What is the white milky stuff under my oil cap?

What is the white stuff under my oil cap??

You take the cap off of your valve cover to top off your oil or fill the crank case after an oil change and discover to your horror that there is some sort of white milky or creamy 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 sludge is moisture.  The real question is how did moisture get in your engine.  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 in the Oil System

This is more common than you might think.  A weather change from warm, moist weather to cold weather or repeated frost (condensation) 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 thanks to the heat of the engine.  However a vehicle that is not driven very far and does not reach full running temperature for very long may not purge out the moisture.  Vehicles that are not driven frequently and sit outside can also acquire moisture in the oil system.  When these vehicles are driven, the engine generates some heat during the short drive, then cools.  The trapped moisture condensates on the coolest part of the engine, the valve cover and oil cap.  Repeated short trips will leave behind more and more moisture on these cooler parts.  You then open the cap and discover this milky white buildup of creamy mocha colored slim.

Is this condensation harmful?

If this is indeed condensation caused from a weather change or infrequent use, then typically it is nothing to worry about.  The key is to check your dip stick and exhaust.  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.  If this is the case, see a mechanic to determine if you do have a more serious problem.  A coolant system pressure test can help to get answers.  However if you see no moisture on the dip stick and a clean, clear exhaust gas coming out of a warm engine, then it may be just be condensation on the metal surfaces of the crankcase and valve covers and oil cap.  Try to wipe it off the cap and out of the 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 off and purge this moisture. 

Other causes 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.  Use caution when cleaning an engine.  Use low pressure and be careful not to spray directly at your seals or inlets like the oil cap, power steering fluid cap, transmission dip stick and air intake.  Moisture in these areas can do more harm than good.  Dirt on the outside of an engine does far less harm than moisture in the engine.

Toyota FJ Cruiser