GM/Detroit 6.5L ENGINE CONTROLS – Glow Plug System Check

Refer to Glow Plug Controls.

Circuit Description
The glow plug system is used to assist in providing the heat required to begin combustion during engine starting at cold ambient temperatures. The glow plugs are heated before and during cranking, as well as during the engine operation. The PCM controls the glow plug ON times by monitoring coolant temperatures and glow plug voltage. This system check will check the glow plugs and the glow plug feed circuit coming from the relay.

Diagnostic Aids
If the glow plug relay is stuck in the ON position, check for proper operation of the glow plugs. When the glow plugs are commanded ON by the scan tool, an internal PCM timer protects the glow plugs from damage by cycling them ON for 3 seconds and then OFF for 12 seconds. Most glow plug system failures are covered by DTC P0380. If no DTCs are stored, the vehicle is hard to start and white smoke is present during cranking or after the vehicle is started, the most likely cause of failure is the glow plugs.

Test Description
Number(s) below refer to the number(s) on the diagnostic table.
1. This step will make sure OBD system check is performed.
2. This step will make sure there are no other DTCs stored that will affect the operation of the glow plug system.
3. This step will check each glow plug for an open.
4. This step will check each glow plug feed circuit for an open.

CDR Valve Test
The purpose of the CDR valve is to maintain 0 to 4 inches of water vacuum in the crankcase at all engine speeds, assuming that piston/ring combustion blow by is not excessive (less than 4 cfm). 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 too high (greater than 4 in. water), dirt or dust can more likely be pulled into
the crankcase cavity (pulled through front/rear crankcase seal, etc…) and contaminate the oil. If the crankcase pressure is positive, engine oil leaks are more likely to occur around engine seals and gaskets.
1. Connect one end of the manometer to the engine oil dipstick hole. The other end of the manometer is vented to atmosphere.
2. Unplug rubber vent tube from turbo inlet elbow.
3. Run engine through no-load speed range (gear selector in park) and observe manometer readings. If manometer reading is +4 in. water or less positive pressure, reconnect CDR system and proceed to Step 4. If manometer reading is higher than +4 in. water positive pressure.

4. Install air cleaner.
5. Start, engine and’observe, manometer reading. It should read one inch (0 in. -1 in.) of water (vacuum) at idle to approximately 3-4 inches of water (vacuum) at 2000 RPM. Add the amount that the manometer column travels up, to the amount that the column travels down to obtain total water pressure (vacuum). An example of a manometer reading is as follows: One-half inch above zero plus one-half inch below zero equals one inch vacuum reading (1/2 in.+1/2 in. =1 in.).

Air Cleaner Diagnosis
A restricted or leaking air intake system could cause loss of power and engine damage.
• Inspect the air cleaner filter for damage or excessive dirt accumulation. Replace if necessary.
• Inspect the air inlet elbow and CDR tube for damage or cracks. Replace if necessary. Refer to Maintenance and Lubrication of the appropriate service manual for change intervals. Operation of the vehicle in dusty areas will necessitate more frequent replacement.



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