I have a 91 Z 305TPI 5spd. I seem to have heat only if i plug the vaccum line that goes to the heater control valve with a bolt and even then it is only mild warm. I know my heater control valve is operating fine because i took a new one and just connected the vaccum line and it opened the valve inside. Once i connect the line to the HCV(BTW it is brand new and i tested the old one and that one wasn't even bad), my heat gets cold. I felt the lines going to the heater core and they are hot and the return line is also hot(i think i am feeling the right ones). What could the problem be? Bad heater core, but how does that work when i think there is hot fluid coming and going from it. my blower works fine also. and there is plenty of vac from the vaccum line. somebody help me out, maybe a picture of the hose that goes to the heater core and the return would help, i am not sure if i am feeling the right hoses. And i also just bought a brand new stock thermo and i know that is working also, because it is pressureing and releasing the pressure in the radiator hose. I don't have a head gasket leak or any other kind of leak and i don't think there are any air bubbles either, i have had no heat for like 2 years now. I don't have an overheating problem either. thanks kyle

 

Have you checked all the vaccum lines under the hood that go to the plenum?

 

maybe the blend door is stuck on the cold setting.

 

...or the heater core is externally plugged with grass clippings, bugs, leaves, pine tree needles, panty hose (don't ask), etc.

 

In your sig you say you are using a 160* thermostat. Up here in the great white north we need to use a 190* for heat.

 

Thanks for the replies, i will have to check these things out. One thing, is it even possible for the heater core to be externally plugged being that it is inside the car? And i am not sure if the vaccum lines to the plenum are plugged in where their sopposed to go. Does anybody have a pic of these vaccume lines to better help me make sure they are at the right spot and does anybody have a pic of the hoses that come and go to the heater core? thanks again guys.

 

Had a similar problem on my kid's 92 RS. At times you got the sense you had hot air blowing, but it wasn't "melt down hot" & as a result the passenger compartment never got warm enough. Like you, the heater hoses were hot to the touch. I thought it was the heater core, so I decided to change it. I did not really pick up on this until I was putting everything back together, but here is what I suspect. When I changed the core, the old one looked & flowed good & it had obviously been replaced before (wasn't a Delco core). So, I had a feeling it was not the core. When I was finishing the job, I was re-installing the heater case cover, and I noticed it was possible to get the blend door pivot shaft not positioned correctly in the case. There was also a possibility for installing the core "holding bracket" incorrectly so that it hangs up the blend door. Bottom line, the blend door might be hung up for the afore mentioned reasons. I should also note, in the summer, the a/c never was right either. After doing the heater core job, both the heat & a/c work fine. The blend door travel was being restricted; never fully open, never fully closed.
FJK