Holding the valve open can cause overheating; it will not prevent it. The thermostat tries to regulate the coolant temperature at 195 degrees. It is possible when under load for the temperature to increase. (When used, electric fans turn on at 212 degrees). That's the point the water in the coolant will boil. By putting the system under pressure, water boils 3 degrees higher for every pound of pressure, so with the standard 15 pound radiator cap, the coolant will not boil and vaporize until it hits 257 degrees. The large flat rubber seal on the bottom of the cap is heavily spring-loaded. Above 15 pounds of pressure, the coolant pushes that rubber seal up and flows into the overflow reservoir.
When the coolant cools down when the engine is off, it contracts and forms a vacuum that could collapse the radiator hoses. The vacuum draws coolant back from the reservoir into the radiator through the small metal disc in the middle of the rubber seal under the cap. Propping that lightly spring-loaded valve open will prevent the system from building normal pressure, and allow the water to boil if it reaches 212 degrees which is common.
More likely, the previous owner wanted to prevent pressure buildup that resulted in a leak and didn't want to tell you about it. If it really was overheating, his trick would make it worse and he would have had to fix it.
Friday, February 12th, 2010 AT 3:40 AM