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Vacuum diagram for my truck? Would you be able to "decode" it for me?

Tiny
TWEBB101
  • MEMBER
  • 1984 CHEVROLET TRUCK
  • 7.4L
  • V8
  • 2WD
  • AUTOMATIC
  • 169,000 MILES
I was wondering if you would mind doing me a huge favor regarding the vacuum diagram for my truck? Would you be able to "decode" it for me, and put it in laymen's terms? I'm looking for something that would say: "run a line from the distributor to the ; run a line from the PCV valve to the carburetor; etc." something like that. Is that something you can help me with?
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Saturday, September 25th, 2021 AT 8:13 AM

31 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

I would be happy to help. By chance, is there a specific one that you are having trouble with?

Let me know.

Joe
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Saturday, September 25th, 2021 AT 8:00 PM
Tiny
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I just rebuilt my engine, Chevrolet 454 in a 1984 C20, and am getting ready to start it for the first time. I have all the pieces ready except for the vacuum system. I just don’t know all the acronyms. Also, I should mention that I have removed the smog pump system.
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Saturday, September 25th, 2021 AT 8:07 PM
Tiny
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Hi,

Okay, let's try this. See if this helps.

Vac act -- vacuum actuator

TRC -- throttle return control

TVS -- thermostatic vacuum system

DTVS -- distributor thermal vacuum switch

EFE -- early fuel evaporation

The others are common, but if you see one that is unknown, let me know.

Take care,

Joe
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Sunday, September 26th, 2021 AT 7:05 PM
Tiny
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Thanks Joe! That helped me be able to draw out a simple diagram to know where to put the lines. In my rebuild, I removed the AIR Pump, and the stock exhaust (I put headers on instead). So, I think that I can remove the spark TVS, the EFE-TVS, and the EFE, and their associated lines. Is that correct? Also, the TRC solenoid cont valve. I'm not sure what that is. Do I need that?
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Sunday, September 26th, 2021 AT 8:55 PM
Tiny
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Hi,

As far as the TRC, take a look at the attachment below. It shows and explains what the circuit is for and what it does.

Let me know if this helps.

Take care,

Joe

See pic below.
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Sunday, September 26th, 2021 AT 9:25 PM
Tiny
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Thanks. I'm not sure I have that part. It doesn't look familiar to me. Is it part of the cruise control system?
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Monday, September 27th, 2021 AT 7:29 AM
Tiny
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Okay, disregard my previous message, I think I found the EFE stuff. My question is now, if I replaced the stock exhaust system with some headers, I wouldn't have the EFE/heat riser anymore, right? So, I'm good to disregard that vacuum line setup?
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Monday, September 27th, 2021 AT 5:50 PM
Tiny
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Hi,

If you installed headers, the heat riser is gone. Just make sure there are no vacuum leaks. If there are hoses not being used, plug them.

Let me know how things turn out for you. I'm interested in knowing.

Joe
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Monday, September 27th, 2021 AT 6:52 PM
Tiny
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Great thanks. I'm still having trouble understanding the TRC portion. Can you help me with that? Do you have a picture of it by chance? Or tell me what it would be connected to?
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Monday, September 27th, 2021 AT 7:09 PM
Tiny
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Hi,

The TRC is basically a stopper. It receives vacuum from the solenoid vacuum which gets vacuum from the intake.

If I remember correctly, this was primarily used to help prevent stalling and dieseling when turning the engine off. It's been a while. LOL

Also, you can run that vacuum hose straight to an intake vacuum. It isn't a ported vacuum.

I hope I'm helping. LOL It's been a while since I had the chance to work on one of these. My old 78 is as basic as they come. LOL

Take care and let me know if I can help.

Joe
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Monday, September 27th, 2021 AT 8:44 PM
Tiny
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Haha, I appreciate your help very much! If my truck has/had this TRC part, I think it hasn’t been hooked up for a long time. Am I okay to run the truck without it?
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Monday, September 27th, 2021 AT 9:15 PM
Tiny
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Also, the sphere-like vacuum canister; can that be capped as well?
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Tuesday, September 28th, 2021 AT 7:56 AM
Tiny
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Hi,

Is it a charcoal canister or is it a vacuum hold canister? Where is it located?

Since you really don't have much of an emissions system anymore, chances are it won't hurt to disconnect it. However, I don't know what the state emissions laws are where you are located. Also, it shouldn't hurt anything disconnecting the TRC. If you have stalling issues, then consider reinstalling it.

Let me know what I Can do to help.

Joe
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Tuesday, September 28th, 2021 AT 6:25 PM
Tiny
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Here you go; it's the round sphere on the right side, driver side by brake booster.
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Tuesday, September 28th, 2021 AT 6:49 PM
Tiny
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Hi,

That is a holding vacuum. Basically, it is a reserve tank. If you didn't have it, the component it supplies vacuum to would lose vacuum under acceleration.

Does that make sense? LOL

Joe
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Tuesday, September 28th, 2021 AT 7:48 PM
Tiny
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Yeah, it does. How do I know what component was hooked to it? Was there a standard configuration for those? I think it went to the cruise Servo, but I have no idea.
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Tuesday, September 28th, 2021 AT 7:57 PM
Tiny
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Hi,

You are correct. It was for cruise control. I actually was able to find a vacuum schematic for it. Take a look below.

I hope this helps. Let me know if you have other questions.

Take care,

Joe

See pic below.
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Wednesday, September 29th, 2021 AT 6:41 PM
Tiny
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Great, thank you! I don't have the servo in yet, it needs to be replaced, so I'll just cap it for now.
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Wednesday, September 29th, 2021 AT 6:54 PM
Tiny
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Sounds like a plan.

Let me know if you have other questions.

Take care,

Joe
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Wednesday, September 29th, 2021 AT 7:21 PM
Tiny
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Hey Joe, one more question: there is a metal vacuum line that runs from my THM400 transmission up to the top of the block, by the distributor. Where does that connect to?
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Wednesday, September 29th, 2021 AT 8:32 PM

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