Coolant Caps

Don’t Just Replace the Thermostat

A slowly failing, worn out or faulty radiator cap can result in overheating, loss of coolant or even major engine damage.

One function of the radiator cap is to keep pressure on the cooling system. If the pressure on the system drops because of a failing radiator cap, the boiling point of the coolant also drops. If the boiling point drops enough, it can cause the engine to overheat.

If you replace a thermostat and the radiator cap is more than three years old, chances are is not holding to the same pressure as a new cap. Explain to your customer that by replacing the thermostat and the radiator cap, their vehicle should run within the specified operating temperature and the coolant system will be properly pressurized.

The radiator pressure cap is a very important component of the system. The cap monitors the pressure of the system and adjusts to maintain the desired pressure (normally in the 15 psi range). If an improper cap or a defective cap is used, the correct system pressure will not be maintained. If the system pressure is too low the coolant will be allowed to boil at unacceptable temperatures. If the system pressure is too high, damage to the system can occur, potentially causing loss of coolant and an engine overheating condition.

A system with the correct coolant ratio and proper pressure will inhibit a fluid boil. If fluid boils within the system, severe damage to the engine and components of the cooling system can occur in the form of cavitation and cavitation erosion. Long term effects of this phenomenon can be detrimental to the water pump seal, impeller, and pump casting along with other engine/system components in direct contact with the coolant leading to catastrophic failure.

In extreme operating conditions or on days with high ambient temperatures, the system pressure may reach pressures above design or desired levels. If the system pressure begins to exceed to maximum acceptable pressure, the pressure cap will release pressure to maintain the design system pressure. The reserve tank functions as a reservoir to capture coolant discharge during a release of pressure from the radiator pressure cap. This prevents the discharge coolant from entering the environment and captures it for replenishment of the system. As the fluid temperature and pressure in the system decreases a vacuum occurs that draws the previously discharged fluid volume in the Reserve Tank back into the system.