PT air from heater goes from hot to cold to hot.

Check the coolant level, especially if it's correlated with engine speed. On our Intrepid (I know, completely different vehicle) when the radiator tank cracked we learned about it because at low RPMs water wasn't getting into the heater core, so the heater went cold at stoplights. Temperature gauge read normal.

I agree with Joe - low coolant/air bubble in coolant. If it has a bleeder, open that after the engine is warm and see if any air comes out. If so, leave open until only coolant comes out. Check level in reservoir and top off if low before and after you bled it. If the reservoir is the pressurized type (I don't know about the PT), don't open it to add coolant while the engine is warm/hot - let it cool down and de-pressurize first.

If coolant level is OK and no trapped air, the heater core may be clogged.

Since you do indicate that you sometimes have some heat, probably ignore my last sentence there.

Thanks all!

I checked the coolant in the radiator. I can not see into the resovoir. I added about a quart of 50/50 antifreeze. It bubbled as it went into the radiator. I figured this was air being replaced. Seems to have solved the problem. One more question. I wonder where the missing fluid went. Nothing on the engine, under the car. And the oil pan has nothing but oil in it.

First place I'd look is the easiest: pressure test the system and radiator cap (you can borrow a pressure tester from Autozone). If it passes that, I'd get worried about the head gasket and related problems.

Once again, Joe and I agree - couldn't have said it better. The pressure caps have a sneaky way of leaking and not leaving a trace.

Would that also apply to non-pressurized system caps also? My 41 Chrysler radiator level goes down, but no leaks on the floor or anyplace I can see. The cap was new earlier this year, but could it have gone bad?

Not sure. The coolant systems back then (10 years before I was born) weren't pressurized? The fact that I ask that question shows that I'm the wrong guy to ask on that one. :)

Wwhen a pressurized system's cap goes bad it doesn't maintain pressure, which results (through a long chain of explanation) in steam escaping through it. I've got no experience with non-pressurized systems, but I'd assume by definition that you couldn't have the same failure mechanism.

It was probably just low a low coolant level. If the 2.4 in the PT is like it is in the cloud cars, the the radiator cap is the highest point in the cooling system. The AA cars had a bleeder on the head to get the air out. If the system was not bled correctly, it would take out the headgasket. I know this from experience.

I would suggest checking the coolant level on a regular basis. When the water pump failed on my cloud car, it would leave puddles of coolant on the garage floor. I would also suggest checking the engine oil and transmission fluid for coolant contamination. Has the water pump and timing belt ever been changed on your PT? It may be time since it is a 2001, depending on the mileage. The belt change on cloud cars was 101,000 miles.

KM

Don't know about a '41, but my '49 had a pressurized system. Pressurized to a *whopping* 4 psi vs. the ~15psi common today :-)