Must say, wow! The rate of response in this forum is incredible. I'd have to wait 5 days in the other forums for all these responses. I'm impressed and humbled. Thanks, guys.

I tried this last night. I ran the truck with the pressure tester attached. From cold, the pressure started slowly from zero and rose steadily as it heated up. I killed the engine when it hit 23 PSI because the tester's cap was not going to vent into the overflow tank and it was rising rapidly at that point. Since the pressure was from thermal expansion and not me pumping air pressure, it dropped slowly as it cooled, back to zero. Saw no leaks. I'm not ruling out head gasket, but I saw nothing unstable as I watched it. It was all very linear. I will be pulling plugs next.

Thanks again.


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RTH, my understanding is that it shouldn't hold pressure when it cools. It should actually be a vacuum, which ends up being zeroed by return flow from the expansion tank. So, there should be no hiss when you open the cap, perfectly cold.