The crankcase ventilation system is used on diesel engines and is designed to maintain a slightly negative (vacuum) crankcase pressure across the speed range. The system consists of a Crankcase Depression Regulator (CDR) valve, located on the right valve cover and to the attaching vent hose/pipes to the engine inlet system. The CDR valve is used only to regulate the crankcase pressure between 0 and 4 feet of water depression over the engine speed range. The CDR valve is Not an oil separator or a crankcase effluent flow regulator. Hence, the CDR valve Does Not prevent the oil droplets/mist from entering the intake system, nor does the CDR valve effect the engine oil consumption.

The intake manifold vacuum acts against a spring loaded diaphragm in order to control the flow of the crankcase gases. The higher intake vacuum (or high intake restriction, e.g. plugged air filter) levels pull the diaphragm closer to the top of the outlet tube. This reduces the vacuum level from getting too high in the crankcase. As the intake vacuum decreases, the spring pushes the diaphragm away from the top of the outlet tube in order to prevent the crankcase pressure from going positive.