HVAC vacuum line diagnosis

[Vetteman61]

Hello,

1987 K5 Blazer. After sitting for 7 years, the air mostly only blows out of the floor. I have checked the vacuum line under the hood and everything is good. I have applied vacuum to the defroster vent and it does operate when I apply separate vacuum. I unbolted the dash selector and all 5 vac. hoses are hooked up (one was empty which I thought was a problem, but according to the manual one is not used and is blocked off internally).

My thought was that If I turn the engine on and disconnect the rubber vac. line fitting on the back of the dash selector I should be able to feel one of the lines sucking, but I do not feel any of the lines providing vacuum no matter how I change the selector. My thought is that there must be something wrong between where the supply line comes through the firewall and where it connects to the dash selector.

Am I correct that with the engine running I should be able to put my thumb over one of the holes and feel suction? It looks like in order to trace the line back to the firewall fitting I would have to remove the entire HVAC box under the dash. Is this correct?

Dash selector vacuum fitting

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The vacuum lines going into the back of the fitting. All 5 are there. According to the manual the empty fitting at the bottom that I am bending with my finger is supposed to be empty and is blocked off internally with the selector.

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Thank you,
Brandon

 

[gmbellew]

lots of good info here https://www.gmsquarebody.com/threads/heat-ac-blend-doors.30548

 

[C10MixMaster]

Hello,

1987 K5 Blazer. After sitting for 7 years, the air mostly only blows out of the floor. I have checked the vacuum line under the hood and everything is good. I have applied vacuum to the defroster vent and it does operate when I apply separate vacuum. I unbolted the dash selector and all 5 vac. hoses are hooked up (one was empty which I thought was a problem, but according to the manual one is not used and is blocked off internally).

My thought was that If I turn the engine on and disconnect the rubber vac. line fitting on the back of the dash selector I should be able to feel one of the lines sucking, but I do not feel any of the lines providing vacuum no matter how I change the selector. My thought is that there must be something wrong between where the supply line comes through the firewall and where it connects to the dash selector.

Am I correct that with the engine running I should be able to put my thumb over one of the holes and feel suction? It looks like in order to trace the line back to the firewall fitting I would have to remove the entire HVAC box under the dash. Is this correct?

Dash selector vacuum fitting

You must be registered for see images attach

The vacuum lines going into the back of the fitting. All 5 are there. According to the manual the empty fitting at the bottom that I am bending with my finger is supposed to be empty and is blocked off internally with the selector.

You must be registered for see images attach

Thank you,
Brandon

yes you should have vacuum / you should be able to access the vacuum lines by removing the glove box . I would start under the hood the vacuum lines tend to rot under the hood not so much under the dash.

 

[C10MixMaster]

Hello,

1987 K5 Blazer. After sitting for 7 years, the air mostly only blows out of the floor. I have checked the vacuum line under the hood and everything is good. I have applied vacuum to the defroster vent and it does operate when I apply separate vacuum. I unbolted the dash selector and all 5 vac. hoses are hooked up (one was empty which I thought was a problem, but according to the manual one is not used and is blocked off internally).

My thought was that If I turn the engine on and disconnect the rubber vac. line fitting on the back of the dash selector I should be able to feel one of the lines sucking, but I do not feel any of the lines providing vacuum no matter how I change the selector. My thought is that there must be something wrong between where the supply line comes through the firewall and where it connects to the dash selector.

Am I correct that with the engine running I should be able to put my thumb over one of the holes and feel suction? It looks like in order to trace the line back to the firewall fitting I would have to remove the entire HVAC box under the dash. Is this correct?

Dash selector vacuum fitting

You must be registered for see images attach

The vacuum lines going into the back of the fitting. All 5 are there. According to the manual the empty fitting at the bottom that I am bending with my finger is supposed to be empty and is blocked off internally with the selector.

You must be registered for see images attach

Thank you,
Brandon

yes you should have vacuum / you should be able to access the vacuum lines by removing the glove box . I would start under the hood the vacuum lines tend to rot under the hood not so much under the dash.

 

[PrairieDrifter]

You don't have to pull the box out, my 84's main vacuum source is a single line through a grommet in the firewall next to the outer box, this supplies the little multiple line connector in your pictures. I replaced all of the engine bay hardline with super small I.D. vacuum hose.

I have the grommet circled in my picture.

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[75gmck25]

You can reach all the lines with the main heater box still in place. However, if you remove the vertical plenum piece (only 3 screws, I think) behind the glove compartment, and then also remove the plenum piece running across under the radio area, you have a lot more visibility of everything.

You can splice the old hard lines with short sections of small diameter rubber line. In the Dorman or Help! Section at the parts store they also sell rubber vacuum T’s that you can splice in for test points to check for vacuum.

Bruce

 

[Vetteman61]

Thanks for the replies. As I stated above, all the lines and connections under the hood are OK.

I have an '87 so the computer is mounted behind the glovebox and on top of the HVAC box. I haven't been able to reach around it to trace the wires. If I remove the computer would I be able to access the area on the firewall where the main vacuum feed comes through. I have found the vac. hose that comes over to the kick-panel diaphragm (it has a T fitting), but I cannot trace it back to its source due to lack of room.

 

[gmbellew]

clips get broken on the blend doors and the rod that ties the two main blend doors together gets knocked loose. apply vacuum and see if you can switch from floor to vent air flow. if not, i would remove the glovebox door and the vertical plenum piece and see if everything is operational with the main doors. if everything is good there, you will probably have to remove more ductwork to get at where the vacuum lines hook into the pods that operate the doors and make sure they are hooked up.

 

[Craig 85]

I ran into a problem with my '85. The pods were all good and I replaced a lot of the under-hood vacuum lines for the HVAC as the PO has cut them. I determined that the valve that the connecter you pictured was my problem. Of course it's discontinued.

The valve has a small O-ring type gasket inside and mine was dried out, causing a vacuum leak. You may be able to use Vaseline or something similar to lube it. I purchased a complete NOS control head for about $200. As of this writing they are $165-300. Problem solved after the new unit was installed.

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[PrairieDrifter]

I ran into a problem with my '85. The pods were all good and I replaced a lot of the under-hood vacuum lines for the HVAC as the PO has cut them. I determined that the valve that the connecter you pictured was my problem. Of course it's discontinued.

The valve has a small O-ring type gasket inside and mine was dried out, causing a vacuum leak. You may be able to use Vaseline or something similar to lube it. I purchased a complete NOS control head for about $200. As of this writing they are $165-300. Problem solved after the new unit was installed.

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I believe I have the same issue, I can literally hear a vacuum leak at certain times operating my hvac controls.

 

[mcjay]

i would check vacuum supply at vacuum reservoir underhood. its round black plastic about 4-5” diameter. reservoir canisters have been known to split. run engine, pull hose off canister, check vacuum on line, reinstall if present, turn engine off, listen closely for air sound when removing hose from canister, a good one will hold vacuum over night if no leaks inside cab

 

[Vetteman61]

To follow up, there was a very small crack on the vacuum line about 1 inch after it came out of the firewall. The only way I could tell is if I started the vehicle, let vacuum build up, and then ran around to listen to the hose while I moved it in that spot with my fingers. A small hissing would come in when moved in the right way.

 

[Vetteman61]

I also meant to add that I had to cut the engine off, then run around to the engine bay, because the sound of the leak was so quiet.