I just replaced the heater core in my 1994 Trans Am. I flushed the old one last year, which got me through last winter. But it failed this year, so we just replaced it.

Here is the problem:
I let the car warm up, about 185, and turned the defroster on. It blew like cool air, nothing that would actually defrost a frosted windshield. So, we tried just setting it to blow hot air through the vents in my trans am, the three in the middle and two on the sides. Well, the two on the sides will only blow lukewarm air, while the three middle vents blow hot air. we checked and there are no heater core leaks.

Anyone got any ideas as to why I am still not getting some hot air for defrost? I run a 160 Hypertech Thermostat, but even in the past with the same thermo I didn't have a problem like this.

Thanks for any help

 

Well your defrost turns on the a/c compressor. Maybe you should look into that? Only getting heat from certain vents isn't right. Pull the dash and see if the air is leaking out before it makes it to the vents.

 

If you're getting hot air out the middle vents, then your heater core is working. Sounds like your problem has to do with the controls to route the air.

 

When you say controls to route the air, are you talking about the dial that changes the air blowing on your feet, face, defrost, etc... or are you refering to the passages under the dash that control the routing of air?

 

Correct. The switch on the dash that controls the air flow is operated with vacuum. It has many vacuum lines attached to it, and depending on what position you have it in it will direct the vacuum to different air flapper doors. The doors will open or close, and direct the airflow to the desired outlet.(feet, face, defrost, etc)

It sounds to me like you have a vacuum leak at the switch, or maybe one of the flapper doors is stuck. Can you hear a hissing sound at the switch? That would indicate a leak. It should hiss a little when you make a change in the position, but the hissing should stop after the selection takes effect. If it never stops hissing, you have a problem there for sure.

 

Nope, no hissing what-so-ever.

 

That does not mean it isnt a problem, because it obviously is if you only get heat from the 3 center vents. Somewhere there is a flapper door that is not responding to vacuum. Either it is not getting vacuum, or it is stuck. Start by looking at the switch. Use a hand pump with a vacuum guage to test each line. It shouldnt be to hard to do if you have any mechanical skills.

Good luck!

 

Now this morning, there is no heat coming out at all from the 3 vents that worked last night. It is still blowing air, it is just not hot.

 

If the air is not coming out where it should, check this
http://shbox.com/1/vac_check_valve.jpg

and also check those lines and make sure they can hold a vacuum

 

This might sound real stupid, but how can you check the lines to make sure they can hold vacuumm???

 

Hand pump with a vacuum guage. You can get one at most parts stores.

 

Diagnostic Vacuum hand pump