Vents blow heat sporadically

[Dr_Snooz]

I tried searching but didn't come up with much on this. For some strange reason, my Suburban starts venting heat into the cab at random intervals, even when the climate controls are set to Off and Cold and have not been touched for days. It likes doing this in the late afternoons when it's hottest, and since I'm still working the bugs out of the AC system (you know, I fix it, it breaks, wash, rinse, repeat for the next 6 years) so this is the pits. When I first got the truck, the vacuum line to the controls was disconnected and I re-attached it straight to manifold vac. Later I learned about the vacuum reservoir, so I teed that in today. I haven't yet tried it, but I don't expect it to help because the heat is cable actuated, so basically I'm just rambling at this point.

All that to ask: why does the heat come on when I didn't ask for it?

[75gmck25]

The heat blend door is operated by a cable, so maybe something is disconnected, or the door pivot is broken and letting the door flap around.   

If you remove the glove compartment door, and then remove the vertical vent that is right behind it, you can see into the blend door area.  If its like most of these trucks, it will have a treasure trove of stuff that has fallen in over the years through the defroster vents.  I found a bolt, match books, rifle ammunition, pencils, and a big mouse nest in mine.  The door on mine would not go full travel because it hit a very hard stop (the bolt that had fallen in).

Bruce

[Dr_Snooz]

x2 on the mouse nests. I found a condo complex in mine when I opened it up a year ago or so. Yikes what a mess! I vacuumed all that out, verified good operation and sealed up the holes real good. I even pulled out the other dash ducts and washed them up real nice like. Even so, who knows what's happened in the year since then. The mice around here are hard little workers. I'll take another look in the next few days. Thanks!

[blazer74]

Check your vac connections in the engine compartment. Engine to vac ball on fire wall.
Check the ball for cracks.

The blend door is cable attached to the heater box behind the glove box has a screw where it attaches to the box, make sure it's tight and is adjusted correctly for full closing.

Everything else is vac controlled.

Only way you get heat from the engine is if the blend door is not closed or the heater box is damaged inside.

Make sure all your vac actuators are working.

[75gmck25]

FYI - if you need to fix any vacuum lines, you can buy GM hard vacuum line from most of the auto parts stores.  Then buy a package of small diameter flexible vacuum line, and use it to connect/patch the hard lines together. 

The hard lines are easier to route up under the dash because they are small diameter; however, they have a short section of soft line at each vacuum motor, and a multipoint soft connector on the back of the dash controls.

Bruce