86 GMC C2500 - Heater blows hot, then cold, then hot, then cold etc.

[Challis350]

Hey guys

The heater in my '86 won't pump out hot air for any sustained period of time. It will blow hot for about a minute, then blow cold air for ten minutes. repeat.

This is the case on any fan speed and location.

I can't seem to find any other threads on this. Does anyone know what the reason might be?

Thanks!

 

[SirRobyn0]

That is kind of an odd one. Does the temp gauge go up and down? I'm wondering if it could be a lazy thermostat.

 

[fast 99]

Low coolant or not properly bleed?

 

[GTX63]

First thoughts- Low coolant, air in the system (see above)

 

[Rickf]

Or, something floating around blocking the heater core.

 

[swe69]

I have the same problem in my K10 1983, And have not found any faults.

 

[WP29P4A]

I had this happen on my Sploder, flushed the radiator and refilled it, then had intermittent heat until I got all the air out of the system.

 

[Snoots]

Vacuum Canister leak?

 

[Itali83]

Vacuum Canister leak?

Even the a/c trucks had cable actuated blend doors so vacuum or lack there of makes no difference on temp. Just location.

Ben

 

[Challis350]

Cool sounds like it might be a trapped air issue which would be a cheap fix.

The engine temp thermostat does jump around quite a lot now you mention it.

 

[85K304SPD]

Flush and new anti freeze with a new thermostat. Thermostat is probably hanging open.

 

[DoubleDingo]

I had this happen once with my Jeep. Except it was below freezing, I was driving home around 02:00, the temp gauge kept getting up to temperature and immediately dropping down, rinse and repeat. I pulled over, popped the hood, and with the engine running, opened the radiator cap. It was full. In my case it boiled down to just being flipping cold! Luckily I didn't break down. I did need to stopped for a nap, though. Not wanting to wake up dead, I turned off the engine. After a time I woke up freezing my butt off. So I got back on the road and made it home before sun-up.

In your case, I suspect it's low on coolant and not properly bled. When bleeding the system always make sure the heater core has full flow so the entire system is burped of air bubbles.

 

[SirRobyn0]

I had this happen once with my Jeep. Except it was below freezing, I was driving home around 02:00, the temp gauge kept getting up to temperature and immediately dropping down, rinse and repeat. I pulled over, popped the hood, and with the engine running, opened the radiator cap. It was full. In my case it boiled down to just being flipping cold! Luckily I didn't break down. I did need to stopped for a nap, though. Not wanting to wake up dead, I turned off the engine. After a time I woke up freezing my butt off. So I got back on the road and made it home before sun-up.

In your case, I suspect it's low on coolant and not properly bled. When bleeding the system always make sure the heater core has full flow so the entire system is burped of air bubbles.

I like the story. At some point I think I have slept in all of my vehicles, at least for a short nap. BTW, the bench seat of the square is quite comfortable to lay across IMO.

 

[Challis350]

Thanks for your help guys. Flushed the coolant and put new stuff in. let it burp all the air out.

Now have hot air on demand!

The engine is also running a lot cooler too (perhaps a bit too cool).

 

[DoubleDingo]

Thanks for your help guys. Flushed the coolant and put new stuff in. let it burp all the air out.

Now have hot air on demand!

Good deal! Thanks for reporting back, now we know that it's fixed and what fixed it.