have an '82 n/a and had heater core replaced back in Nov. The guy did a poor job so the heat doesn't work properly. At first it wouldn't come on at all so took it to Pep Boys and had a vacuum test and they found nothing. So I kept trying it and still nothing. I messed around with it and found that if I let the cold air run for about a min. then turn to hot that the heat would run for about 3 minutes then it would cool off. so i would run the cold again for about a minute and then switch to heat and it would run for about the same time. does anyone know what this may be?

 

Oh, you probably have the Auto Climate Control system on your Z. That whole heater/AC thing is just messed up on mine too.

Sounds like its as simple as your water valve closing after a while. I'm not sure on this, but that valve may not be manually controlled by the heat selection in the dash. It may also be dependant on a vaccume system and thermostat. Maybe try opening up the heater valve that you can see from the passenger side. jam the arm that opens it to keep it open. Then try running your heat. If that keeps the hot air flowing all the time then you've found your problem. To properly resolve the issue will take some planning on your part. You can swap the whole system to a manul heating control found in most 1979-81 280ZX's. I'd like to do that some day on mine.

 

I did the swap on my 83 and it was easy. Everything plugged right back up and I like the manual so much more than that auto.....