LO 11-6 Identify and describe the construction features, types, and applications of the diesel engine cooling components

1.3. Radiator Caps

Radiator Caps
While early radiator caps simply prevented coolant from spilling out of the cooling system, today’s radiator caps have several additional functions. First, a calibrated pressure valve inside the cap increases the pressure of the cooling system to raise the boiling point temperature of coolant. This feature prevents coolant loss through boil over Figure 42. The pressure valve also allows the coolant to boil over if the temperature of hot coolant exceeds the cap’s pressure rating. Hot coolant escaping as superheated steam will carry heat away, leaving cooler liquid coolant behind in the radiator. To prevent damage to the heater core from excessively pressurized coolant, which can sometimes be pressurized by chassis air, some heater core circuits use a pressure bypass valve. The pressure-regulated valve blocks high-pressure coolant from reaching the core and instead creates a bypass circuit to return coolant back to the engine before it reaches the core Figure 43.

A	B		FIGURE 11-43: Chassis-pressurized cooling systems require a heater core bypass valve to prevent damage to the heater core from excessive cooling system pressures.
FIGURE 11-42: A. The pressure rating of a radiator cap, the point at which the pressure relief valve opens, is stamped on the cap. Radiator pressure cap rating: 7 psi (48 kPa). B. Radiator pressure cap rating: 15 psi (103 kPa).

A second function of the radiator cap is to allow air or coolant to re-enter the cooling system after the engine cools. Another valve in the radiator cap that enables coolant return is the vacuum valve. Coolant volume in the engine contracts when the coolant temperature is lowered, resulting in a pressure drop Figure 44. This allows the vacuum valve to open and draw coolant from the overflow tank back into the engine. If the system has only a surge tank but no additional remote-mounted overflow tank, only air is drawn back into the cooling system through the vacuum valve to prevent negative system pressure from forming. The radiator cap is also important to water pump efficiency. When using the radiator cap to pressurize the cooling system, the pump will operate over the maximum 85% efficiency that is achievable without the cap. Finally, pressurizing the cooling system minimizes cavitation of the entire cooling system because vapor bubbles caused by low pressure are less likely to form.

Faulty vacuum valves are often identified when the upper radiator hose collapses after the engine cools or when coolant is lost Figure 45. If the valve is faulty, coolant will either not be allowed into the system and remain in the surge tank or negative pressure will be applied to the entire cooling system. A coiled spring inside the lower radiator hose, or a solid metal tube, is required to prevent its collapse when the engine is running.

FIGURE 11-44: A. Operation of the pressure-relief function. B. Vacuum valve operation of a radiator cap.

FIGURE 11-45: The upper radiator hose of this engine is collapsed and blocks coolant flow. The collapsed hose is likely due to defective operation of the radiator cap’s vacuum valve.