CDR Valve Basics
In more than one manner, the CDR valve is the diesel equivalent of a gasoline engine's positive crankcase ventilation system. On a 6.5L GM diesel, the CDR valve is located on the passenger side valve cover and extracts fumes through the valve cover and into the compressor (intake) side of the turbocharger. The CDR valve is necessary to control negative pressure in the crankcase. In other words, a simple tube running from the crankcase to the turbocharger risks creating an unfavorably high vacuum in the crankcase. Additionally, a check valve in the crankcase ventilation hose is necessary to maintain a unidirectional flow. The CDR valve serves both these purposes.
The need for the CDR valve and/or a positive crankcase ventilation system altogether is often questioned. While older vehicles (gas or diesel) vented fumes into the atmosphere, all modern vehicles recycle blow-by fumes in the crankcase to eliminate pollution by these fumes and it is therefore technically considered a component of the emissions system. However, the function of the crankcase ventilation system also produces favorable results with regard to oil contamination and general engine operation. Venting crankcase fumes to the atmosphere is therefore ill-advised for a combination of these factors.
All 6.5L GM diesels feature a CDR valve and the OEM service recommendation is to replace the valve, grommet, and hose at 60,000 miles intervals. It is relatively common to replace a CDR valve much sooner as they have a tendency to "gum up". Excessive engine oil pooled in the intake or bottom of the turbocharger compressor housing is a reliable indication that the CDR requires replacement. When replacing the CDR valve, thoroughly inspect the valve cover grommet and supply hose; replace as necessary.
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• The CDR valve is located on the passenger side valve cover. The outlet hose of the CDR valve connects to the inlet pipe (air intake) for the turbocharger compressor.
• Locate the single bolt that secures the metal CDR valve tube to the the bracket on the turbocharger; remove it with a 10 mm socket.
• Disconnect the rubber CDR valve elbow at the intake tube. There is no need to loosen the hose clamp, the nipple at the end of the hose will disconnect from the intake tube with slight pressure.
• The CDR valve and hose assembly can be removed together by simply pulling the CDR valve straight upwards from the valve cover, or you may choose to remove the tube from the valve (hose clamp at the CDR valve) and then the CDR valve by itself.
• Inspect the grommet/seal that protrudes through the valve cover. It can typically be reused so long as it is not cracked or deteriorating.
• Remove the rubber elbow and hose coupler from the metal CDR tube, then clean the interior of the tube thoroughly with a solvent. Clean the coupler (which is being reused) with a clean, lint free rag (do NOT use solvent on the rubber hose).
• Reinstall the hose coupler and NEW hose elbow to the metal CDR valve tube, then install the CDR valve in reverse order. Snug all hose clamps once the CDR valve is installed and the retaining screw is tightened down.