Vacuum system -how exactly does it work?
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Instructor
Joined: Jan 2020
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From: NYC
Vacuum system -how exactly does it work?
Hi,
I have not been able to find good info anywhere so far.Does anyone know where to find info on how the vacuum system actually works, what it does where, what exactly creates the vacuum, how it's affected and by what etc?
I have an 87 and can hear a hiss when letting off throttle but it disappears when I give it gas. So I started wondering how the entire system actually works.
I know about the vacuum diagram, testing for leaks etc. But that's not what I am asking. It is more the general setup of the system and working principle. Hope that makes sense.
I have not been able to find good info anywhere so far.Does anyone know where to find info on how the vacuum system actually works, what it does where, what exactly creates the vacuum, how it's affected and by what etc?
I have an 87 and can hear a hiss when letting off throttle but it disappears when I give it gas. So I started wondering how the entire system actually works.
I know about the vacuum diagram, testing for leaks etc. But that's not what I am asking. It is more the general setup of the system and working principle. Hope that makes sense.
Le Mans Master
Joined: Oct 2004
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From: Everett WA
C4 of Year Finalist (appearance mods) 2019
Thread Starter
Instructor
Joined: Jan 2020
Posts: 188
Likes: 35
From: NYC
Perhaps this will help with the basics:
https://itstillruns.com/car-engine-c...-12193724.html
https://itstillruns.com/car-engine-c...-12193724.html
And yes, good start, thank you. Still need to find supporting info on the individual systems then, like the hvac line for instance.
Never thought about the entire system before, to be honest. It's usually 'you have a vac leak' and you try to find and fix it without ever bothering to understand the bigger picture.
Reading that the vacuum decreases on throttle was something I never really thought about before but this helps. Now I can imagine why I only hear a hiss off throttle.
Team Owner
Joined: Dec 2000
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From: SE NY
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It's the pistons going down on their intake stroke that create the manifold vacuum when the throttle is nearly closed, actually.
The "plumbing" is just about distribution to systems like brakes, HVAC, cruise and vapor canister.
The FSM for your '87 will pretty well cover the vacuum tube routing for your Vette.
To find that "hiss" I'd start by checking the ends of all visible vacuum hoses for cracks as the rubber tends to dry and split with time and temp cycling.
The "plumbing" is just about distribution to systems like brakes, HVAC, cruise and vapor canister.
The FSM for your '87 will pretty well cover the vacuum tube routing for your Vette.
To find that "hiss" I'd start by checking the ends of all visible vacuum hoses for cracks as the rubber tends to dry and split with time and temp cycling.
Thread Starter
Instructor
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From: NYC
Thanks.
no issues with/about routing (easy to follow sticker and manual) and lines were all replaced.
Last time I checked the vacuum was also where it should be.
I did have a weird whooshing sound at some point bc the intake coupler was not tight enough. But it did go away after reseating it. I'll check that area again though as well as all other lines, just to be sure.
no issues with/about routing (easy to follow sticker and manual) and lines were all replaced.
Last time I checked the vacuum was also where it should be.
I did have a weird whooshing sound at some point bc the intake coupler was not tight enough. But it did go away after reseating it. I'll check that area again though as well as all other lines, just to be sure.
Drifting
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Finalist 2020 C4 of the Year - Unmodified
Test the check valve, up behind the distributor. It's purpose is to keep vacuum in the vacuum tank when there is no vacuum from the intake and it does go bad, stay open. Wouldn't expect that to hiss, unless it's pulling from the intake.
