Howdy
I got a 1992 Jeep Cherokee. Dunno if anyone has ever run into this problem but here it goes. My heater control valve only seems to open when I'm running down the hwy ,,,,,I get heat,,,,I come to a stop light,,,,starts blowing cold. Ive sorta got the same problem with my air blowing out my defroster vents on the dash,,,,,,after running about 5 min it will come out the vents on the front of the dash. I"ve check for vacum lines broken but cant find any.....There a place where all these come together. Where do I need to look?

Nate

 

Hi Nate,

search, there are plenty of threads with great diagrams and pictures that cover this.

eliminate the valve, an un-needed evil wating to burst.

 

thought about just putting a manual valve but if its simple to fix which it may not be I'd like to have the luxury of controling it from inside my jeep. Weather is so drastic here in Texas can be 80 one day and 35 then next. Your right I have had to change that thing 2 times in the past couple years from it leaking.

 

He meant for you to completely eliminate the valve. You don't need it. Late models didn't even come with one. Just bypass it with some straight unions.

 

Wont my heat stay on all the time? Or not?

 

No. Warm coolant will circulate through the heater core. When you slide the HVAC temp slider to 'cold' a blend door closes air passage to/through the heater core.

Eliminate the valve and the only side effect will be a working heater....

However, the bypass valve is vacuum controlled, as are the actuators that operate the vent selection. It sounds like you have a vacuum leak somewhere due to the defrost change. I would suggest tracing the two vacuum lines from the firewall (where the heater hoses and AC lines enter the cab) and inspect for cracks, leaks, damage, etc. One of the vacuum lines should feed the heater bypass and the other should end up at the vacuum ball behind the bumper. Then trace the vacuum line from the ball back to the manifold.

 

....and if u have a manual valve in the "closed" position for very long, u will likely have corrosion form inside the heater core from stagnant water. As stated, just elimnate the HCV.....the factory did '97 up.

 

Very True!!!
My 99 Tahoe has the standard heater control valve and a manual valve. When it started to get cold here I had no heat until I found the manual valve. I opened it and the heat was fine till two days later and the heater core started leaking. I got the Tahoe from my brother and he had the heater core put in a year before I got it from him. Worse design I have ever seen.

 

Behind your right front bumper? There is a vac. Res. "ball" there, with a large line that comes all the way back to the firewall, across to the drivers side, then connects to intake vacuum.

Then another small line comes back from the ball that controls the heater and cruse control, if you have that. They break/crack, fall off....in my sig you will find diagrams that apply to your Jeep.

 

Come on Don, give him a stolen photo to look at!!!

 

And BTW, ditch that POS heater valve!!!!!!!!!!

 

Do you recommend putting seafoam in your vacuum lines to find leaks?

 

Seafoam doesn't impress me in any way, shape or form. Actually it's best use MAY be finding vacuum leaks. I've never used it for that.
An aerosol can of Throttle body cleaner,sprayed around suspected areas with the engine running, has worked for me for over 40 years though.

 

Have you checked the coolant level in the radiator when it's cold... not just the overflow tank? you are describing classic low coolant level just because it's not overheating doesn't mean its not low. if it's not coolant level then i would inspect the vacuum lines and such. then maybe try flushing the heater core. start with the easy stuff first.

 

We know he has a vacuum problem because of his vents changing to defrost on their own. That heater control valve is also vacuum controlled, so odds are they are both related. I'm sure there is a cracked/broken vacuum line somewhere.

It won't hurt to check your coolant level, but that's not causing your vents/heater valve issues