If a unit has recently experienced overheating while generating power, and the fault occurred suddenly, the problem may be with the thermostat. However, after inspection, the thermostat appears to be functioning properly. What are the possible causes of engine overheating?

A very common cause of engine overheating is a faulty radiator pressure cap. The radiator pressure cap is designed to maintain the cooling system pressure at 151 psi. When the cooling system pressure increases, the boiling point of the coolant also increases. If the cooling system pressure decreases (due to a faulty radiator pressure cap), the boiling point also decreases.

Radiator Cap Testing Method

Use a pressure cap and cooling system tester to inspect the pressure cap. If the radiator pressure cap cannot maintain the pressure of the cooling system, the tester will emit a sound.

The pressure cap also protects the hoses from expanding and collapsing. When the engine is turned off, the coolant begins to cool. As the coolant cools, its volume decreases. Eventually, a vacuum forms in the cooling system. This means that the pressure outside the radiator is higher than the pressure inside. The pressure difference on both sides causes the hoses to collapse. Continuous expansion and collapse of the hoses can cause them to rupture and eventually leak.

The pressure cap has a vacuum valve that allows atmospheric pressure to enter the cooling system when there is a slight vacuum. During operation, a small spring connected to the expansion tank keeps the vacuum valve closed. When a vacuum forms in the cooling system, the vacuum valve is pulled downward and opens. This reduces the vacuum pressure in the cooling system. When the vacuum valve opens, it can also draw coolant from the overflow tank. Therefore, as the coolant in the engine continues to cool and its volume continues to decrease, it draws in additional coolant rather than air. The increased pressure also reduces cavitation. Cavitation is caused by small bubbles formed by the rotation of the water pump. Increased pressure reduces this phenomenon. The pressure in the cooling system also reduces surges, which are rapid impacts of water from the water pump. This can be caused by a sudden increase in engine speed. Surges can create bubbles and agitate the coolant. The pressure in the cooling system reduces this phenomenon.

Radiator Cap Failure

Failure Symptoms: The engine has overheated. This failure occurs very quickly.

Diagnostic Method: If the engine has overheated and this failure occurs rapidly, the radiator cap may be faulty. If the radiator cap cannot maintain the pressure in the cooling system, the boiling point will decrease, potentially causing the engine to overheat.

Repair Method: Follow the procedure below to inspect the radiator cap. Ensure that the cooling system is at room temperature before beginning work.  

(1) Remove the radiator cap.  

(2) Use the pump on the pressure tester to increase the pressure on the cap.  

(3) Ensure that the pressure cap releases pressure when the pressure reaches the pressure setting displayed on the cap.

(4) If the cap begins to release pressure at a pressure lower than this point, the cap is faulty and should be replaced.

(5) Note the pressure at which the cap begins to open. As pressure is increased with the pump, the pressure will eventually reach a point where it cannot be increased further.

(6) Place the cap on the pressure tester. As with the radiator, turn the cap to seal it on the pressure tester.

(7) Also inspect the pressure cap's vacuum valve. One method to check the valve is to examine the hose on the radiator. If the hose on the radiator collapses after the engine cools down, the vacuum valve may be faulty or blocked. If the valve is blocked or damaged, it cannot balance the pressure in the cooling system during cooling. If this occurs, replace or clean the vacuum valve to resolve the issue.