how to fix 1994 gmc sierra heater to blow out hot air


Asked by Feb 01, 2013 at 06:24 PM about the 1994 GMC Sierra 1500 K1500 SL 4WD Standard Cab LB

Question type: Maintenance & Repair

put new thermostat and the heater is not working

8 Answers

Assuming the radiator is topped off, the most likely your heater core is not getting hot water to it. The valve that controls the flow may be stuck, or the linkage to it. Locate that valve, inline on heater hoses and try opening it by hand. Let's start there

18 out of 18 people think this is helpful.

You must FLUSH the heatercore now. takes an odd combination of a pressure fitting on a hose fitting to flush 'er clean. Problem is getting the air out of the system. once a heatercore becomes air-bound, it is pratically impossible for it to see the light of day. We've joked amongst our fellow mechanicsm "just park it on a hill" which has a strange wisdom to it....continuing problem for heater core short flush the core with water, blast the core with air...and say a little prayer :)

1 out of 1 people think this is helpful.
Chris Austin

did you flush out heater core?, check to make sure there is no line blockage, and change coolant, chances r its green so its fairly cheap to do this, at the shop i work at its 70 $ plus tax for green coolant flush, fully drain and hose out rad and lines as best you can and refill with 1 jug of pure green coolant and then fill with 1 jug of water, top off with 50/50 mix of coolant and water, preferably distilled but not necessary.

1 out of 1 people think this is helpful.

Is the heater core clogged? Check both heater hoses for heat. If the hoses are hot, the core bypass door is open. Check the HVAC fuse - the core bypass door is electrical - the temperature knob controls it

7 out of 7 people think this is helpful.

Them 3 dudes are right also fyi

tenspeed is right. if really you don't need to do this, you shouldn't. perhaps the control just needs to be 'goosed' a bit disassembling and wire brushing things that can be cleaned off.


if heater hoses are both hot to the touch you probley have actuator motor bad, remove glove boc liner and there are to actuator there and 1 actuator above accelerator on side of heater case, see if acttuator move when changing from hot to cold, if not remove actuator and move door by hand and if it changes to hot actuator motor bad

4 out of 4 people think this is helpful.

i forgot its electrical. i was thinking cable. i'll check the fuse, if not i'll be taking the clove box out. thanks

2 out of 2 people think this is helpful.

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