QUESTION - I have found a chocolate
brown emulsion on my engine's dipstick or the underside of the valve
cover. Do I have a cracked head?
ANSWER - It is uncommon for
a cracked head or blown head gasket to open a passage between the
engine oil and coolant. The common head crack failure mode creates
a passage between the water passage and a cylinder. Blown head gaskets
commonly create a passage between adjacent cylinders or between
a water passage and a cylinder.
It usually takes a crack in the engine block to mix coolant and
oil. When this happens you get lots of steam coming out all over.
It is very noticeable. Or at least it was the one time it happened
Your most likely cause:
Hot air is less dense than cold air. This means that when an engine
is at operating temperature there are less molecules of air inside
the engine than when the engine is cool. Hotter air can also carry
more water vapor molecules than cold air can.
As an engine cools, the air inside the engine block cools down
becoming denser. This creates a slight vacuum. Air is drawn into
the engine block to equalize the air pressure.
If the air sucked inside the engine is moist and the engine gets
cold overnight, a dew can deposit water from the air onto the inside
surfaces of the engine.
If you drive the car long enough for the engine to come up to full
operating temperature, the deposited water goes back to being a
vapor then gets "burned" out of the engine.
If you do not drive the car long enough to "burn" the water out
of the engine during periods of high humidity days and cool nights
you get a build up of moisture in the cooler parts of the engine
such as the valve cover and dip stick tube. This moisture combines
with the oil creating a chocolate brown emulsion.
A normal product of engine combustion is water vapor. A little
gets forced through the rings from the combustion chamber aggravating
Basically you are looking at a normal condition that occurs during
certain weather conditions, usually in cars that are driven short
distances. The additional moisture in the crank case will hasten
the formation of acids in the oil.
The appropriate response is to change the oil and filter a little
more frequently and wipe the emulsion build up off the dipstick
to keep it from rusting.
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