Question for anyone that has used a combustion leak tester on their cooling system to check for a head gasket failure.

Does the thermostat need to open for the test to be accurate? The instructions state just to let it idle for 10 minutes then to do the test. I let it idle for more than that, and left it on the rad for more than the 2 minutes it suggested, but my thermostat never did open (its cold out here). The liquid stayed blue, didn't change colour at all. Just wondering if anyone that has used them has noticed if they are most reliable once the thermostat opens, and there is proper flow through the system, or if it doesn't really matter, and the exhaust gasses will work themselves out of the system even with the thermostat closed.

Thanks!

 

My guess the thermostat has to be open. Run the truck until it does but don't let the radiator build any pressure.