I've been trying my best to understand what's going on here, and since there isn't much info about my specific issue, I'll try and go into as much detail as I can about it.

Okay, so, for starters, when I got my 95, the air was coming out of all the vents. (Defrost, panel, and floor vents) The air speed was the same from each vent, and from my understanding, is a vacuum-related issue.

So, I went to AllData (if you don't know what that is, it's a subscription service that has any and all info on pretty much everything), and I found a diagram of where the vacuum lines are, and what door they control. The first thing I checked was if there was a vacuum present in the line, and if all the check valves were working and installed correctly.

The check valve at the end of the intake on my 2.5 was working, and I had a vacuum there. I then traveled further to the pass. side firewall, to the little T junction, and checked the check valve there.

At that T junction, I have a vacuum going into the firewall, as well as to the vacuum canister. Now I didn't have enough time to check the actual canister to see if it can hold a vacuum, but from what I understand, is that it's only there to hold a vacuum when the car is off, so it doesn't have to re-generate a vacuum.

Then, I dove into the firewall.

The first thing I noticed was that the recirculation door stayed closed on the Normal setting, and did not change when I slid it over to Max. So I checked to see if there was a vacuum at the diaphragm, and there was none.

Now, I can confirm the slider is functioning right because I can switch between A/C and heat, but the vent operation doesn't change. So it looks like the issue is somewhere near the source of the vacuum. But at the same time, why am I getting a vacuum going into the firewall, but no vent operation?

So, next up, I took a look at everything behind the HVAC control unit and found something weird. It looked like I was missing a wire connecting the left side of the control unit. There were 5 prongs, but only 4 wires. The connecter also wasn't in good shape, so that makes me think that could be the source of my problems.

What should I do next?

 

Turns out, that the missing wire isn't actually missing.

Back to the drawing board. Again.

 

Wish I could help. My 96 has a problem with the doors as well. I have heat and air, but vents don't work as they should. I've checked the vacuum lines, reservoir, and check valves up to the firewall and have no issues. I replaced both check valves with AC Delco ones since they're inexpensive and replace one very short section of hose off the intake manifold. It holds vacuum very well. So next is to dive into the dash and look at the control unit and vacuum lines inside. Unfortunately I won't be able to that for another 2 weeks. I believe I found a vacuum diagram somewhere for under the dash, so if I find it again I'll post it.

 

Hopefully this will help. It's for an '88 but might be the same for '95 and '96.

 

The vacuum can is there to hold vacuum under acceleration. Likely the vacuum can is bad or the hoses/pipes to it.

 

It is more likely under the dash if the car runs fine and doesn't act like it has a vacuum leak. I have no issues under the hood with vacuum lines, reservoir, or check valves. The blend doors have vacuum lines controling them so if the doors don't work properly and air doesn't flow where it should its in the control unit, vacuum lines, or actuators. The OP has also checked under hood first so I believe we both have an issue under the dash. I'll report back in a couple weeks once I can have a look and trace the lines.

 

Not true. ^^^

 

I've seen a lot of guys treat that vac reservoir like an appendix and toss when they put a custom bumper on there, and then later wonder why they have vacuum problems. Pretty common for it to break the mounting bracket and be hanging loose too.

 

Okay. I did some more testing. The vacuum reservoir holds a vacuum, but the line going into the firewall cannot hold a vacuum. I plan on smoke testing that line to see where it’s leaking from.

I also discovered that all the diaphragms are not getting vacuum, so I think this issue is localized.

And as a side note, I cannot figure out where the main “vacuum” box is that controls the vacuum to each diaphragm. I thought it might be the HVAC control unit, but I was wrong. It seems that it must be hidden deeper in the dash

 

It's the hvac control in the dash that routes the vacuum. The blend door is cable operated and sometimes the clip breaks where the cable attaches down by the pivot arm for the door.
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