Initial Inspection and Diagnostic Overview

1. Isolate the area by probing the engine with a sounding bar (large screwdriver).

WARNING: The radiator 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 blockage. Blocked radiators generally occur on vehicles with higher mileage and not on newer 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 water 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 cylinder heads and check the block with a pen light flashlight. Never replace a block unless the restricted area can be seen.
8. Inspect the cylinder heads if the problem is not found. Cylinder 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 cylinder head cooling passages are intricate, and all the passages may not 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 cylinder head and inspect the replacement head before installation.