Vacuum Leak Question

[Warren99]

I suspect I have a vacuum leak somewhere as when I change throttle position signficantly, the HVAC doors loose vacuum and all the air reverts back to the defrost vent. This is especially noticeable when cruising in 5th gear. Also, when you turn the engine off, the HVAC door motors loose vacuum within a second, and I would assume they would hold some vacuum in the reservoir for longer than that.

https://cruiser54.com/?p=121

I haven't inspected my vacuum harnesses thoroughly yet, but I do know that both connections on the valve cover sit at an angle rather than flush, enough to let oil vapor build up over the valve cover over time. My question is could these also be the source of a vacuum leak big enough to cause these issues, or do I need to look for something bigger?

[rokinn]

I recommend looking between the ball behind the front bumper and where it comes through the body up toward the battery.  Common place for it to break.

[Warren99]

I know it has vacuum all the way to the ball but I guess it could have maybe cracked

[Gojira94]

My rig doesn't have A/C so it has the small ball. The glued seam definitely leaks now after 36 years. Dithering a bit on exactly what to use to seal it.

[Warren99]

I still haven't installed the new cruise control motor and vacuum lines and such yet, so that may end up fixing it when I install it. After I install the rest of my CC, if I still have issues then I'll look further into it.

[rokinn]

8 hours ago, Warren99 said:

I know it has vacuum all the way to the ball but I guess it could have maybe cracked

The line just breaks completely in two.

 

[cruiser54]

[Warren99]

On 6/14/2023 at 11:49 PM, rokinn said:

The line just breaks completely in two.

 

I forgot but a long time ago it broke there and I fixed it with a rubber hose, that is not my issue tho.

 

Major change, I finally installed my cruise control motor, and the associated reservoir and lines. After this, the problem with the a/c doors didn’t change at all. Note: rather than replace the line all the way to the intake, I replaced everything after the fork( red in picture)

 

 

[Warren99]

Welp, I had the idea of manually pulling a vacuum on the black vacuum line that goes into the hvac box, and blocking off the white/pink line that comes out of the hvac and onto the heater core valve. When doing this, the vacuum doesn’t really hold at all… I’m assuming this means I have a leak somewhere under the dash. This is even more frustrating considering I had the whole dash and hvac box out a few months ago and didn’t figure this out til now. 
 

Can someone confirm is that’s what that means? And if so, can I test from the plug in the back of the HVAC controls, or am I SOL?

[AZJeff]

Looking for vacuum leaks isn't that hard, assuming you have some sort of vacuum pump.

 

If you think the vacuum leak is in the dash area, intercept the line were it goes through the firewall into the heater box and plug it.  Then use the vacuum pump at the point where the vacuum is sourced at the intake manifold.   If the vacuum holds, then you have confirmed your suspicion that your leak is in the heater box. 

 

If the vacuum does NOT hold with this test, then at least your concerns about having to tear into the dash are over, and now it becomes a case of blocking off various parts of the system and retesting for holding vacuum until you find the area where the failure is located.