish car magazine advised their reader. 

They asked him to drive a long distance so that the trip would last 30 minutes and the engine would warm up well. The idea was that instead of using low engine speeds, as is usually the case in everyday life, the car owner should have used medium engine speeds during the trip, that is, shifting gears later than usual.

After such a test, the motorist told the editors that most of the “butter” had disappeared. The experts of the auto publication claim that the reason for the formation of an emulsion that looks like butter is in the underheating of the engine, and not in the failure of the cylinder head gasket.

However, if you find this in your car, we at would still recommend that you change the engine oil as soon as possible since water does not have a positive effect on protecting the engine from friction.

How to prevent the formation of an emulsion?

Most often, this happens in the period from autumn to spring at low temperatures. Especially in the morning when starting the engine. In summer, such a symptom should be treated with more caution, because the “emulsion” in engine oil rarely occurs at higher positive temperatures.

If you find something that looks like “butter” under the engine oil filler cap, we would advise you to change your route or driving style at least once a week if your trips are in


Leave a Comment