Here's the deal, if my car is not under load I don't have vents. To clarify my HVAC slider only lets air come from the defrost vents until I am moving. I can put it at any position and the air comes from the right spot so I'm stumped. Last week I replaced the vacuum ball and the heater diverter valve. Any ideas?

The car is a 1991 Z28.


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Loss of vacuum would put the air out the defrost as a default.
Check from your vacuum source to the check valve to the resivour and to the line going through the center of the firewall to the controls.
Maybe one of the hard plastic lines has a hole.

Not long ago I fixed that for my brother on his F150. He cleaned the battery but left all the dried up acid to fall onto some plastic lines and it ate through. His vents still worked but would witch to defrost at times.

 

is there a certain spot/place I should have the vacuum ball attached? Right now it's going into the side of the plenum.

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I think that is where it goes into mine as well. The fittings right above the FPR?
Has it always done this? Do you have a check valve? It should be close to where the line connects to the plenum and the plastic line going through the firewall.
The check valve is not really needed when its vacuum right from the manifold but maybe when its from the upper plenum a check valve is more needed.

 

Hmm I don't have a check valve. There is no smog stuff on my car so if that valve is part of the charcoal canister it's long gone. can you get me a picture or a part number?


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Its not smog related stuff. I think I have a pic on photbucket but I have been having troubles with that. All different now!
I think I have one on the shelf in my garage, I'll go see if I can take a pic of the one in my car and if not maybe the one sitting on the shelf (if its still there!)

 

Alright thanks, hopefully that's something I can pick up from work and finally get this straightened out.


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The valve allows vacuum one way but not the other. So if its not there and when vacuum drops it may be the cause of your issue.

 

perfect, last dumb question. which direction should the valve be going?

 

If I remember right the other side is flat. So the taper will go towards the manifold. There will also be a third nipple on the valve that goes to the vacuum resiviour ball that you cant see in the pic because its to the back.
You can also suck or blow on the valve to see which way the air flows (just do that in the privacy of your own home ). And remember you want the intake side to be able to draw the air through.

 

update:

today I bought a one way vacuum valve and installed it. now it's not the same OE one because that one is discontinued. so I still don't have vents until I rev the car up in park to 2k rpm, then the air blows from wherever I selected but eventually defaults back to defrost. the valve is in between the manifold and vacuum ball.

where are the hoses going on your valve?

 

As you see in the pic the one hose goes to the source.
The other hose you see to the left goes into the harness which goes inside the car for the switches.
Then there is a hose on the backside which is not seen in the pic. It goes to a T which spits off and goes to the ball and the cruise control. If you dont have cruise you would not have that T in the line.

Also there is another vacuum line in the harness from the switches. It goes to the heater control valve. Its a much longer plastic line that the one that goes to the check valve. But even it that one is disconnectd it would not cause trouble. When I got my car that heater control valve was missing and the line was left open.

Maybe, if you have Cruise, pinch the hose off that goes to that unit and see if it works.

 

I had the cruise line just hanging in the back so that was one problem, but on my egr solenoid there's a nipple on the back. should i have that blocked off or also going to the manifold?

 

The valve shouldn't be discontinued. I just bought one off the rack at Advance Auto 3 days ago. Cost about 4.50. If you were looking up the original GM part number, that number has likely been superceded a dozen times by now.

 

What's the part number. And yes the gm part is discontinued but was only $1.88