Hot cold hot cold goes the heat in my truck.

[CCSS]

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.

 

[Camar068]

not that I know much about it and just a thought, what about the vacuum hose? Isn't there vent's in the side panels that open? Just throwing stuff out there, sorry I can't give you a more educated answer.

 

[CCSS]

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

 

[75gmck25]

Is the engine temp staying stable during the changes, but only the heater is going from hot to cold and back? Engine temps usually change fairly slowly unless something major is not working. Does the temp usually run at about the first hash mark (quarter scale) on the dash gauge?

I assume a non-A/C truck has similar controls to my A/C truck. Even in an A/C truck, the vacuum doors don't have any impact on heater temp (they just change the path of air between dash, foot and windshield, or open up the recirculate flap on passenger footwell).

If its just the heater that is changing temps (not the engine), it sounds like the heater blend door. However, that's a very simple mechanism that you should have worked on when you put it back together when replacing the heater core. In heat mode the air always comes in through the cowl and down into the heater plenum. The core is always hot, and the heater blend door controls how much incoming air goes through the core vs. how much bypasses the core. The door is cable driven, directly from the slider on the dash, and the mechanism is easy to see if you just pull the glove compartment door.

Bruce

 

[CCSS]

My truck has idiot lights no gauges so I am not sure what temps are in engine compartment while driving. Installed auto meter gauges in dash but haven't had time to wire them yet. Blend door seem to be functioning correctly. I have had the passenger watching arms for the doors while temp changed but there was no noticeable movement. I have even added foam stripping to edge of doors so they would "seal" when closed. No change.

 

[tanyourhide]

Are you certain your getting consistant antifreeze running through tge heater core? Ask your self what operates the bypass?

 

[CCSS]

I thought I may have had air getting into heater core but I checked the output hose from core and there was steady flow before and after changing heater core. Even changing from it being routed to radiator and now just engine I should have noticed some change. This has been my issue for 3 winters so far. I have used and infrared thermometer at the heater vent and temps range from 50-110. I am just glad my drive to work is just 15 min.

 

[CCSS]

Any other ideas?

 

[shiftpro]

Like Bruce mentioned above, sounds like the heat/cold blend door is 'flapping in the wind'.
When you move your dash heat controls can you hear the doors opening and closing? Simple matter to
take the dash pad off and you can see if the cables are still in place and the doors work.

 

[CCSS]

Blend door not moving.

 

[Honky Kong jr]

Blend door not moving.

Make sure you have sufficient vacuum from the engine to move the flap. It will be a small single vacuum line going in threw the fire wall passenger side.

 

[gpmorgan]

I've never owned a square but have worked on some. Does it have a heater control valve in one of the heater core hoses? Do you hear any vacuum hissing sound at the controls?

 

[Honky Kong jr]

I've never owned a square but have worked on some. Does it have a heater control valve in one of the heater core hoses? Do you hear any vacuum hissing sound at the controls?

There is no valve just a blend door.

 

[gpmorgan]

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.

 

[CCSS]

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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