Abstract: A device and the use of, are disclosed for pressure testing the integrity of coolant systems of the internal combustion engine, and more particularly to a compressed air operated pressure tester for performing diagnostic services on water cooled engines and their related components which include the radiator, the coolant flow passages within the engine itself, the pressure type radiator filler cap with available adapters, and the connecting hoses. The device includes a connecting means and restriction component for discharging compressed air into the coolant system with an adjustable pressure relief valve for limiting the maximum pressure.