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HVAC vent to defrost uphill, cruise works

D

Desert4x4

NAXJA Forum User
Location
Phoenix
So my vents switch to defrost when going up a hill (basically flooring it).
But the cruise control works flawlessly. Also the check valve on the manifold and the one going to the firewall by the heater hoses both work fine.
Also the system held a vacuum overnight; when I took the vacuum hose off the manifold I could hear the vacuum realeasing.

So is my problem in the dash?
 
For the love of god, when you find this please let me know!

I've been chasing the same issue with the exact same symptoms for a while now.

The typical suggestion is to check for cracks in the vacuum ball and cracks in the hoses under the battery tray. All of that has been replaced on mine.

I'm thinking the actual vacuum "motor" is bad at the Vent/Defrost mode door on mine but I haven't pulled the whole thing apart yet on a hunch. When it finally switches back to vent you can hear the door slam shut which leads me to believe the motor is worn.

I also have an idle issue that I think is connected somehow.

I've searched every inch of vacuum line in the engine bay and I've found nothing.
 
Last edited:
^^^^ Me too! It's driving me nuts in my '98. It cuts out under moderate to heavy throttle on the freeway and I start frying because of no AC. Checked every vac line I can find, been this way for years.
 
Lowrange2 said:
For the love of god, when you find this please let me know!
I'm thinking the actual vacuum "motor" is bad at the Vent/Defrost mode door on mine but I haven't pulled the whole thing apart yet on a hunch.
Click to expand...

Done this in the 97 i have, DIDN'T work. still does it if i have my foot to the floor and no other time. I am talking high RPM's
sorry i am not much help.
 
scottmcneal said:
Done this in the 97 i have, DIDN'T work. still does it if i have my foot to the floor and no other time. I am talking high RPM's
sorry i am not much help.
Click to expand...


Mine does it at around 2000-3000 RPM while pulling a hill. It has to be under load though. If I'm on flat ground at 3k rpm it doesn't do it.

I've replaced all the lines from the T at the firewall (At the hose that goes to the HVAC box) down to the vacuum ball. I've checked all the fittings and hoses from that T over to the intake. I replaced the elbow at the intake since it was collapsing at idle.

Perhaps a have a massive vacuum leak on another portion of the system?

Isn't there a line that goes to the fuel tank? PCV system?

I have great vacuum at the T at the firewall while at idle and WOT. I don't get it.
 
Ill hop on the "me too" bandwagon with my 98'. Anything over 11" of MAP and it does it. It just goes to defrost until I turn the fan motor down. My best guess is the vacuum motor itself. It normally just goes to the defrost vents but sometimes the blend door just starts closing completely making a horrible hissing noise until I cycle it off then on again. Ive just learned to live with it because Im not about to tear out the dash hunting for broken vacuum lines. I have thought about doing the smoke machine trick to see if I could pinpoint the leak.
 
I can live with it but my kids can't. 98* and 90% humidity is too dang hot in the summer time to be roasting the kids when I drive up a hill. lol
 
Mine has done this for years as well. I just figured the ol' tired 4.0 coupled with the bigger tires and 3.55's was to blame. Like many others, I have replaced every damn vacuum line and tee in sight, -surprise, -it didn't do a dang thing. After reading that everyone else is dealing with the same issue, it has got me to thinking (I know, -dangerous!). I'll bet the vacuum diaphragm is pulling the damper open against a spring (which is why it returns closed when there is insufficient vac to the diaphragm).

Ok, well, I'm just making stuff up now, but if this is the case, I wonder if the problem could be remedied with a spring that has less tension? If the geometry of the pullback spring isn't retarded/ridiculous, maybe a lighter spring could be used? Maybe the spring lives inside the diaphragm can? Maybe there is a check valve somewhere that isn't holding and allowing the vac to bleed back the other way when manifold vac dips?

Does anyone know where the vac-can and damper are?

Personally, I'd have no issues with just operating the damper mechanically by some means (vice grips, etc.), -no AC or defrost going up even the slightest hill sucks..
 
Use one of those mityvac hand pumps to test for a leak in the HVAC system. Find the line going to the HVAC, attach pump there and give it a few pumps until it holds.. if it doesn't hold there is a leak in the system.
 
Ok. So I found at least one of the problems. The actuator linkage for the defrost vents keeps falling out. I can push it back in, but then after driving around it falls out again. I'm thinking of gluing it in there now.
 
Lowrange2 said:
You can access the piece you're referring to though the radio hole?

What year is your Jeep?
Click to expand...

Yes. It's a 1999. When you remove the radio, you can see one of the pivot lever thingys on the left side in the back of the radio hole. Switch the HVAC controls between defrost and vents to see the piece move. For me, it was obvious that the pivot piece was not pushed in all the way and I could see it slopping around. A good thick coating of superglue on the end of the pivot kept it from slipping out.
 
Hey new to the site was reading a little bit and I don't know about the heater problem but I just replaced the head gaskets on my 5.2 and I was having a problem with the idle and it bucked at high rpms and I figured the bucking out and a lot of the idle had to do with my pvc. Valve int the valve cover don't know of its any help
 
This was always a big issue for the older renix XJ's. Generally it was being caused by not enough vacuum. (FYI, when you're running at high RPM under load, you get less vacuum) probably why you mentioned the cruise working ok.

The vacuum reservoirs usually have different sides used for different things, IIRC one side controls the HVAC motors (there is one inside the dash, and another on the outside controlling the flow of hot water). The other half of the system is used to control other things like your cruise.

It sounds to me like half of your vacuum system may be leaking.

As far as retaining that rod, it probably originally used something like this:

http://www.bakfin.com/starlock/imperial-roundshaft.html
 
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