The thermostat of a vehicle’s engine manages the flow of coolant. It only allows the coolant to flow when the engine heats. The main valve of a thermostat stays shut if the engine is cool or idling. 

For some reason, if the thermostat can’t regulate the coolant flow, it can result in engine heating. This happens when the thermostat fails to open the valves, preventing the engine radiator from receiving the coolant. 

The coolant flows through the radiator after absorbing heat from the engine. The radiator’s cooling fan brings down its temperature by allowing the coolant to transfer the heat to the atmosphere. 

In case the cooling fan is not functioning at an optimum level, it reduces the cooling efficiency of the coolant. This problem may occur due to a mechanical failure in the electric motor that runs the radiator fan. 

Engine oil in your Hummer provides lubrication and helps to reduce excess engine heat as well. If your Hummer is running low on engine oil, it can also cause excessive engine heating. 

Almost all vehicles contain a coolant expansion tank these days. It regulates the entire cooling system’s pressure. But if the expansion tank is unable to release the pressure from the radiator, it can cause overheating in the engine.