Leak Test Kit with Fluid Co2 in radiator leak test kit checks for head & gasket leaks Spots and locates combustion leaks quickly in cracked or warped heads, a leaky gasket or a cracked block. The test is done by inserting the leak-check unit into the radiator filler opening, and doing the test with the engine idling. The blue test liquid will turn yellow in 60 seconds if there is a combustion leak, no matter how small. For pinpointing which cylinder is leaking disconnect one spark plug at a time and retest.