Hot cold hot cold goes the heat in my truck.

[Honky Kong jr]

No valve, the blend door seals off the heat.

 

[75Monza]

Just curious, does this have a fan clutch and if so, is it working properly?

 

[75Monza]

Also curious, is the water pump correct rotation for a 4.3 in a C10? I've heard the 4.3l in an s10 is reverse rotation, never looked to see if you could mount either in the same place and don't remember if the rotation difference was just different for v-belt vs serpentine.

 

[CCSS]

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.

I have pulled everything out and checked I can't find anything loose or moving without being moved by the controls.

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

That is very true.

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?

I have had several t stats in it. The first one I pulled out was not seated correctly and thought that maybe be the problem, but three t stats later no change. It currently has 190 in it. I did change heater core. I check fluid level quite often and level is good.

 

[CCSS]

Just curious, does this have a fan clutch and if so, is it working properly?

It does.

Also curious, is the water pump correct rotation for a 4.3 in a C10? I've heard the 4.3l in an s10 is reverse rotation, never looked to see if you could mount either in the same place and don't remember if the rotation difference was just different for v-belt vs serpentine.

I have also changed water pumps no change.

 

[4WDKC]

With new tstat, pump, radiator and heater core only things left are the coolant hoses, vacuum hose if any, loose door or not connected completely. The goes cold when turning makes sense for the loose door or ducting not seated, the only other thing I can think of is low coolant level sloshing away from the heater outlets. But you said it always has a full level correct? What about out put from the heater core it could be blocked from being built wrong or something fell/left/crawled inside during manufacturing or storage.

 

[CSFJ]

Is there a provision on the radiator for one of the heater hoses? I think I would try moving one up there if there is.

 

[HotRodPC]

Not on mine. I think that is on w/AC models.

Yes, being a non AC truck there is no vac diaphragms to open and close the side vents. They'll manual pull levers for the occupants to open ad close as desired instead.

 

[HotRodPC]

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

So you think his truck going through menopause? Maybe !!! Having and hot and cold flashes is usually a sign.

I'm curious to what your temp gauge is doing. Is it holding the 190 degrees or does it fluctuate alot up and down?

 

[Azathoth]

I'm betting it's the little plastic clip where the actuator attaches to the blend door.

 

[RustyPile]

Background info .... 1986 C10 4.3 /th400 w/o AC. The heat in my truck has done this for years now but it works fine until temp outside is below 40. Colder than that and the air coming out of the heater is warm one minute and the next minute it is like you have turned the temp control to cold and then a little later it goes back warm and then back to cold (notice I never said hot). Temp may even change to cold when turning a corner, wth? Things I have done that have made no difference:
Changed to 190 thermostat
Changed heater core
Routed heater hoses to engine only (not through radiator)
Checked for loose doors and cables are tight.
Removed entire heater duct assembly doors are moving to proper positions.
Install new radiator (because tank broke not for heat issue).

Any ideas on what to try next? Hope I didn't forget anything.

What's the latest, Daniel?? Have you solved this mysterious problem yet??

 

[CCSS]

I haven’t found anything that has helped so far. But engine has been burning oil and has a few leaks so I decided to pull it and install a fresh engine.

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[CCSS]

So you think his truck going through menopause? Maybe !!! Having and hot and cold flashes is usually a sign.

I'm curious to what your temp gauge is doing. Is it holding the 190 degrees or does it fluctuate alot up and down?

Another thing I am to do is install gauges while doing engine swap. It only has idiot lights currently.

 

[Rusty Nail]

My thought is something about ... a weak, one-way check valve.

 

[Honky Kong jr]

I haven’t found anything that has helped so far. But engine has been burning oil and has a few leaks so I decided to pull it and install a fresh engine.

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Where is the rest of it? Lol