Tan Colored Oil Residue on Oil Filler Cap

Probably a combination of water vapor and oil - from driving short local trips that don't allow the engine to come up to its normal operating temperature - OR - a thermostat that's stuck open and so creates the same effect.

A nice long trip - like NY to Arizona - will quickly eliminate the deposit.

... or a bad head gasket .

"Probably a combination of water vapor and oil - from driving short local

A friend of mine just bought a pre-owned BMW X5 from a dealer in NYC. When checking the oil, which was a quart and a half low, I noticed the same thing on the bottom of the oil fill cap. Also, the bottom of the dip stick had some reddish looking crud on it that I had never seen before.

I came to the conclusion that the brownish stuff on the fill cap was likely from driving short trips in NYC by the previous owner. It's still remarkable that a dealer would sell a certified car with 36k miles on it and not only not change filthy oil, but send it out a quart and a half low.

The comments on the thermostat got me thinking. I've noticed it takes quite awhile to get heat of this beast. Maybe the thermostat is out of whack too? But one would think this would generate an error message from the computer. On the other hand, since its not critical, maybe it just logs it and the dealer service guys ignore that too!

Some oils have various polymers added to "boil" off and attach themselves to the various nasties that appear in the oil - carbon, metals, etc. Usually these are blown out through the breather back into the inlet and re burnt in the engine. If the engine is not getting hot enough (short trips) the gunk collects in various places inside the engine - filler cap, valve covers - as the vapour does not get out through the breather.

One of the reasons that modern engines "use oil" is that the oil is designed to remove the impurities, but you have to top up every few thousand kms. I've had vehicles that are driven hard and in high temperatures that will have gone through enough top up oil to have had a complete oil change between "real oil changes".

The detergent/dispersant will hold the water in suspension creating a milky white mayonaise like substance on the bottom of the oil filler cap when the oil isn't getting warmed up enough.

When the engine is hot, the oil will eventually pass through the piston rings, the hottest part of the engine's lubrication system, and evaporate the moisture out of the oil.

A blown head gasket which lets enough water get in the oil will eventually turn the oil milky white.

Try to drive the car at leat 10 miles once a week in the cold winter months to get the oil warmed up.

I first noticed the white crud on my Porsche 911 engine's oil cap during the winter after driving only 7/10 mile from home to work week after week . . . with a 14 quart dry sump system. Obviously the oil was never even starting to get warm.

Pete Cowper