coolant system pressure?

The overflow bottle is a two-compartment item. There is the pressure side of it that has the cap, then there is an area where the cap screws in that have a vent. What is supposed to happen is that under normal use the coolant can expand and contract in the surge tank. It holds the pressure at 13 psi to prevent it from boiling. If it is properly filled it should only go from cold to hot and back. However, if it gets too hot the coolant may expand farther than normal and exceed the 13-PSI rating of the cap. That will force coolant out through the pressure cap and out a vent on the bottle, so it is controlled. I suspect that the tester you have either has the wrong adapter and isn't sealing properly or it is damaged. Sadly, that is a common thing with rental tools.

thank you, Steve, W.

I'm novice mechanic but next I'm investigating clausis Clapeyron equation about boiling points.
next trying to narrow down my problem.
have agood one.

What is the issue? Coolant boiling or loss? With proper coolant in that engine it should normally run around 200 degrees with the fans coming on at 215 or so.

i put in a new thermostat myself, i am changing from all vehicle to Dex cool to see if it makes a difference.
problem is after new thermos seems heats faster holds good, then sometimes goes much higher then back down.

Type of coolant won't make much difference, what would make it different is if you used water and not coolant. Your description sounds like the thermostat isn't moving properly, is it a GM - Delco 12650241 part?

no its a AutoZone Duralast.

Could be binding or defective. Unfortunately that is something seen daily now. These days I trust used parts more than new, even on OEM!