Radiator caps are relatively simple in appearance but they are extremely important components of the engine cooling system. Like radiators and other cooling system parts they are exposed to harsh environments while functioning relatively maintenance free for several years and many miles of service. But the metal, plastic, and especially the rubber materials in radiator caps will deteriorate or become contaminated over time. Therefore, radiator caps should always be inspected and tested carefully whenever performing cooling system maintenance.
The radiator pressure cap serves five important functions in the cooling system.
- Filler cap to access the cooling system
- Closure cap to keep coolant in the radiator
- Pressure cap to prevent overheating by maintaining system pressure to raise the boiling temperature of the coolant thereby allowing the engine to operate at more efficient higher temperatures. The higher operating pressures also provide a margin of safety at the water pump inlet to prevent damaging cavitation which can lead to loss of coolant circulation and/or pump impeller erosion.
- Pressure relief valve to safely and harmlessly vent excess coolant and air into the overflow reservoir as the coolant expands when it is heated. By preventing excessive internal pressure it protects the radiator and other cooling system components. The pressure relief valve functions in conjunction with the vacuum relief valve to facilitate the removal of air from the cooling system after the initial factory fill and after cooling system service. Entrained air needs must be purged from the cooling system to maximize cooling performance and to control internal corrosion of the engine and the radiator.
- Vacuum relief/siphon valve to allow coolant to return from the overflow reservoir back into the radiator. When the coolant in the hot system returns to ambient temperature it contracts creating a vacuum which draws coolant back through the valve and into the system from the overflow reservoir.
There are several components in the radiator cap that must work in harmony for the cap to function properly. The parts in a conventional radiator cap are illustrated in the diagram shown below.
Two of the most important components are the two elastomeric seals (gaskets) that mate with features in the radiator filler neck to either contain the coolant under pressure or direct coolant to and from the overflow reservoir. The lower gasket provides a seal between the pressure valve and the radiator filler neck and also between the radiator cap vacuum valve and the pressure valve. The upper seal assures that pressurized coolant is directed through the overflow tube on the filler neck when it is released past the lower seal. If either of these seals are compromised due to age or by corrosion product contamination then the 5 radiator caps functions noted above will be affected.
While the radiator cap is a relatively inexpensive component of the cooling system its proper operations is critical to the efficient and reliable operation of the entire power train system. Most service experts believe that replacing the old radiator cap is inexpensive insurance when performing cooling system maintenance and always recommend a new cap whenever a new radiator is installed. At a minimum the cap should be inspected and tested for proper sealing and pressure release functions when a new radiator is installed and when the coolant is changed.
Some of the domestic and foreign original equipment manufacturers are now choosing caps made of plastic components over the more conventional caps made from stamped metal components. But regardless of the design, all caps use elastomeric seals which are subject to aging from heat, chemical, and ozone exposure. So while the plastic parts may still look good, the working components may have past their useful life expectancy and the cap assembly should be replaced. Again, inspect and test, or install a new cap as cheap insurance when performing routine maintenance or repairs.
Look for more application information and service tips about radiator caps in our next installment of “Keeping it Cool Tech Tips”.
Iceman Cooling Journal 
Comments
Overflow tanks are old tech. No modern car uses them. Notice a rad now days doesn’t have a cap instead it’s topped over through the expansion tank/degas bottle. In racing applications once water overflows you’re done and in a death spiral of air and not enough water. That overflowed water isn’t coming back until you turn off the car and cooling can happen and that almost magical vacuum that brings the water back into the rad from the overflow tank can happen. Am I missing something?
Tom, your understanding of the advantages of pressurized coolant reservoirs is correct. But many current model passenger cars and light trucks still use non-pressurized overflow tanks. Even those that use an expansion tank/degas bottle have pressure relief caps.