Another Heater Vent Problem...(searched)

I have a 91 XJ 4x4.

I have been trying to figure out this problem for some time now (almost a year!) My heater/ac only comes out of the defrost vents. I HAVE checked the vacuum canister and all of the lines under the hood. My jeep is currently getting 20-22 MPG so this leads me to believe this is not a vacuum issue.

The unique thing about my problem that I have not heard anyone mention, in the ten or so pages I came up with when I searched, was that air comes out of the defrost vents even when the switch is in the off position. The fan works in all 4 settings but only comes out the defroster.

I also noticed that there is a flap under the glove compartment that never opens, regardless the settings (although I have only tested this parked in the garage and not very long).

What else should I look into? The air comes out nice and warm BTW.

Thanks in Advance for any Help.

Jeep914x4

Ok, I dug around some more. I found the actuators (that's what they are called right?) under the dash on the driver side. I pushed on them until I found out which one should route the air to the floor. I put it in the "vent" position and unpluged the line to see if there was vacuum... nothing.

Could it be the check valve right before the vacuum line goes from the engine compartment through the firewal into the jeep?

TIA

Sounds like you have no vacuum going to the heater assy. to me .Did you check the vacuum lines and reservior just below and behind the right side headlight ......fairly common problem.....get a vacuum gauge and start testing should almost have manifold vacuum to the heater .....i changed out my lil plastic hose for bigger rubber hose when i had that problem i found small cracks all over espesially where its closest to the engine....brittle crap anyhow

Get a vacuum gauge and measure the vacuum. Until you measure the vacuum, all bets are off.

I think you might have to trace the vacuum line that goes through the firewall to the heater controls. First step would be to find the right line and ensure that vacuum is getting as far as the firewall. From there you should be able to trace back to the heater control and see if you can find a line that's either pinched or broken, because it sounds as if nothing is reaching your controls at all. When I had a leak in the controls themselves, I heard it hissing. If you have no hiss at all, I'm guessing a kink, perhaps from a radio installation or other work behind the dash.

However, the "vent" position of the heater should route air to your dashboard vents, not to the floor. The flap down by the passenger's feet is not a heat vent at all, but a cold air vent, which should open only when you turn the controls to "off."

Thank you for the replies.

majic_tech:
In my original post I mentioned that I already checked the reservoir, which isn't in my front bumer anymore because I don't have a front bumper per say. It was replaced with a "tube" bumper after a fender, well bumper bender, a few years back. I have already had this problem before and replaced the plastic lines with rubber hoses which fixed it. I double checked them and they seem to be in good shape.

old_man:
That is my next task. Where can I get a good vacuum tester at a reasonable price? Do I just start at the manifold and work my way out from there? What kind of readings should I get?

Matthew Currie:
I do hear a hissing noise, but only when the AC compressor is on (defrost and AC modes). I have a feeling you are right about vacuum not getting to the heater controls as I do have (or so it seems) vacuum under the hood. Like I said before, my jeep is probably running the best it has since I bought it 6 years ago. 22 mpg with 31's, aw4, and stock 3:55 gears (commuting to work @ 65 mph ~50 miles a day). Wouldn't a vacuum leak under the hood hinder my MPG and engine performance? I have installed a radio but it was long before this problem arose.... come to think of it, maybe it was when I had an alarm installed.

Thank You again for the help and insights. I shall look into it more sometime this weekend and keep you updated. has been a great resource for me since I got my beloved XJ. I just graduated from college and should get a "real job" soon and I plan on becoming a member with a hefty donation to make up for the last 5 or so years.

Jeep on!

Yeah, you have to watch those alarm and stereo install guys like a hawk. I repaired a late model chevy this week that had inoperative 4x4 due to the cd player installer (they didnt plug the 4x4 switches back in) So henceforth and invariably they took the bill to the stereo shop for reimbursement.......hmmmm i wonder how that went???????? By the way when somone says vacuum problem that means just that . What i said in the ealier post were some suggestions but you will find that it is in fact a vacuum problem remember we cant see your jeep from here........

Well did you find the problem dude? silly me, you did say youve been lookin at this for a year already .......why would someone telling you what to look for help?

majic_tech said:

By the way when somone says vacuum problem that means just that . What i said in the ealier post were some suggestions but you will find that it is in fact a vacuum problem remember we cant see your jeep from here........

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I appreciate you taking the time to reply to my inquiry, however if you had read my original post more throughly, you would have noticed I had already checked the things you mentioned (canister, hoses under the hood).

majic_tech said:

Well did you find the problem dude? silly me, you did say youve been lookin at this for a year already .......why would someone telling you what to look for help?

Click to expand...

I have been chasing vacuum problems for some time with no luck. That's why I posted, I thought it might be an actuator or something. My jeep is running great and getting good gas mileage so I don't think it's a leak under the hood. I have been able to narrow down the problem even more from "people telling me what to look for". It makes sense that I have a kink and not a leak.

I didn't figure it out yet. I decided to go skiing this weekend instead of mess with my heater. Plus, the local auto parts store didn't have a vacuum guage in stock (small town) I had to order it.

Good news. I figured it out.

See this thread. for more information.

Old_Man, your answer was the most helpful. I had checked and rechecked all the vacuum lines... I had just never unplugged them while the jeep was running to make sure they actually had vacuum.

Thanks again for everyones help!