The purpose of the CDR valve is to maintain 0-0.996 kPa (0-4 in of water) vacuum in the crankcase at all engine speeds, assuming that the piston/ring combustion blow by is not excessive (less than 4 cm). Too little vacuum will tend to force oil leaks.

The CDR valve is checked with a water manometer. The U-tube manometer indicates pressure or vacuum by the difference in the height of the two columns of fluid.

If the crankcase vacuum is more than 0.996 kPa (4 in of water), dirt or dust can be pulled into the crankcase cavity through the front/rear crankcase seal, etc. and contaminate the oil. If the crankcase pressure is positive, engine oil leaks are likely to occur around the engine seals and the gaskets.

1. Connect one end of the manometer to the engine oil dipstick hole. The other end of the manometer is vented to the atmosphere.
2. Unplug the rubber vent tube from the turbo inlet elbow.
3. Run the engine through a no-load speed range (gear selector in PARK) and observe the manometer readings. If the manometer reading is +0.996 kPa (+4 in of water) or less positive pressure, reconnect the CDR system and proceed to step 4. If the manometer reading is higher than +0.996 kPa (+4 in of water) positive pressure, this indicates that there is an internal engine problem.
4. Install the air cleaner.
5. Start the engine and observe the manometer (vacuum) reading. The manometer reading should be approximately 0.249 kPa (0-1 in of water) at the idle to approximately 0.747-1.245 kPa (3-5 in of water) at 2000 RPM. Add the amount that the manometer column travels up, to the amount that the column travels down, in order to obtain the total water pressure (vacuum).