I have a problem with my radiator cap "opening" allowing a lot of coolant to escape into the overflow tank. Any ideas why this is happening?

Background:
-Robertshaw high flow thermostat (160 degrees)
-Stewart high flow water pump
-Original factory radiator
-Radiator cap has been replaced 3 times, but to no avail
-Car is driven on road race courses so the motor sees a lot of 3000-5500 rpm duty
-This same cooling system ran OK for 2 years after I installed the Stewart water pump. It just started happening suddenly.
-On the track, I run a few slow laps to warm up tires, brakes, fluids before I drop the hammer.
-Enough coolant escapes to the overflow tank that it spills out of the cap (I start with the tank almost empty)
-This problem happens only occasionally when I drive the car on the street
-The coolant temp increases slightly on the race track but stays below 200 degrees.
-When I shut down the engine with excess coolant in the overflow tank, the coolant stays there. It is not drawn back into the radiator as the engine cools.

Chuck
85 IROC

 

have you tried a different manufacturer of cap?

you may just be getting unlucky

 

another thought...

Since the system is pressurized I'm wondering if there is a blockage somewhere which is increasing the pressure beyond the caps capacity. The blockage may be in the lower rad hose and is causing a pressure increase as the pump tries to force coolant through the rad.

 

Thanks for the advice Sleeper. I haven't tried another brand of cap. I'm using a Stant cap now.