See Figures 1 and 2
The Crankcase Depression Regulator (CDR), found on 1981 and later diesels, and the flow control valve, used from 1978 to 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 filters, one per valve cover. The filter assemblies should be cleaned every 15,000 miles by simply prying them carefully from the valve covers (without damaging the grommets underneath), and washing them 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 controls on these diesels. Instead, clean the valve cover filter assembly and vent pipes and check the vent pipes.
Replace the breather cap and assembly every 30,000 miles. Replace all rubber fittings as required every 15,000 miles.