Author Topic: '80 C10/305 Broken vapor canister: what should I cap off?  (Read 1187 times)

Offline deadllama

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'80 C10/305 Broken vapor canister: what should I cap off?
« on: March 07, 2019, 10:30:49 AM »
First confession: I've never worked on a vehicle in my life, other than replacing wiper blades and stuff like that. Never even changed oil. I'm learning all this stuff on the fly.

My vapor canister is in a bad way. (See attached photo.) The other lines are attached, but clearly this one isn't. I now know that this probably explains the faint fuel smell around the front of the truck (it's parked under an outdoor carport, not in a garage). Would this broken-off connector be causing a vacuum leak? I see mentions of leaks related to the canister causing issues with idling, and my truck idles pretty rough at stop lights when it's warm.

Is there a way to patch all of this back together, or should I just find a replacement? Is there a something I can cap off until I find another canister that will temporarily solve whatever leak this is causing?

Offline Henry

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Re: '80 C10/305 Broken vapor canister: what should I cap off?
« Reply #1 on: March 09, 2019, 12:34:17 PM »
Hi Deadllama:
Yeah, if you are smelling faint hints of gas and you have a rough idle and your cannister has a disconnected hose, they could all be related. One of your photos shows a hole and crack in the plastic cannister...looks possibly like a rodent chewed on can use some JB Weld adhesive or other epoxy type adhesive for plastics to seal this up. Was your other issue that you had a hose missing on one of the cannister ports? If so, just locate that hose and connect it back on if it has not deteriorated. If you cant find it, go to the Autozone website and check their tech section for what your emissions hose routing should be for your year and engine config truck...if you have trouble figuring it out you can just go to an Autozone when they are at a slow time and one of the sales guys should be able to come out and help figure out what the connections should be from the hose routing diagram...they also should have replacement vac hoses.

As a temporary cap-off until you get everything solved, you need to just follow every hose from your carburetor and make sure it is not hanging loose or leaking. If you find one that is loose cap it off with a golf tee or similar and you can continue to drive around until you fix or replace the cannister and/or hoses. If you only find a split or cracked hose leaking just tape it up temporarily with electrical tape. Inspect all your hoses that go to the cannister...they are probably so old now they are cracking and falling apart...all may need replacement.

Offline bd

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Re: '80 C10/305 Broken vapor canister: what should I cap off?
« Reply #2 on: March 09, 2019, 01:41:58 PM »
Replace the canister.
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline 75gmck25

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Re: '80 C10/305 Broken vapor canister: what should I cap off?
« Reply #3 on: March 18, 2019, 01:24:43 PM »
GM redesigned the vapor recovery system over the years, so different years have different combinations of purge valves, check valves and other features to make them work.   My understanding of the purge valve system is that it allows the system to pull vacuum and evacuate tank vapors under certain conditions, but then closes off the rest of the time to avoid a vacuum leak.  Its best to fix all the pieces and parts with the right parts from your year and engine combination.

If you don't need to have an emissions inspection, the other option is to plug off the hoses in the engine compartment and switch to vented gas caps. 

I found my '75 system did not work right with my newer carburetor (old carburetor had a separate port for the vapor line), so I just connected the vapor canister hose to a fitting I added to the air cleaner body.  It just draws fuel vapor out of the canister and into air cleaner, but since its not on a carburetor vacuum port it does not create a vacuum leak or change the mixture. YMMV.