Hello, last week the a/c suddenly would only blow from the defroster and floor vents. After some YouTube research, I discovered that the vacuum hose running from atop the engine to a reservoir under the battery tray had disintegrated and become detached. I replaced the hose with new, took for a short drive and thought all was good...very exciting! Later, when accelerating on the highway, noticed the same thing was happening, but would return after cruising on level road. I started watching the vacuum readings on the Torque Pro app and noticed that the vacuum went down to zero during those normal acceleration instances, then returned to 14-15 Hg when cruising. It did this our whole trip...very annoying! Does anyone have any idea what I should check next?

Thank you
2019 G3500

 

Vacuum goes down to zero when the throttle is wide open. There is a check valve I believe built into the splitter that sends the vacuum to the vacuum reservoir ball under the battery and the AC. On my 2015 it's behind the receiver drier. Green arrow is the location, red arrow is the valve / T.

 

This should be the valve.

GM 1550568

 

wow your hose disintegrated after only 5 years?

Triaged is correct. When the throttle is wide open there's no restriction, so air rushes in through the intake as fast as it's ingested by the engine. Hence no vacuum. Hence why there's a checkvalve.

 

I'm not surprised... I had those pictures ready because I just replaced mine a few weeks ago.
broken-exhaust-manifold-bolt-116653/
My van is older but very low miles and I've seen less rotted vacuum lines from the 70's. I'm not sure what they made that hose out of but it's complete junk and was falling apart in my hands.

 

yikes I should check mine. They're 11. I have new hose ready.

 

7/32" is a bit of an odd size. I thought about looking for some silicone but I guess I've got a few years before I have to replace it again.

https://standards.globalspec.com/std/10053546/gm6148m

"Construction. The tubing shall consist of a single wall of non-blooming epichlorohydrin copoly-mer."

Not sure why I looked it up because I have no idea what that is!

 

Thanks for the quick replies Gents. The rotted hose was literally falling apart in my hands...nasty black residue as well. Yesterday I checked the valve to make sure I hadn't mis-routed the tubing, but it appears correct to me. The black end of the T that goes to the manifold is one-way. I did a test drive, getting the same result. Arrrrggghhiie!

I do have a long-standing issue where the a/c doesn't get cold unless it's been running a few minutes, despite having a good charge. Not sure if these two issues are related. I did check the hoses in the passenger footwell and everything seems connected, but I am not an expert on these HVAC systems.

 

I take it you tried to blow through the check valve to make sure it sealed? Sounds like you have a leak somewhere. Vacuum leaks can be hard to find but a hand vacuum pump can help you track them down so you can hear without the engine running.


edit: you have the check valve / T facing the right way.

 

It only goes up to 2013 but GM hasn't changed much or anything since 2003 when it comes to the body of these vans.

https://charm.li/Chevy%20Truck/2013/...0Conditioning/