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

Challis350 · Feb 23, 2023

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 · Feb 23, 2023

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 · Feb 23, 2023

Low coolant or not properly bleed?

GTX63 · Feb 24, 2023

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

Rickf · Feb 24, 2023

Or, something floating around blocking the heater core.

swe69 · Feb 24, 2023

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

WP29P4A · Feb 24, 2023

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 · Feb 24, 2023

Vacuum Canister leak?

Itali83 · Feb 24, 2023

Snoots said:
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 · Feb 24, 2023

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 · Feb 24, 2023

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

DoubleDingo · Feb 24, 2023

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 · Feb 25, 2023

DoubleDingo said:
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 · Mar 13, 2023

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 · Mar 13, 2023

Challis350 said:
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.

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

Challis350

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fast 99

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