Hi Lester. Welcome to the forum. A stuck thermostat will stop coolant from flowing to the radiator so the engine will overheat. The system pressure will build up too but once it reaches the rating of the radiator cap, coolant will flow past the cap into the overflow recovery tank. The most common value for the cap is 15 pounds. The boiling point of the water in the coolant increases 3 degrees for every pound of pressure, so since water boils at 212 degrees, a 15 pound cap will increase that by 45 degrees to 257 degrees. That's the same principle your wife's pressure cooker operates under.
So while the thermostat could be responsible for overheating and pressure buildup, the cap is responsible for pressure limiting.
Radiators are extremely efficient so it is possible on some models to never build very much pressure. A defect could develop in the radiator, typically due to corrosion, that could make it unable to contain much pressure, but the problem won't occur until the day comes when the pressure does build up enough to cause it to leak. The fact that a plastic part gave out suggests corrosion was not involved, and that more than likely the pressure relief valve in the cap didn't open soon enough.
Monday, April 26th, 2010 AT 12:32 PM