• Dirty, clogged radiator. You'll be amazed at the grime that collects between the fins on the radiator. Because the radiator (which is a heat exchanger) has a large surface area, this can considerably reduce airflow and efficiency. Clean the radiator thoroughly with kitchen soap and water, or use a commercial grade degreaser if necessary. Do not use a high pressure water source such as a pressure washer, as this can actually bend the fins and restrict airflow. A low pressure water source, such as a garden hose, is recommended when cleaning radiators.
• Radiator plugged internally. If the radiator has not been maintained properly, it may be clogged internally. It is common for the 6.2L/6.5L engines to experience debris build up between the AC condenser and radiator, so check this first before replacing the radiator. Routine coolant flushes greatly reduce the risk of particle/debris development in the radiator. Using a coolant that includes a corrosion inhibiting additive will also reduce the risk of debris forming in the radiator.
• Stuck thermostat. The thermostat is a mechanical device that controls coolant flow through the engine. It is designed to open when the coolant reaches a predetermined temperature. If the thermostat has corrosion or build up on it, replace it. The part is inexpensive enough that it's worth replacing at the first signs of an engine running hotter than normal. They can be tested by placing in a pot of water and bringing it to a boil; if the thermostat does not open, it needs to be replaced. A more efficient thermostat design was introduced mid-year for 1996. Make sure the thermostat is installed correctly. If installed backwards, it will not function as intended.
• Worn fan clutch. The fan clutch is used to provide airflow through the radiator when the vehicle is stopped or not enough airflow is produced from driving. A faulty fan clutch can lead to overheating issues, especially in low speed, high load driving conditions.
• Faulty water pump. The water pump design on these engines is not particularly impressive. An updated water pump was released for 1996. It is often the culprit behind overheating issues.
• Blown or leaking head gaskets. Coolant loss will be present, engine performance may suffer, and engine may overheat.
If you're having reoccurring overheating issues with your 6.2L or 6.5L GM diesel, look into replacing parts with improved versions; the aftermarket has responded to many common problems by designing higher efficiency components.