928 Vacuum Troubleshooting
 

I have a 1985 928S that has HVAC issues. I'd like to start looking in to this problem by performing a comprehensive test/repair of the vacuum system. Are there sources that lay out a process for such a project?

Thank you,

pits928s

This is great: Dwayne's Garage HVAC vacuum (1987)

It is for an '87 S4, so there may be differences, I don't remember. My car is an '86.5, so more like yours, and I think this was pretty close, if not identical. There are other good resources as well, including some threads here.

Thank you!

Here's another helpful tutorial as well. https://members.rennlist.com/pirtle/svc_act.html

Update and more questions
 

Thank you both for the links to the info I requested. I have gone through the vacuum troubleshooting procedure and found the following:

- The braided line that supposedly leads to the intake flap actuator was disconnected, causing the vacuum leak in the HVAC system. I capped this line and tested for vacuum and it held vacuum OK, Therefore my HVAC, reservoir and cruise control connections are holding vacuum. I actually have a very slow leak-down when the engine is turned off, but otherwise it maintains about 17 inches of vacuum once the reservoir is filled upon start-up. Is this within range of where the system is supposed to operate? Should I be chasing down this small leak? (I lose about 1 inch per minute when the engine is shut off)

- Do I have an intake flap on the '85? I've had the intake manifolds off and cleaned them thoroughly and don't recall the flap. Of course that would lead to the question of where this line should be connected. I took a close look to see where this vacuum line might go but could not find it. Can you help here?

Thanks,

I bought all my vacuum actuators and diaphragms from Roger. My car is an S3 and will hold vacuum for days. I can come back to the car two days after it's last run and hear the flap close when I turn on the ignition.

Thank you for your input. I am still looking for the answers to my questions:

1 Is 17 inches of vacuum within the expected operating range, and if so is the slow vacuum leak I have worth chasing down and fixing?
2. Where does the braided vacuum line from the check valve connect?

Thank you!

I think if I remember correctly between 15-20 is within range. All vacuum leaks should be chased down. Is the check valve the blue and black one? This may help you I hope:

https://cimg6.ibsrv.net/gimg/rennlis...fb2fa725c0.jpg

Thank you Scott

I found the slow leak - turned out to be the vacuum manifold. FSM refers to it as the header. I was able to find a vacuum diagram of the HVAC system in the FSM. The diagram shows the connection I am looking for is just a test port. My FSM only shows the 84 and newer AC update, so I am going with this in absence of any known update in 1985 (my model year). I have attached the diagram for reference.

Dave (pits928s)