Radiator caps are a simple and inexpensive but vital component of a properly Operating cooling system. A faulty cap can result in overheating, loss of coolant or major radiator damage, so should be checked periodically and replaced if necessary.
To prevent overheating and coolant loss, cooling systems are pressurized, which raises the coolant's boiling point about 3-degrees F for each additional PSI above atmospheric pressure. The cooling system is pressurized as the engine warms up and the coolant expands. The radiator cap's pressure relief valve allows pressure to build up to a specified maximum level, then lets excess pressure to escape. In older cars, an "open" systems found in older cars, excess pressure escapes to the atmosphere through an overflow tube. However, as the system cools, air enters through the overflow tube and some coolant is lost.
In a "closed" or reservoir cooling systems used in later model vehicles, as the coolant expands, excess goes through the over- flow tube into a reservoir and a siphon valve allows coolant to be siphoned back into the radiator as the engine cools and the coolant contracts.
Radiator caps act as pressure relief valve to prevent excessive pressure in the cooling system after the engine is turned off, to prevent damage to the radiator, heater core, hoses or water pump seal. Conversely, the radiator cap also lets air in as pressure drops, to prevent radiator hoses and tanks from collapsing due to the partial vacuum created by fluids cooling.
Check the radiator cap during routine maintenance, and whenever coolant is tested or replaced. Check he cap only when the engine is off and cooled down. Place a rag over the cap and remove it. First turn it counterclockwise about 1/4-turn, until it reaches the safety stop. This allows internal pressure to vent before fully removing the cap. Then pressing down on the cap and continue turning it counter-clockwise. On some vehicles, the radiator cap is located on the overflow reservoir.
Once the cap is removed, they should be pressure tested to ensure they meet their rated specifications. The cap should be replaced if it fails to hold the rated pressure for one minute. Caps come in a variety of pressure ranges: 4-pound caps (3-5 lbs.), 7- pound caps (6-8 lbs.), 13- or 14-pound caps (12-16 lbs.) and 15- or 16-pound caps (14-18 lbs.). Using a cap with the wrong pressure rating -check your owners Manual-- can either cause over-pressurization or if too low, lower the coolant boiling point.
While doing this, inspect the overflow tube connecting the filler neck to the overflow reservoir for looseness, cracking or obstructions. Check the radiator filler neck sealing surfaces for any nicks, dents or corrosion that could impair a proper seal. Inspect gaskets for looseness, cracking, hardening or other damage that allow pressure leakage and coolant to escape.