The Crankcase Depression Regulator (CDR), found on 1981 and later diesels, and the flow control valve, used in 1980, are designed to scavenge crankcase vapors in basically the same manner as the PCV valve on gasoline engines. The valves are located either on the left rear corner of the intake manifold (CDR), or on the rear of the intake crossover pipe (flow control valve). On each system there are two ventilation filters, one per valve cover.
The filter assemblies should be cleaned every 15,000 miles by simply prying them carefully from the valve covers (be aware of the grommets underneath), and washing them out in solvent. The ventilation pipes and tubes should also be cleaned. Both the CDR and flow control valves should also be cleaned every 30,000 miles (the cover can be removed from the CDR; the flow control valve can simply be flushed with solvent). Dry each valve, filter, and hose with compressed air before installation.
Do not attempt to test the crankcase con