Well known issue on PTCruisers (ask me how I know) are coolant /cooling problems. Cracked heads, warped heads or blown head gaskets are common, as are radiator fan failures. Do the compression tests, as suggested, and take it from there.
Doesn't matter that you never saw coolant, once the engine gets up to temperature and cooling system pressure it will find it's way wherever it can go. Ideally it can only go where it's supposed to, but regardless, if there is a path it's not supposed to go, it will go there too.
Buy a bottle of coolant dye ($10) and a blacklight bulb ($3) for your trouble light, and you will see every coolant leak, slow or fast.
'57 FL Straight 50 wt
'90 Miata 1.8L w/Rotrex Supercharger [Mobil1 0W-40]
'96 Ram 1500 [3.7L Mobil1 0W-20 / 1L 15W-50]
'01 PT Cruiser [Mobil1 0W-40]