340 Overheat / Radiator Geyser

If it doesn't move coolant the thermostat is closed. Should be open at 3/4 gauge.

Take it out and put in a pan on the stove and see when it opens. To see if it is the problem.

Or just replace it anyway.

What is the temp below t-sat and the front of the heads?

Put the cap back on before you turn it off next time.