Hot cold hot cold goes the heat in my truck.

[Honky Kong jr]

Scratch the valve question. Thanks Honky Kong, I didn't know. I worked for a used dealer in the early 80s and there were a lot of problems with the vacuum switches on the controls.

And still is today.....then they got super stupid and went electric motors.....and climate control to even further over complicate a simple operation of opening a door.

 

[Honky Kong jr]

I could be wrong, but I haven't notice anything vacuum operated on mine wasn't that model's with air condition? This is what mine looks like:

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I assumed it had A/C my bad.

 

[CSFJ]

Iirc, the non ac trucks were all cable operated. I was going to ask if you were possibly getting a random draft through the cab, the '79 I had in HS was terrible about that. But I see you measured vent output temp, so obviously not that. Where are the heater hoses plumbed coming off the engine? is there any chance that they were swapped to, or can be swapped to different ports? It seems you've got the mechanical controls covered at this point, I would have to suspect like was mentioned above, that here may be an issue with coolant flow through the core.

 

[CCSS]

I will check and maybe take pics of how it is plumbed currently hopefully tomorrow.

 

[tanyourhide]

Make sure the springs in your heater hoses havent colapsed. It sounds like your not getting enough hot coolant from your water pump. That may be ypur next step

 

[CSFJ]

The only hose I've ever seen with a spring in it, was the lower radiator hose. Never heard of heater hoses having springs. Have you run across springs in those before?

 

[74 Shortbed]

I've never heard/seen heater hose with springs inside???, have seen it on the outside on sharp bends but I'm sure it was owner installed though..

 

[PrairieDrifter]

Your inlet to the heater core should come off of a port on the front end of the intake and if you don't put the outlet to the radiator it should go to the port on the top of your water pump. It has to be the blend door or you're low on coolant.

Especially since you said it happens on corners sometimes. There's no other part to the system to cause this. Are you sure your blend door seal is good? And there's no play in the mechanism? You might have a faulty heater core but that's doubtful.

The other thing I would check is your blower motor. It could be possible it turns on and off, due to a loose connection or wiring issue.

 

[RustyPile]

.......{snip}.......Temp may even change to cold when turning a corner......{snip}.....

That might be a clue to what's happening.. You've checked to be sure the temperature cable isn't moving and it still controls the door.. Have you checked to be sure the door is still connected to it's shaft???

 

[gpmorgan]

This is how I had my heater hoses on a sbc and it worked great. One from the intake. The other to the radiator.

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[CCSS]

That might be a clue to what's happening.. You've checked to be sure the temperature cable isn't moving and it still controls the door.. Have you checked to be sure the door is still connected to it's shaft???

I have check cables and they are not changing. I have removed duct work and checked the arm that moves door to ensure there is no lost motion between the two. Everything is working properly there.

 

[CCSS]

This is how I had my heater hoses on a sbc and it worked great. One from the intake. The other to the radiator.

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Mine was originally set up the same. But now it is set up like this.

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[da_raabi]

I must also agree that there seems to be some issue inside the heater box, not with the coolant. Even if coolant flow was cut for a short time for some unknown reason (I see absolutely no way for cornering to affect this) the heater core would remain hot enough to keep the air warm until coolant was re-applied.

I think there is something going on inside the box. There's got to be a door swinging around somewhere. Cornering/sudden temp changes/ etc all point towards that.

It's a simple thing to do. Just pull the box and do a complete inspection.

 

[Honky Kong jr]

Face it your truck is a bi-polar female not to uncommon these days. Lol

 

[4WDKC]

There are only so many things it can be ie blend door, heater control valve if it has one ( I think it should or you would have constant heat on in the summer) bad t stat not getting hot enough, air pockets in the cooling system, low fluid level in cooling system. Thought I saw you replaced the heater core already right?