Heat.....but not really

[funkingonutz]

Every once and a while ill cold start the truck (3.0L 5speed 4x4) and it'll get up to normal operating temp. Then ill notice that there still is'nt any heat comming out of the heat vents. So then i pop the hood and pull the vacumn line off of that thing (forgot what its called) between the heater hose and i hear a short "pssst" noise, that little valve thingy opens and ta-da I have heat. What the hell is causing this??

 

[cocoasranger]

A vacuum actuated heater control valve.

 

[funkingonutz]

yes thats what that thing is called...i remember now cause i had to replace it last Sept. when it was 38 degrees in the morning and i found out i have no heat lol!!!
Now since i replaced it, that should'nt be the problem right?? maybe its the vacumn line itself thats leaking??

 

[Wicked_Sludge]

if the valve defaults to hot when you pull the vacuum hose, then it sounds like it shouldnt have vacuum to it while the heat selector is on hot....but the "pssst" sound would indicate that it does have vacuum for some reason. sounds like whatever controles the vacuum input to the valve isnt doing its job.

 

[cocoasranger]

I think WickedSludge is onto something. Maybe the valve is set up so a vacuum line failure does not result in loss of heat and therefore vision out the windshield which would be a definite safety issue. My son's Maxima was set up this way.

If that's the case you could disconnect the line and plug it during the cold weather and fix the problem when things warm up.