Heater Box Blows Cold Air When Off

[k1n3k]

I'm going to be pulling the dash out here in a week or two to fix a few annoyances and while I'm at it I'd like to fix this heater problem. It works as expected when the heater/ac is on. vents blow hot or cold as desired, dash vents, defrost, feet all work fine. Maybe not as hot as I'd expect but I'm thinking thats a blend door vacuum issue. The one I've not been able to find any info on is that when I turn the heater off, it then starts to blow cold air out of the passenger footwell area, I'm thinking through the recirculation door. messing with it when parked though it seems like its fine.

Any advice as to wtf is going on and what to check to fix this before I yank the whole thing apart?

 

[macleanflood]

Is the fan running or it's just coming through the cowl, through the box?

Definitely sounds like vent doors to me.

I'd be checking vacuum, for leaks, measure with a gauge or check the ODB PID...think it is supposed to be 16 inches. Checking the one way valve. Using something like a mighty vac hooked to the HVAC box to give it enough vacuum or more to get the doors moving.

-Mac

 

[NskLJ]

I'm going to be pulling the dash out here in a week or two to fix a few annoyances and while I'm at it I'd like to fix this heater problem. It works as expected when the heater/ac is on. vents blow hot or cold as desired, dash vents, defrost, feet all work fine. Maybe not as hot as I'd expect but I'm thinking thats a blend door vacuum issue. The one I've not been able to find any info on is that when I turn the heater off, it then starts to blow cold air out of the passenger footwell area, I'm thinking through the recirculation door. messing with it when parked though it seems like its fine.

Any advice as to wtf is going on and what to check to fix this before I yank the whole thing apart?

I would not pull the dash until you have a solid diagnosis, once the dash is out it will be hard to test. Your profile is not filled out with vehicle specifics, check the service manual for the vacuum door diagram to see if the recirculating door changes to inside air when shut off. You should be able to diagnosis this just by removing the glove box.

 

[k1n3k]

I would not pull the dash until you have a solid diagnosis, once the dash is out it will be hard to test. Your profile is not filled out with vehicle specifics, check the service manual for the vacuum door diagram to see if the recirculating door changes to inside air when shut off. You should be able to diagnosis this just by removing the glove box.

Its a 1999 TJ Sport, 4.0 and manual. Those just might be important details to include....

Is the fan running or it's just coming through the cowl, through the box?

Fan off, coming through the cowl and through the box while driving.

 

[NskLJ]

Its a 1999 TJ Sport, 4.0 and manual. Those just might be important details to include....



Fan off, coming through the cowl and through the box while driving.

Here is a vacuum diagram. The green hose ( recirculating door) should have vacuum in the off position. Drop the glove box out and look for disconnected or cut hose.