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

Hi,

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

Let me know.

Joe

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.

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

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?

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.

Thanks. I'm not sure I have that part. it doesn't look familiar to me. Is it part of the cruise control system?

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?

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

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?

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

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?

Also, the sphere-like vacuum canister; can that be capped as well?

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

Here you go; it's the round sphere on the right side, driver side by brake booster.

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

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.

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.

Great, thank you! I don't have the servo in yet, it needs to be replaced, so I'll just cap it for now.

Sounds like a plan.

Let me know if you have other questions.

Take care,

Joe

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?

Is it coming from the vacuum modulator on the transmission? If it has a steel line, chances are that is what it is. Look at the transmission and locate the module. See if it attaches to the steel line. If I recall, it will be mounted near where the transmission dipstick tube is mounted. If that, is it, it simply gets engine vacuum.

If the vehicle has a carb (I don't think it does, but don't recall asking), it will mount to the bottom of the carb at a vacuum port.

Let me know. See pic below.

Joe

Great, thanks. Yes, it’s a steel line, as you described, and yes, I have a carb. Does it matter what port it connects to on the bottom of the carb?

Hi,

I said that backward. The base of the carburetor is ported vacuum. You need a manifold vacuum. There should be a nipple behind the carburetor on the intake that it goes through.

Take care,
Joe

Okay, cool. So, the distributor should hook up to the large vacuum port in the middle of the carburetor in the back side, and then there is a small nipple just next to it, that’ll be for the transmission line?

If it is on the manifold, yes. It's been some time, but I remember the vacuum being attached to the intake manifold behind the carburetor.

The only one I have on the manifold behind the carburetor is a large vacuum port, and I have the brake booster connected to that.

Okay, try using one from the base of the carb. It shouldn't make a difference. The worst case is a slower response from the modulator. However, I can't imagine it would even be noticeable.

That's a whole lot of engine! LOL

Hopefully, when you get it running, you can videotape it and upload it for me to hear.

Take care,

Joe

To confirm; distributor needs ported vacuum, and transmission line needs manifold vacuum? I think I have them reversed right now.

Hi,

Yes, the distributor should be ported. The transmission responds to a drop in vacuum, so you want a manifold vacuum.

Looks great. Let me know how things turn out for you.

Take care,

Joe

Thanks Joe. What about the rear vacuum break assembly? Ported or manifold? Or just plugged?

Hi,

It appears you circled the choke pull-off. That will be a manifold vacuum.

Joe