Reducing cooling system pressure

June 1, 2008 | By Richard Prince


My question has to do with my son’s 1996 Collector Edition Buick Roadmaster with the Chevy 350 LT1 engine. Since the cooling system has no provision for an overflow and relies on a closed system recovery tank residing at a high location well above the engine, it traps air and residual pressure. Is there a mechanism by which to introduce an overflow cap somewhere in the system? I’ve had excellent success utilizing 7-pound caps so as to preserve the hoses. If one pays attention to the thermostat via the temperature gauge then 15-16 pounds seems absurd. I’ve followed this precept since the late 1960s when we installed aftermarket overflow tanks.


As with all liquid-cooled vehicles made for the past 40 or 50 years, the cooling system in your son’s Buick Was designed to operate within a certain range of pressure, and reducing the operating pressure (for example, by substituting a lower-pressure cap for the recommended higher-pressure one) can adversely affect the cooling system’s effectiveness. This is because the boiling point of a water/antifreeze mix increases as the pressure it is subjected to increases. By lowering the operating pressure of a car’s cooling system you also lower the boiling point of the coolant. Once the coolant starts to boil, its insulating properties increase and its ability to transfer heat out of the engine decreases.

If you do modify the cooling system with the addition of an overflow cap, it should be at the system’s high point. You can install an aftermarket recovery tank or use an OEM tank from a different vehicle.