Home » GM 6.5 » GM/Detroit 6.5L – SYSTEM CHECKS


Check the drive belt for looseness, damage, or fabric delamination. If the belt is loose, check the belt tension­er for proper operation. The belt tensioner is not adjust­able. If it is not working properly, it must be replaced. If the belt is delaminated or damaged, replace it.

• Cracks across the rib material are a normal part of the belt aging process. Cracks do not affect belt operation, and do not indicate a need to replace it. Check the belt length scale on the tensioner. If it reads outside the “acceptable” range, check for proper routing as shown on the belt routing label and in this section. Also check for proper tracking of belt ribs in the pulley grooves, correct pulley sizes (including idlers), and correct belt length.

Multiple rib belts stretch and wear very little. Nor­ mal stretch and wear rarely exceeds 10 mm (3/8 inch) on the belt length scale.

To check for exhaust leaking into the cooling system, drain the system until the coolant level stands just above the top of the cylinder heads, then disconnect the upper radiator hose and remove the thermostat and belt. Start the engine and accelerate several times. At the same time, note any appreciable coolant rise or the appearance of bubbles that may indicate that exhaust gases are leaking into the cooling system.

NOTICE: A worn head g aske t m ay a llo wexhaust gases to leak into the cooling sys­tem. This can damage the cooling system as the gases combine with the water to form acids that are harm ful to the radiator and engine .

Check coolant pump operation by running the engine while squeezing the upper radiator hose. When the engine warms, a pressure surge should be felt. Check for a plugged vent hole in the pump.

Test for restrictions in the radiator by warming the engine up, turning the engine off, and feeling the radia­tor. The radiator should be hot along the left side and warm along the right side, with an even temperature rise from right to left. Cold spots in the radiator indicate clogged sections.

Make an operational check of the thermostat by hanging the thermostat on a hook in a 33 percent glycol solution, -128C (10T) below the temperature indicated on the valve. With the valve submerged and the coolant agitated, the valve should close.

Restrictions in the cooling system can cause engine overheating and/or cooling system noise.
Components prone to this condition are the cylinder head, coolant pump, block, thermostat housing, and inlet manifold.
Symptoms are:
• Engine may make snapping/cracking noises.
• Heater core may gurgle or surge.
• Radiator hoses may collapse and expand.
• Heater hoses may vibrate and thump.
• Overheat light may or may not come on.

Symptoms are caused by coolant boiling at some localized area and may be noticed after extended idling and/or while driving. Determine which side of the engine is involved and whether it is at the front or rear of the engine.

1. Isolate the area by probing the engine with a sounding bar (large screwdriver).
CAUTION: The radiator or diesel surge tank cap should be removed from a cool engine only. If the radiator cap is removed from a hot cooling system, serious personal injury may result.
2. With the radiator cap removed, observe the coolant being circulated in the radiator. Feel the front area of the radiator for cold spots which indicate block­ age. Blocked radiators generally occur on units that have accrued miles and not on new vehicles.
3. Inspect the thermostat to see if it opens.
4. Inspect the thermostat housing to make sure it is free of obstructions.
5. Remove the coolant pump from the vehicle and remove the back cover on the pump. Inspect all internal passages using a flashlight.
6. Inspect the crossover at the front of the inlet mani­fold. This entire passage can be seen only with the thermostat removed.
7. Remove the heads and check the block with a pen light flashlight. Never replace a block unless the restricted area can be seen.
8. Inspect the heads if the problem is not found.
Heads with blocked coolant passages generally have more than one area that is blocked. Look for signs of overheat discoloration (a dark blue or black area). If none are found, look in the coolant passages for blockage and probe all accessible passages. The head is intricate, and all the pas­sages cannot be reached. Use a substantial wire to go through or around a partially blocked area. If nothing is found by visual inspection and probing, inspect the passages for a rough, ragged appear­ance. The roughest internal passages are probably the ones that are blocked. Replace a blocked or suspect head and inspect the replacement head before installation.



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