Have noticed in my new to me XJ that when I am cruising at 50+ my air system will act weird. Say I am going down a hill it will stay in the vents into the cabin but when I give it gas to go up a hill or the transmission shifts from 4th to 3rd it will blow air through the defrost vents and then if I let off the gas it will go back to the vents where I have it set. It does this in AC mode and heat mode. If it's in defrost mode it will stay on defrost no matter the speed or gas. 1998 Sport 4.0 with AC

 

I think I'm having A similar problem I can hear the vent doors in the –dash opening and closing sometimes. Hope somebody chimes in with a solution without having to rip open the dash or tear out the dash? 90 xj i6 aw4 242 Tc. Does your heat blow hot .?and AC cold?

 

The AC does blow cold and the hear works.

 

Sounds like you have a vacuum leak. Start by checking the vacuum ball behind the front bumper on the passenger side. Check it for any cracks or loose hoses. If none fallow the hoses all the way to the firewall. The vacuum is what controls the blend door which controls where the air comes out of. Hope this helps.

 

^^^ Agreed on the vacuum leak. FYI to the OP, with no vacuum in the system, the HVAC will default to blowing out the defroster. That's why you lose your vacuum when you're going up hill. Since the throttle is open, there's no vacuum in the intake manifold.

 

Yep I have a vacuum hose that's cracked and another flopping around on the intake.
Not my thread but thanks!
Ryan

 

Here's some photos of the HVAC/Cruise vacuum system in the engine bay - mixed years.


Click the image to make it larger and click it again to make it larger yet.


https://www.cherokeeforum.com/g/album/3212409

 

Found it
The guy I bought this from must just tapped it and I didn't notice till now.

 

That ain't it mate. Keep looking.

 

My god I love this thing but it's proving to be a real pain in the a## as a first vehicle.

 

This?

 

This? Could that cracked hose be the problem? That's between the body and battery.

 

You're looking for a cracked or broken plastic tube.

 

First vehicle? You mean to say you have never owned a vehicle before?


My first vehicle was a 1940 Ford sedan. That was 1960.

 

Do you see that split corrugated tubing resting against the end of the fuel rail? Post #8.


Move it up and away from the fuel rail. Secure it up with zip ties.