When the engine is shut off after a good run and the coolant is at max temp (whatever the thermostst temp is), consider this:

The engine is still fully heated, but the coolant no longer circulates. So it gets overheated, and maybe enough to boil - a big pressure increase. The pressure will go somewhere, and that's what you see.

Check to see that your coolant reservoir pressure cap is the correct color.

Let the engine idle for several minutes before shutting it down.

A head gasket can blow in several ways: Between two cylinders, in which case the loss of compression on two cylinders will be noticeable.

Between a cylinder and outside, which can show as a coolant leak at that spot.

Between a cylinder and the water jacket, which can be seen with this test: When the engine is cold, open the hood, remove the pressure cap, get in and sit so that you can see the open top of the coolant reservoir. Start the engine while watching that open top. If the coolant wells up and overflows you have a head gasket problem. Compression pressure is getting into the cooling system.

Such a leak may not show after the engine heats up, as the heat expansion may close off the leak. The engine will usually run hot, though.

Good Luck,

Bob

:>)