Automotive Systems

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Radiator Pressure Cap

Radiator Pressure Cap

The radiator pressure cap (fig. 6-6) is used on nearly all of the modern engines. The radiator cap locks onto the radiator tank filler neck Rubber or metal seals make the cap-to-neck joint airtight. The functions of the pressure cap are as follows:

  1. Seals the top of the radiator tiller neck to prevent leakage.
  2. Pressurizes system to raise boiling point of coolant.
  3. Relieves excess pressure to protect against system damage.
  4. In a closed system, it allows coolant flow into and from the coolant reservoir.

The radiator cap pressure valve consists of a spring-loaded disc that contacts the filler neck. The spring pushes the valve into the neck to form a seal. Under pressure, the boiling point of water increases. Normally water boils at 212F. However, for every pound of pressure increase, the boiling point goes up 3F.

Typical radiator cap pressure is 12 to 16 psi. This raises the boiling point of the engine coolant to about 250F to 260F. Many surfaces inside the water jackets can be above 212F.

If the engine overheats and the pressure exceeds the cap rating, the pressure valve opens. Excess pressure forces coolant out of the overflow tube and into the reservoir or onto the ground. This prevents high pressure from rupturing the radiator, gaskets, seals, or hoses.

The radiator cap vacuum valve opens to allow reverse flow back into the radiator when the coolant temperature drops after engine operation. It is a smaller valve located in the center, bottom of the cap.

The cooling and contraction of the coolant and air in the system could decrease coolant volume and pressure. Outside atmospheric pressure could then crush inward on the hoses and radiator. Without a cap vacuum or vent valve, the radiator hose and radiator could collapse.


Always remove the radiator cap slowly and carefully. Removing the radiator cap from a hot pressurized system can cause serious burns from escaping steam and coolant.

Figure 6-6.—Radiator pressure cap.
Published by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy

Copyright 2001-2004 SweetHaven Publishing Services
All rights reserved