2001 Chrysler PT Cruiser

A blown head gasket won't always show symptoms as you described.

What happens is:

When your engine is under load, exhaust gas will blow through any leak in the head gasket, causing bubbles to enter the cooling system. These bubbles will begin to collect at the highest point in your system, which in your case is the heater core.

Once there's enough bubbles, it will block the circulation of your coolant. (Thus no heat) And once your coolant stops circulating, your engine will soon overheat.

You can usually drive around town at low speeds without overheating. But once you hit the highway, sustained high speed will quickly cause your engine to heat up.

To quickly check for a blown head gasket, remove the cap to your surge tank or radiator. (Engine cold) Start the engine and have someone rev the motor up and down to at least 1,500 r.P.M.S while you watch inside the coolant for bubbles. (NOT FOAM)

If you see bubbles everytime the engine revs, then this is exhaust gas.

Also, when you replaced the radiator, did you flush the system? Sometimes the heater core will get clogged up.

Remove the upper radiator hose and run a garden hose through the system. If water doesn't flow freely, then somethings clogged, and it's probably your heater core.

One last word, you should never, ever hunt down a problem by replacing parts until you hit the jackpot. This only waste your time and eats a hole in your wallet.

The proper way to diagnose any problem is to isolate the system at fault and thoroghly testing each component until the guilty part is found.