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Crankcase Ventilation

Internal combustion engine crankcases need to be ventilated to remove harmful blowby gasses and moisture. This reduces the formation of sludge and acids to help prolong engine life. Many early automobiles used a road draft tube system for crankcase ventilation. Forward motion of the vehicle through the air caused a suction on the road draft tube. This suction circulated air through the engine carrying the contaminated gasses out to the atmosphere. The slow speeds of an off road vehicle do not generate much air flow and therefore less crankcase ventilation.
Willys was one of the first manufacturers to use a closed crankcase ventilation system providing a positive flow contained system that does not require vehicle motion for operation. Contaminated gasses are not released to the atmosphere providing another benefit from this system. The following cutaway engine illustration from the service manual, with red highlights added, indicates the air flow.

A crankcase ventilation valve (5) is mounted on the intake manifold and connected by a tube (10) to the side valve cover (2).  Clean air is drawn into the oil filler tube through a hose from the air cleaner crossover pipe. Air flows down the oil filler tube to the engine crankcase. Any vapors in the crankcase are mixed with the air and carried out the tube (10) at the side cover (2), through the crankcase ventilation valve (5), and into the intake manifold where they are combined with the incoming combustion mixture and then burned in the cylinders.

The ventilation valve (5) regulates the system air flow dependent on intake manifold vacuum. High vacuum, such as found at idle, closes the valve reducing the flow volume. Under load, low manifold vacuum causes the valve to open for maximum air flow.
In the United States during the 1970's a closed ventilation system became mandatory for emission reasons. This closed system is now commonly called a PCV system, which stands for Positive Crankcase Ventilation.

In 1943 Delmar G. Roos, a Willys-Overland engineer, filed a patent for the crankcase ventilation system used on the Jeep. Click on the patent drawing for a larger view.

The crankcase ventilation valve is serviceable.
The outer housing is a two piece threaded assembly. Unscrew the housing half's to remove, clean and inspect the valve and spring.

A valve that is stuck closed can cause sludge buildup over time and possibly external engine oil leaks due to excessive crankcase pressure. If the valve sticks open the result is likely to be a poor and/or unstable idle.
Some military vehicles were equipped with underwater driving capabilities. The engine ventilation system was used to pressurize the crankcase and gearboxes to help keep water out. See the Siblings section for more information about Willys military vehicles.

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