Which part is this ?

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Godwinaustin

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Hi everyone, new to the forum ! I have a 1983 K20 with the 350

I’m wondering if anyone can tell me what part this is and where a vacuum hose should run off it to ... right now , it appears to not have any vac hose associated with it.

thank you !

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highdesertrange

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the vacuum modulator on the transmission. there is actually a hard line with rubber at both ends. the tranny can't possibly be shifting properly and the engine has to be running lean as hell. highdesertranger
 

Godwinaustin

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I have the SM 465 manual ...
 

Bextreme04

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It's an accessory manifold vacuum. It either needs to be capped or it could run to A/C, Cruise Control, or maybe a vapor canister. I have an 80' K20 and mine runs to the A/C, Cruise Control system, and Vacuum modulator on the TH400
 

1987 GMC Jimmy

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^^^ This except the charcoal canister will use ported vacuum via a carb port, not manifold.
 

WebMonkey

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my '85 k20 has a line coming off that very port.
about 3 inches going up to a tee junction.

it then goes to the driver's side 'vacuum ball' (cannonball shaped container mounted on the fender), and the other side runs through the firewall and onto the back of the AC/heater control 'cluster'.

good luck
 
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just connect it to vacuum ball. it will be okay. no need to micro diagnose.
 

Godwinaustin

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my '85 k20 has a line coming off that very port.
about 3 inches going up to a tee junction.

it then goes to the driver's side 'vacuum ball' (cannonball shaped container mounted on the fender), and the other side runs through the firewall and onto the back of the AC/heater control 'cluster'.

good luck

that’s the one ! Thank you everyone !!
 

dvdswan

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Welcome to the site.
 

Shorty81

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Welcome from the Buckeye State.
 

AuroraGirl

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my '85 k20 has a line coming off that very port.
about 3 inches going up to a tee junction.

it then goes to the driver's side 'vacuum ball' (cannonball shaped container mounted on the fender), and the other side runs through the firewall and onto the back of the AC/heater control 'cluster'.

good luck
Is that so you have vacuum canister? Like to turn air vents when truck is off or have enough while throttle is open? I remember in my 77 LTD If i wanted to change heat vents in use while driving i would move it and hit the gas really quick to snap closed the throttle plate quick.
 

WebMonkey

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Is that so you have vacuum canister? Like to turn air vents when truck is off or have enough while throttle is open? I remember in my 77 LTD If i wanted to change heat vents in use while driving i would move it and hit the gas really quick to snap closed the throttle plate quick.
yes, pretty much just a 'reserve' of vacuum to 'smooth out' any fluctuations from stopping/going/fast/slow etc etc.
 

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Does EVAP use the port that vacc advance does not?

That’s a complicated question because it depends on year/engine/tonnage and federal/high alt./CA emissions schemes, but in short I would say yes. You want to run your vac advance to a manifold source, and your canister control to a ported source. The canister has two jobs: to store vapors when the vehicle’s turned off or idling and to help facilitate the purging of those vapors when the throttle opens. If it was hooked to a manifold source, it would constantly purge when the vehicle was running, and I guess the eggheads at GM/emissions overlords c. the 1970s deemed purging at idle unnecessary.

In a perfect world, vacuum advance would have constant vacuum access, and that’s how it was until things turned smog complex. Then you had distributor delay valves and thermal vacuum switches (TVS) switching from ported to manifold respectively to exert greater control of emissions between cold start and operating temp.

Emissions got way too complicated on these trucks from about 1981-86. I’m in the camp that I think some of this equipment is good to have (e.g. PCV, canister, sometimes EGR), but a lot of it is junk (e.g. EFE, air injection, myriad of TVSs/valves, and an overall spagettified vacuum system), and it should be simplified as much as possible. Some people want to get rid of all of it, and some people live in places where you can’t so I guess I’m somewhere in the middle leaning towards practical deletion.
 

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