P.C.V. valves offer the advantages of reduced crankcase emission and increased fuel economy. However, if not maintained properly, they develop a number of malfunctions: starting problems, poor idling, and oil leaks. The cause is the development of sludge and contaminates in the oil filter. The author emphasizes that P.C.V. valves must be tested each time the vehicle is serviced to determine if the whole system is operating properly. The system must be thoroughly cleaned periodically to prevent the buildup of sludge. This conclusion is based on the author's experience with Post Office vehicles.