You could bring the engine up to full operating temperature, shut it off and then slowly open the radiator cap very slowly with a big towel around it (so it doesn't spray) to slowly release all the pressure. Once the pressure is relived, then remover the radiator cap, attached the pressure tester and pressurize the system. That's what the article is basically saying.I asked about this because I recall reading it in my auto book.
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I think the distinction I missed is that the engine is warmed up , but not running.
My coolant temp was 160F and the engine was off. With it pressurized in this state, I was able to see coolant leaking from spots I suspected. Once the temp had dropped much lower and I re-pressurized the system, I could not see the leaks at the same spot.
Increasing the pressure to 16psi with the engine cooled down I was able to see a clear leak from a crack in a different spot I suspected.
So I think the idea the book is saying is to connect the pressure tester, bring the car to a warmer temp, turn it off, then pressurize it and check for leaks.