I have a 99zx2. Towards the end of last winter the heat in the car started to get worse and worse. Had the coolant flushed in it by a local garage, which seemed to make no difference. He did not that there did seem to be stop leak in the system. Made do for the rest of last winter and didn't worry about it until know its freezing out again. I'm trying to correct this issue. I replaced the thermostat. Made no difference. Both the lines going to the heater core are very hot, so i'm assuming that hot coolant would be flowing through the heater core as well? I did not specifically try to flush the heater core itself yet, but it seems everywhere suggests that if both lines are hot the core is most likely not the issue. All the controls seem to work, and I checked under the dash to make sure the hot/cold line was properly attached and working, and everything seemed to be in order. The only way I can seem to get the car to produce any heat is while driving if I turn the heat off for 5-10 min, and then turn it back on it wall produce warm air for about 45 seconds or so. The coolant is at the proper level, and i'm not really sure where to start checking next. And suggestions? Possibly try to flush the heater core even tho both lines feel hot?

 

if the hoses are hot then the problem is not likely the heater core as others have noted. instead, your problem is likely due to the vacuum system not operating the doors to allow the heat to come into the cabin. By default the door shut and allow the vent air through. The likely culprit is a small vacuum line under the hood. If you look on the back of your intake manifold, near the throttle body, you will notice 4 hoses. Follow the smallest diameter of those hoses until it comes to a t connector. This small hose leading from this is likely cracked from age and now needs replacement. Once this is replaced it will allow the vacuum to work as it should and will allow the doors to move letting the heat flow.

good luck, i hope this helps