What Causes Moisture Build Up Under My Truck's Oil Cap?

by Robert Moore

You remove your engine oil fill cap to find the dreaded white sludge that has accumulated on the bottom of the cap. The first thought that comes to mind is that water is mixing with your engine oil to create this vile sludge, which is absolutely true. Determining the root cause of this mixture and repairing it is imperative if you wish to prevent expensive future repair bills. There is a variety of reasons for this buildup of sludge ranging from simple moisture build up to severe internal gasket or component failure.

Natural Moisture Buildup

Moisture naturally builds up inside an engine, especially in cooler areas and damper climates. Generally as the engine is put into service and the temperature rises, any built up moisture is burnt off by the heat the engine produces. Making short trips, on the other hand doesn't allow the engine to warm up completely, giving the opportunity for moisture to accumulate and mix with oil on the bottom of the oil cap. Moisture can also accumulate and create sludge on the cap if the seal on the cap is worn or damaged. The best practice is to clean the bottom of the cap with brake cleaner and always allow the engine to warm up when taking short trips.

Blown Head Gasket

If the vehicle has recently overheated or started running funny, the white sludge on your cap could be a preview of the oil traveling through the engine and sitting in the oil pan. A blown head gasket will allow coolant to leak into the oil passages and/or the combustion chamber. Coolant entering the oil system creates the milky substance almost instantly and is very damaging to internal engine bearings. If you don't repair a blown head gasket right away, you will be stuck with a rather large bill to perform a complete rebuild of your engine from the bottom up.

Cleaning Habits

Naturally anytime you spray and engine down with De-greaser and water it presents the opportunity for water and moisture to work its way into the engine. Using a high powered pressure sprayer can literally force water through engine seals and under the oil cap, power steering cap, air filter housing and dipsticks. The best practice for engine cleaning is to use a lower pressure spray and avoid engine sealing areas like valve cover to head mating surfaces and intake to head mating surfaces. Never spray the air filter housing or air intake tube, dirt on the outside of the engine will do far less harm than moisture that finds its way into the engine.

Determining The Cause

To determine whether the sludge is something to be concerned with or not there are several things you can check. Inspect the oil dipstick and examine the color and quality of the oil from the crankcase. If the oil appears normal, chances are you were subject to moisture and have nothing to worry about. If the milky substance exists on the oil dipstick you have a more serious problem and shouldn't drive the vehicle until it can be repaired. Check the coolant level and quality, if you appear to have been loosing coolant and you don't have any apparent leak, chances are you have a failed head gasket or intake manifold gasket. If coolant is leaking into the engine oil, the only way to find the direct source is to begin tearing down the engine and searching for the cause.

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