White Stuff, under my oil filler cap – What is it ? Is it really bad ?

White Stuff, under my oil filler cap - What is it ? Is it really bad ?
White Stuff, under my oil filler cap - What is it ? Is it really bad ?

So, you take the oil cap off of your valve cover, to top off your oil.

Then, you find milky creamy white stuff, coating the underside of the oil cap and down into the filler hole.

So, what is this white stuff and what is it doing in my oil system? In most cases, the white stuff is moisture.

The real question is, how did moisture, causing white stuff, get in your engine. Actually, it is something which is very common. Determining how the moisture got into the engine; will tell you whether this is nothing to worry about or if it’s something more serious. Causes can range from, condensation from weather changes, to a blown head gasket.

Condensation Causing White Stuff
Condensation Causing White Stuff

Sometimes you might just end up, topping up your engine oil between oil changes. It is during such times, that you might notice that your oil cap has a milky, creamy white stuff.

Condensation In The Oil – Finding, White Stuff, Under Oil Filler Cap

This Is More Common, Than You Might Think

A weather change from, warm moist weather, to cold weather or; repeated frost on the vehicle and frequent high dew points, can create moisture to form in the crankcase.

This can appear as condensation in the oil system and under the valve covers. Most of the time, vehicles are driven long and far enough, so that this condensation is burned off.

Therefore, a vehicle that is not driven very far; does not reach full running temperature, to purge out the moisture. Vehicles that are not driven frequently and sit outside, can also acquire moisture in the oil.

White Stuff Under Valve Cover
White Stuff Under Valve Cover

When these vehicles are driven; the engine generates some heat during the short drive, then cools. Then, the trapped moisture, condensates on the coolest part of the engine; the valve cover and oil cap. Therefore, repeated short trips will leave behind; more and more moisture on these cooler parts.

Is This Condensation Harmful ?

If this is indeed condensation, then it is nothing to worry about. The key is to check your dipstick and exhaust. So, if you see beads of moisture on the dipstick and; white smoke coming out of the exhaust of a warm engine; this can indicate a head gasket leaking coolant into the oil system, which is not good.

Consequently, a coolant system pressure test, can help to get answers. However, if you see no moisture on the dipstick and a clean; clear exhaust gas coming out of a warm engine; then it may be just be condensation.

Try to wipe it off the cap and filler tube and check it again in a few days. If it is condensation; it is generally minimal moisture in the system and heat, can help burn it off.

Another Cause Of Moisture In The Engine

Cleaning an engine with a high pressure spray, is a good way to force moisture into the seals; under the oil cap and into places, where moisture shouldn’t be. As a result, always use caution, when cleaning an engine.
Washing Car Engine
Washing Car Engine
Furthermore, use low pressure and be careful, not to spray directly at your seals or inlets like:
  • The oil cap
  • Power steering fluid cap
  • Transmission dipstick
  • The air intake

Consequently, moisture in these areas, can do more harm than good. Also, dirt on the outside of an engine, does far less harm, than moisture in the engine.


To determine whether the white stuff, is something to be concerned with or not; there are several things you can check:

  • Inspect the oil dipstick and examine the color and quality of the oil, from the crankcase.
  • If the oil appears normal; chances are you were subject to moisture and have nothing to worry about.
  • But, if the milky substance exists on the oil dipstick, you may have a more serious problem.

Check the coolant level and quality. If you appear to have been loosing coolant and you don’t have any apparent leak; chances are you have a failed head gasket or intake manifold gasket.

Oil Filler Cap Leaking Causing White Stuff
Oil Filler Cap Leaking

It could also be that your oil cap seal is either, damaged or worn out. In case there is a break in the cap’s seal integrity; then it is possible, that the moisture, will be able to enter into your engine.

Thank You !