Carbon Canister to Fuel Tank Hose

[ea6driver]

I was reinstalling all the hoses leading to/from my carbon vapor canister today and I noticed something interesting.  The short hose that connects to the steel line leading back to the fuel tank isn't simply a rubber hose.  In the attached photo, i'm referring to the rubber hose connected to the canister port labelled by a blue dot (Fuel Tank). As I was about to reinstall it I thought I'd blow into it just to make sure that it wasn't clogged and, surprisingly, I felt resistance that seemed like a clog.  Upon further investigation, I discovered that there is an obstruction in the hose that seems purposeful. This obstruction seems to be metallic and has a small hole through the center of it (see photo of hose end).  I couldn't find any reference as to the purpose of this hose restriction anywhere. Could anyone shed some light on exactly what this is? What is it called? What is its purpose?

 

Sorry if Fuel Injection isn't the best place to post this question...It was either here or Engine/Drivetrain.

Thanks!

Edited September 9, 2020 by ea6driver

[Captain Obvious]

They used restrictors like that in several locations over the years. Most times added later as an "afterthought" fix for some sort of issue they were encountering. And I believe they were put in to slow a signal down or smooth a rough one out. Sometimes they talked about the little restrictors, and sometimes they didn't. Small detail that sometimes didn't get any press attention.

I don't know what the specific purpose is for the one you found there. I would expect that to already be a smooth signal, so I don't know.

What year are you working on?

[ea6driver]

It's a 78.  I purchased it in 1980 from the original owner so I'm pretty sure it's the original factory hose.  Thanks for that insight.

 

[Zed Head]

That might be the mysterious check valve, #17.  I've often wondered where, exactly, it sits in the car.  We've had conversations about it in the past, what it is and what it's for and how it works, but I don't recall anyone ever finding it.  Maybe you've made a critical discovery.

They show it as a separate fixture but, like the flame arrestor in the PCV hose, maybe they just jammed it in to the hose.

Edited September 9, 2020 by Zed Head

[AZDatsun]

That’s what you find when you go blowing on hoses

 This sounds bad but can you suck on it? Is it a one way valve?

 I am guessing it allows vent from the tank to the canister but not back to the tank?

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[Zed Head]

Wait.  CO found it a while ago.  I forgot.  Yours might just be a flow control device.

 

[ea6driver]

Yeah...after blowing and sucking on both ends...it seems like it's just a "restrictor" because I encountered the same amount of resistance no matter what....either that or, if it is some kind of check valve, then it's broken.

[Captain Obvious]

It's just a restrictor. As mentioned above, the check valve a different device.

A little history... 1974 is the first year for the carbon canister system to capture and contain fuel tank vapors. Prior to that, they used the "flow guide valve" system in 73 and earlier.

So, 74 was the first year for the CARB can, and on page EF-33 of the 74 manual there is a diagram of the evaporative emissions control system. Note that they call out the existence of that orifice. Also note that in subsequent years, they dropped that diagram from the FSMs. So my guess is that orifice was carried forward from 74 on, but they just didn't bother describing it in the manuals.

I didn't study each and every manual from 75 to the end, but I took a quick look at a couple of them and didn't find any mention.

Here's the diagram from the 74 manual that shows the restrictor orifice:

 

:

 

 

[ea6driver]

Wow!  Thank you for that not so obvious Z emissions systems lesson Captain Obvious!  I always try to do some research before posting a question on the forum, but I admit, there wasn't a chance I was gonna discover that answer. I'm just glad you didn't find that diagram in the 78Z FSM!

Until your response this morning I didn't think we were gonna get a likely answer to this orifice mystery...My hat's off to you sir!

[Captain Obvious]

What? It wasn't obvious?         Kidding aside, glad to help.

Yeah, they took stuff out of the manuals over the years, and sometimes they added stuff. But it seems more removal of detail than added.

(And I didn't look at the 78 manual (I stopped at 77), but you better believe I will now!!)) Hahaha!! 

Edit - It's not in the 78 manual. No surprise there.

Edited September 12, 2020 by Captain Obvious
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