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 unusual for a cracked head of blown head gasket to open a passage between the engine oil and coolant. It usually takes a crack in the engine block. 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 to me.

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 where the oil resides 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 vacume. 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 into the inside surfaces of the engine.

If you drive the car long enough for the engine to come up to full operating temperature the water vapor 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 when the nights get cool you get a build up of moisture in the cooler parts of the engine such as the valve cover and dip stick tube. This combines with the oil to create 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 the problem.

The additional water in the crank case will hasten the formation of acids in the oil and you should change the oil and filter a little more often. It will not cause immediate problems.

Basically you are looking at a normal condition that occurs during certain weather conditions usually in cars that are driven short distances. The appropriate response is to change the oil a little more frequently and wipe the emulsion build up off the dipstick to keep it from rusting.

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