If you don’t see any coolant in the reservoir, or at the top of the radiator, add coolant to your system and run the vehicle ...

Turn it upside down with the radiator cap on and do the same procedure through the upper hose. A clogged radiator will pass a pressure test, but not a flow test. 3.

With the radiator cap off, the engine should be warm and allowed to idle during the test. The coolant must be warm and circulating while the test is in progress.