[djsamuel]

Does the cooling system have to be opened to change this timing belt? Or is the water pump driven from it, and replaced as a precaution at the same time?

Yes, the water pump is usually changed as part of this as the timing belt drives the pump. My guess is the mechanic did not properly fill the system; especially if there are no signs of leakage. It can be a bit of a challenge getting the water pump gasket right given there is such little room to work. However, I would think that link would be fairly obvious.

 

[djsamuel]

I found the PT Cruiser 2.4 to be very easy to fill. I simply filled at the radiator, started her up, topped it off just a hair, then filled the tank to the hot level, never had an issue whatsoever, almost the easiest one I ever did. Check the tightness of the hose clamps all around, shouldn't have been touched to change the pump itself, but you never know. Stock ones are the squeeze clamps, just make sure they are intact and look to see if any of them were moved. Also, turn the heater on full blast, sometimes when they drain the heater core becomes an air bubble and very slowly it empties of air, usually it only takes a couple minutes of full flow to purge it.

Normally, you are right. the thermostat that came with my PT originally, had a "bleed hole" in it that allowed air to escape. However, the replacemnent thermostat did not have that hole, so I needed to open the bleed valve right below the thermostat housing to let air escape while adding coolant. Still very easy, but if that valve isn't open, and the thermostat doesn't have the bleed hole, you may end up with an air bubble stuck in there.

 

[djsamuel]

One reason I always drill a 1/8th inch hole in every thermostat I install. Enough of a hole to allow auto bleeding (you can hear air come out of it when filling it the first time), but always allows a little bit of pressure under the thermostat to pass without compromising the warming of the engine.

That is a great idea. I wish I had thought of that when I installed the new thermostat.