Heater Problem

[WheelsWY]

I have a heater problem. My heat blows normal and very hot once the vehicle starts to warm up. The heater works just fine until the vehicle has been running for around 20 minutes or so. The heater instantly switches to cold air and blows nothing but until the vehicle has been shut off and has a chance to cool substantially. I haven't been able to pinpoint if the the thermostat is opening or closing at this time. I just know after an extended amount of running time the heater stops blowing hot air. I have recently installed a new thermostat and radiator. Problem existed before and after both changes.

It's a 96 Tahoe w/ 5.7l

Thanks.

 

[MinnesotaChevy]

Sounds like my f150 that i had that was a simple thermostat replacement and it was good to go, i would think that your still having problems with your engines cooling system check things like antifreeze/coolant, thermostat again, water pump, heater core or anything to really do with the ac and or heat

 

[90W7]

That's weird that it just starts blowing cold air. I would think if your temp gauge and engine are at operating temperature and it's blowing got air initially then your coolant level would be fine. If it was hot air with higher rpm and blew cold at idle then it's low coolant.

But for it to switch to cold leads me to believe you have a blend door issue possibly.

 

[WheelsWY]

That's weird that it just starts blowing cold air. I would think if your temp gauge and engine are at operating temperature and it's blowing got air initially then your coolant level would be fine. If it was hot air with higher rpm and blew cold at idle then it's low coolant.

But for it to switch to cold leads me to believe you have a blend door issue possibly.

Almost like once the module that runs the blend door (whatever it's called) gets too hot and stops working and the door goes to default which is open or cold I think. Anybody heard of these things doing that?

I watched last night on my way home. According to the temp gauge the thermostat opens and closes just fine. Drove about 15 miles and the heater never stopped so it's sporadic to boot.

 

[90W7]

blend door actuators would generally be Dead in the water in whatever position they broke in or would click because of the stripped/cracked gears. I've just never had one do what you're is doing.

the part your going to be after is the blend door actuator.

 

[WheelsWY]

Plot thickens with this issue. I didn't think too much about it, but my temp also fluctuates between 160-ish and 195-ish. Its consistent and constant. The temp gets to 195, then drops instantly to 160, slowly back to 195, then to 160. Fan clutch never kicks off either, and it's new. I have since tried yet another thermostat, changed the coolant temp sending unit (to the gauge), and changed the blend door actuator. No go. Fluctuating temp, and heater switches to cold after a certain amount of time. Afraid it may be a head gasket, but oil is good in color, and it doesn't overheat.

Tonight I'm actually going to test for exhaust in the gas, and actual coolant temp to see if it's really fluctuating or if it's the gauge.

 

[98_k1500]

Heater core may have trash in it and just be plugging up. Have you flushed it?

 

[90W7]

Plot thickens with this issue. I didn't think too much about it, but my temp also fluctuates between 160-ish and 195-ish. Its consistent and constant. The temp gets to 195, then drops instantly to 160, slowly back to 195, then to 160. Fan clutch never kicks off either, and it's new. I have since tried yet another thermostat, changed the coolant temp sending unit (to the gauge), and changed the blend door actuator. No go. Fluctuating temp, and heater switches to cold after a certain amount of time. Afraid it may be a head gasket, but oil is good in color, and it doesn't overheat.

Tonight I'm actually going to test for exhaust in the gas, and actual coolant temp to see if it's really fluctuating or if it's the gauge.

Hopefully you aren't getting combustion gases in the cooling system, but air in the cooling system might explain the fluctuating gauge.

Did the new thermostat you put in have the little bypass hole at all?

Sent from my throttle body injected galaxy note3