need help tracing a vacuum leak in the HVAC system

[ciffer]

i noticed and replaced a broken vacuum line under the hood a few weeks ago while doing some regular maintenance. since then i can hear a vacuum leak in the dash which seems to be from the HVAC system. i can hear a hiss in all positions of the HVAC selector, including "off", except for "Bi-Level" position. in the bi-lev position, it seems to work correctly, i can feel warm air coming out of the vents.

where do i start looking? i am not any good at looking for this type of thing.

 

[lectric80]

On the back of the controller is a large vacuum line plug, make sure that it is fully seated. From there you need to trace those lines down to the actuators that adjust your doors. Make sure they are all connected and not leaking. There are several lines, but few actuators, so it really isn't hard.

Since it works in one position, I would assume that some of the lines were removed during a heater core install, and then never hooked back up properly. Probably the ones that are the hardest to get to, but even then, if you take your time you shouldn't have any trouble finding them.

 

[ciffer]

that is entirely possible, i replaced my heater core last year. probably never noticed it because of the cracked vacuum line under the hood. are the vacuum lines for the HVAC system available if any need to be replaced?

 

[lectric80]

I don't believe that the lines themselves are available, but that is mainly due to their mostly rubber construction that lasts a long time. Pretty rare to have to replace the lines, but I have never seen them. You could just use some small rubber hose to replace any that are bad.

 

[ciffer]

i have the the dash apart, of the vacuum lines i can see, i can't see anything disconnected. i didn't take out the box that houses the heater core and mix door; i see that the vacuum lines run behind it.

the hiss sounds like it is coming from the area of the HVAC controls. is there anything else that could cause the hissing in the HVAC controls?

i'll try the smoke trick tomorrow and see if i can find anything that way.

 

[lectric80]

If it is strictly coming from the controls, it could be that the selector is broken internally and allowing a leak. It could also be that the connector itself is so stiff that it cannot close correctly over each point. The smoke trick should give you an indication of where the leak is. If it is the controller, it is about $27 from LMC, and is called the rotary controller.

 

[ciffer]

i can't see the smoke doing anything around the controls. it is definitely a vacuum leak, i pulled the hose that feeds through the firewall from the engine, won't hiss when disconnected.

it sounds like it is from the controls, but also sounds a little like it is coming from behind the heater box. i'll grab a cheap stethoscope and see if i can get a better bearing on where the leak is. if it is behind the heater box, then the heater needs to come out again to fix. if that is the case, it isn't going to be fixed this year, to close to winter to risk screwing up my heater.

 

[lectric80]

I can't blame you there. I will say that leak anywhere further down the line could cause you to hear it in the control, because the vacuum is still hissing through the lines.