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
A nice long trip - like NY to Arizona - will quickly eliminate the deposit.
"Probably a combination of water vapor and oil - from driving short
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.
1987 Mercedes-Benz 300E
1974 Porsche 911S
1962 VW Convertible
1960 Austin-Healey 3000
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