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I'm tracing a vacuum leak that's causing the reservoir to drain down to 0 psi in about 5-10 seconds after shutting the engine down.
Everything seems to be working OK when the engine is running and I'm pulling about 18 psi - except for the defroster/heater modes (non-A/C car).
Only If the mode wheel is in DEFROST, the vacuum doesn't leak out after shut-down. Any other mode, and it bleeds off to zero pretty quick.
I confirmed that both vacuum hoses are connected to the Defroster Door Valve as well as the Air Door Valve (see Willcox diagram below - but ignore the red circle from another thread).
It does seem to be switching modes "somewhat", but not what I'd expect.
"OFF" is definitely OFF.
But I get strong air blowing from the Defroster in all 3 other modes ("Defrost", "Defog", and "Air"). I would expect NO Defrost air up top when it's set in full "Air" mode. And then a blend when it's in "Defog"
So, I'm, suspicious of the Defroster Door Valve.
So, I next clamped off both hoses just before they go into the Defroster Door Valve, and I can get the Vac Reservoir to hold vacuum much better after engine shut-down.
Could it be a bad Valve?
What I don't look forward to, is trying to replace this Valve while the HVAC Module is still installed. Looks like it will be the 2 vacuum lines, some form of fasteners, as well as the linkage to the door itself.
Any tips or hints on getting this done while still installed and the dash is in place?
Or am I barking up the wrong tree?
Is there some other reason for this vacuum drain coming from that area?
Or, even further - is this normal behavior for that valve? Should it let the system drain off to atmosphere after the engine is shut down?
Also wondering if adding a check valve somewhere in the line that supplies vacuum into the whole interior HVAC system would be an easy fix?
Last edited by Bergerboy; Sep 8, 2023 at 09:22 PM.
I just replaced the Defroster Door Valve , in my 69 with no A/C ,for the same reason , you must remove heater box to replace it. You will also need a heater box seal kit .NO ,the system should hold vacuum with engine off .Also replace all vacuum hoses since its all apart .When sill in car, hook up a hand held vacuum pump, to check each valve to make sure all are working as they should .
Steven
I disagree that the system SHOULD hold vacuum after shutdown. It's more like.... how slow can you get it to leak down with 50 year old switches & motors. Short of replacing them all.
Might test your canister while yer at it. How much trust would you put in a rusty 50 year old Folger's can? And yer 0ne-way valve. BTW the same valve for a Camaro is $20 less on Ebay.
Go get 20' of tubing at your FLAP and replace every hose you can while you're in there.
Seals on the flapper doors. No fun at all.
Last edited by wadenelson; Sep 10, 2023 at 11:44 AM.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Start by cutting off a 1/2 inch from the end of each hose so they fit tight.
If that doesnt fix it unplug all the hoses and plug the ends and see if the leak persists. Hook them up one at a time until you find which one is bad
You may want to find a small electric vacuum pump so you can easily draw down the system. I run a harbor freight pump in a different room so i can here the leak. Use a vacuum gauge, dont go beyond 20 inches Hg
Vacuum Leak - Under-Dash
