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I just purchased a '98 Trans Am this weekend and I noticed that it's not blowing hot or even warm air when I turn on the heater. A/C works but for some reason the heat is not. Once I start seeing cooler days in this car I plan to use the heater. Previous owner installed a new water pump right before I purchased the car. I noticed the temp gauge hovers close to 160 but sometimes gets near 180.
Is there a common issue that these late 4th gens have with this problem? I'm new to these cars so any information would help a lot.
There should be no problems other than what any other car would have. Check for:
- A properly working temperature selector. On our cars, it is just a flapper in a plastic duct under the passenger's side dash. The flapper is connected to the **** by a cable. When you turn the **** all the way from cold to hot, you should hear a "thunk" sound. If you hear that when you go all the way from hot to cold, that should be fine. (The lower dash is insulated, so you may have to take off the cover to hear this.)
- Look for a blocked heater core. The water pump has two hoses that go off of it to the heater core, which supplies the heat to the inside of the car. (You should be able to find articles on this and how to deal with it.)
You should also look for funny business at your water pump. It sounds like your previous owner put in a cooler (than normal) thermostat when they changed the water pump. Maybe they also didn't hook up the heater core lines?
Your lines should look something like this: (Yours should have a few more bends, but they will run towards the back of the car from the water pump and to the firewall.)
Take off the lower IP panel under the passenger side dash. If you look up under the dash, you should see the ductwork for the HVAC system. There is a round disk with a cable going to it. This is the cable that controls your temperature. There is a flapper inside the duct that diverts hot/cold air and adjusts your temperature accordingly. The other end of the cable is attached to the temperature control dial.
With the cover off and once you can see the cable and disk, try moving the control hard over to cold and hot and see if you can hear the "thunk." If not, then your cable could be broken or disconnected at the control in the dash or under that disk.
The procedure for fixing all that is in the Factory Service Manual, which you can purchase from Helm Inc. This site gives you a sneak preview of the procedure and then you can see the rest for a subscription: https://www.repairprocedures.com/ser..._8_513443.html
With all that in mind, and knowing that your heater core is hooked up to your water pump, it would probably be more common for your heater core to be clogged - so you should hear the "thunk" once you take the lower cover off.
Just pulled the panel this morning. My temperature control is moving the disc under the dash and I just got the heater core disconnected. I also see that the door is moving now that I yanked out the heater core.
My biggest issue now is that I ripped my finger nail off while disconnecting one of the heater core hoses...not happy about this one.
Last edited by F-Body International; Sep 10, 2016 at 11:50 AM.
Got the new heater core in and it now blows heat. I was, however, unhappy about the tubes being shaped different on the heater core itself. The one I got was a Spectra sold through Oreilly Auto Parts.
Next thing will be draining system so I can refill with the GM Dex Coolant. Previous owner changed the water pump and I'm not sure if they put in the correct coolant.
Thanks wssix99!
Also, I ended up finding that the A/C system runs while the heat is on. Is this normal?
It was that bad? You couldn't clean out or unclog the old core?
The heat "runs" all the time and the heater core is always ready to give heat when air is diverted over it. You don't actually turn the heater "on", you just pull air over it when you want to. The A/C will actually kick on and off but that should only happen when your temperature modes are turned to an A/C mode or a defroster mode.
It was that bad? You couldn't clean out or unclog the old core?
The heat "runs" all the time and the heater core is always ready to give heat when air is diverted over it. You don't actually turn the heater "on", you just pull air over it when you want to. The A/C will actually kick on and off but that should only happen when your temperature modes are turned to an A/C mode or a defroster mode.
Looking back, I think attempting to back flush the original core would've been a better idea. It must've been really clogged up to not pass any heat at all. For now, I'm going to clean up the original and box it as a spare in case this new one has issues.
I didn't know the a/c actually stays on when it's in defrost mode. That explains that one. I will check and see if it stays running on the other settings tomorrow.
Sounds like a good plan. Sometimes, it's not possible to unblock the clog and these cores can corrode and leak. So, if that thing has been plugged for a long time; changing it may have been a good preventative thing to do.
Here is a good drawing of the physical system so you can see how it works. When your selector is the "vent" modes, yoru A/C just turns off and fresh air just runs over the A/C evaporator, which is "room temperature" because the A/C is off in those modes.
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No heat from from ventilation system 
