PRESSURE CONTROL CAP
COOLANT PRESSURE CONTROL CAP
The radiator (or expansion tank) has a pressure control cap with a normally closed valve. The cap, with a
number 7 stamped on its top, is designed to permit a pressure of approximately seven pounds in the system
before the valve opens. This pressure raises the boiling point of the cooling liquid and permits somewhat
higher engine operating temperatures without loss of any coolant from boiling. To prevent the collapse of
hoses and other parts which are not internally supported, a second valve in the cap opens under vacuum when
the system cools.
Figure 1. Pressure Control Cap (Pressure Valve Open)
Use extreme care while removing the coolant pressure control cap. Remove the cap slowly after the engine
has cooled. The sudden release of pressure from a heated cooling system can result in loss of coolant and
possible personal injury (scalding) from the hot liquid.
To ensure against possible damage to the cooling system from either excessive pressure or vacuum, check
both valves periodically for proper opening and closing pressures. If the pressure valve does not open between
6.25 psi (43.1 kPa) and 7.5 psi (51.7 kPa) or the vacuum valve is not open at .625 psi (4.3 kPa) (maximum),
replace the pressure control cap.
Figure 2. Pressure Control Cap (Vacuum Valve Open)