Overheating question

[KWKloeber]

My usual auto parts store doesn't have the radiator cap pressure tester to loan/rent.

Usually Advance Auto, Autozone, O'Reilly and sometimes NAPA rent them.
Harbor Freight sells a kit.

Or, buy a gauge, bicycle pump, and a couple pipe fittings and test thru the thermostat cap.  You'll probably want a valve in line to isolate the pump once you pressurize the system because the pump might leak a little.  You also need to know the pressure rating of your cap and not exceed that. I'd buy a high psi cap - like 20 psi - just for the test.

[Craig Illman]

I rented a radiator pressure tester from a general large rental shop (many years ago).

Craig

[Breakin Away]

All the local AutoZones near me have them for free loan. You buy it (used) and upon returning get a full refund. They're pretty generic kits with lots of adaptors for different radiators.

A few years ago I wasted a 3 hour trip to the boat because the AutoZone kit was missing the required adaptor, so I decided to buy my own for a coolant replacement.

https://www.amazon.com/gp/product/B0067UE6IY

Since then I've had no need to use it, so it's sat around unused. But it's there for when I need it.