HVAC Vacuum Leak Questions/ random hole in the firewall

[Warren99]

When driving with the air on, it seems using more than 20% throttle is enough to force all of the vacuum motors to close, making the air come out of the defroster, then they return to how they are supposed to be when I let off the gas. I assume this is the product of a vacuum leak somewhere. I haven't really checked under the hood yet, but right now my whole interior and dash is out, so is there anything I can look for/test on the HVAC box since I have access to it? Figured if there was a time it would be now.

 

Also, there is this big hole diagonally above and to the left of both my fuse block and hood latch cable. What is this for? What is the best way to plug it? Will it be easy to find factory plugs for it from XJ's at the junkyard?

 

[Pete M]

most vacuum leaks are in the engine bay. 

[schardein]

For the vacuum leak, it's common to find the vacuum line to the vacuum reservoir not plugged in.  The vacuum reservoir is mounted to the back side of the front bumper on the pass side.  The hose must be disconnected to remove the bumper, and it's easy to forget to reconnect it.  I've done it myself.

[howeitsdone]

7 hours ago, Warren99 said:

Also, there is this big hole diagonally above and to the left of both my fuse block and hood latch cable. What is this for? What is the best way to plug it?

 

It's a grommet. Measure and buy a grommet plug and you'll be good to go.

 

[pizzaman09]

I covered that hole with a piece of gorilla tape.  

I also had the same HVAC issue under throttle.  When I installed cruise control I got a full new vacuum harness and accumulator ball in the front bumper, that fixed the vacuum leak that was causing the HVAC to not work.  If I was to take a guess, I think I had a bad accumulator.

[cruiser54]

[Warren99]

13 hours ago, howeitsdone said:

 

It's a grommet. Measure and buy a grommet plug and you'll be good to go.

 

Ok thank you.

[Warren99]

5 hours ago, pizzaman09 said:

 

I also had the same HVAC issue under throttle.  When I installed cruise control I got a full new vacuum harness and accumulator ball in the front bumper, that fixed the vacuum leak that was causing the HVAC to not work.  If I was to take a guess, I think I had a bad accumulator.

 

14 hours ago, schardein said:

For the vacuum leak, it's common to find the vacuum line to the vacuum reservoir not plugged in.  The vacuum reservoir is mounted to the back side of the front bumper on the pass side.  The hose must be disconnected to remove the bumper, and it's easy to forget to reconnect it.  I've done it myself.

For the longest time my controls wouldn't leave the defroster until I eventually found my accumulator ball and reconnected it, which then resulted in what I have now, where it works unless I am under throttle.

I have a whole new system and ball that I got with the cruise control from an 87 Chief. I was going to put that whole thing in and see if it fixes it. Right now, I have the entire dash off so if there was any potential cause for this problem on the HVAC box I wanted to look before putting the new dash on.