I have been battling a no vent only defrost issue forever now, and I think I may have found the problem. It's on my '91 XJ. The fitting furthest to the right of the intake manifold, where the vacuum hose connects that runs down to the vacuum ball, has a little check valve in it. Now, it would be my understanding that since this is a VACUUM ball, when the engine starts and runs, it is constantly allowed to pull all air from the line, but the check valve prevents any air from flowing the opposite way back in. Here's the thing; when I pulled my little check valve, I think it was reversed because I could blow air through the valve, towards the resevoir, but I could not suck air back, which would be away from the resevoir. Basically, instead of allowing the engine to suck the air from the system, my guess is it was reversed and only allowing air to flow into the system, but not be sucked out. Does this sound right? Thanks everyone!

 

This is a diagram of the vacuum lines:



I am referring to component #1, the check valve. It seemed it was allowing air to flow back through the system, but preventing the engine from pulling it out.

 

I went ahead and switched the line so that air can be pulled towards the engine but cannot be blown back through to the resevoir. Can I start the Jeep this way, or am I gonna have some unexpected spontaneous failure?

 

Also, kind of a stupid question, but as long as the valve allows flow one way but not the other, it's still working, right?

 

Yes the valve is good so long as it only allows vacuum to flow one way....and as long as that one way is the correct way. Did you start your Jeep up to see if that fixed the problem?

 

Uh... no lol. I guess I'm worried something will go wrong. Pathetic, huh?

 

Give it a try and let me know what happens. Make sure your vacuum ball is hooked up before you start it. We can figure this out. I have had just about every problem known to man with my 91 XJ at one time or another.

 

It worked!! I have vents! YES! Lol the neighbors looked at me funny because when I pulled the switch to vent and it worked, I just stared at the dash and said loudly "Holy Crap! It actually worked!" Thanks for the help!

Another quick question... Is it normal for the air to blow at the leg vents when I put it on heat? When I put it on Vent, it blows from the vent, and when the XJ warmed up a tad (first white mark) it blew toasty, not deathly, air. When I go to heat, it switches to the leg vents. Normal?

 

I tried this. Didn't work for me... vacuum leak maybe?

 

Did you make sure the check valve is oriented so air can pull into the intake manifold but not blow back out?

If this didn't work, then yes, vacuum leak. Start at the vacuum ball and work your way up.

 

Yes, and I checked for leaks for an hour the other day, could my door actuators for the hvac box be the problem?

 

I have intermittent cruise loss and not sure about vents working. That diagram shows where the vacuum ball is usually mounted by the bumper, but mine was relocated to just in front and below the wiper fluid bottle/below and the to the side of my heater control valve. Mine is a large cylinder, about the same size as an alt maybe.

If that line still runs down by my bumper, and it looks like it was tied into the middle of the line, would that be causing my issue?

 

the check valves are so vacuum pressure is not lost under acceleration to the controls this link should help with trouble shooting http://www.jeep4x4center.com/jeep-se....swf#toolbar=0

 

That's what I was told also, however don't give up on the check valve. If your check valve is switched like mine was or failed and blocking the engine from drawing vacuum into the resevoir, then none of the vacuum driven stuff will work. Just do what I did.

Take your check valve off.
Hold it how it in the orientation it was in while attatched to the engine.
Put the intake manifold side into your mouth, and suck. (giggity)
If it doesn't suck, it's either bad or the wrong way.
Repeat for the otherside.
Spit on ground multiple times to get nasty taste out of your mouth.