Fuel Sending Unit Connectors

[68_Step]

This fuel sending unit is from my 78 K30. I know the shorter connector is for the fuel line, but what is the longer one for? Return line or vent?

[68_Step]

Trying again...
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[Sodak]

It's the vent line.

[motornut]

Originally Posted by Sodak

It's the vent line.

yup

[68_Step]

How do I vent this. The PO had it plugged with a short piece of hose with a bolt stuck in it.
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[motornut]

Use a diff vent
Should it be that close tho?
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[tucsonjwt]

X2. Classic Parts sells the one way vent that goes on the differential and also on the sender vent nipple, with a short piece of hose. I extended the fuel line to the back of the truck and terminated it above the spare tire to keep the gas fumes out of the cab. I have dual tanks, so you can see 2 vent lines terminated with the gray plugs. You can also see the differential vent hose capped with the same plug in the foreground of one pic. I have a dump hoist conversion, so that is why it is all visible.

[68_Step]

Originally Posted by motornut

Use a diff vent
Should it be that close tho?
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Didn't vents like this go to the charcoal canister? My truck doesn't have a canister, removed at some point in the past. Maybe run a length of hose (with a vent on the end) to. Protected spot under the flat bed?
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[ray_mcavoy]

Originally Posted by 68_Step

Didn't vents like this go to the charcoal canister? My truck doesn't have a canister, removed at some point in the past. Maybe run a length of hose (with a vent on the end) to. Protected spot under the flat bed?
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It depends on your truck's year and GVW. The cut-off point changed over the years, but trucks under that GVW were considered "light duty" as far as emissions were concerened and they had the gas tank vent line(s) hooked to a charcoal vapor canister. Trucks that were over that GVW were considered "heavy duty" and just had the gas tank vent line(s) capped off with those plastic "differential vent" type caps as described above.

Edit ... just noticed in your signature that your truck is a 78 K30. I believe that would have been classified as heavy duty for that year and therefore wouldn't have originally had a canister. If your flat bed leaves the top of the gas tank somewhat exposed, running the vent line up to a protected area under the bed & capping it off with one of those diff vent covers would be a good idea. The factory setup on HD emissions trucks with a fleetside pickup box just used a short (3 or 4 inch) section of rubber hose between the sending unit & vent cap.

[motornut]

my 78 a Jimmy it runs to the front right where the charcoal canister is in the 79

[ray_mcavoy]

motornut, I think that's actually the vent for your front diff.

Dad and I each have a 77 GMC K15 and that's exactly where the front diff vent is on both of them. They're both 6200 GVW trucks so they're in the HD emissions range for that year (no canisters). And the factory tank venting setup on both of them just has a short 3 or 4" piece of hose connecting the sending unit to the plastic vent cap. Mine has a single tank with just the one vent cap. Dad's truck has dual tanks with a vent cap for each one.

[68_Step]

Originally Posted by tucsonjwt

X2. Classic Parts sells the one way vent that goes on the differential and also on the sender vent nipple, with a short piece of hose. I extended the fuel line to the back of the truck and terminated it above the spare tire to keep the gas fumes out of the cab. I have dual tanks, so you can see 2 vent lines terminated with the gray plugs. You can also see the differential vent hose capped with the same plug in the foreground of one pic. I have a dump hoist conversion, so that is why it is all visible.

I went to Classsic Parts and found this vent, is this it? http://www.classicparts.com/1947-91-.../#.UMfu_Gt5mSM
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[motornut]

Originally Posted by ray_mcavoy

motornut, I think that's actually the vent for your front diff.

Dad and I each have a 77 GMC K15 and that's exactly where the front diff vent is on both of them. They're both 6200 GVW trucks so they're in the HD emissions range for that year (no canisters). And the factory tank venting setup on both of them just has a short 3 or 4" piece of hose connecting the sending unit to the plastic vent cap. Mine has a single tank with just the one vent cap. Dad's truck has dual tanks with a vent cap for each one.

Yup your right, I thought about it,the 78 dosn't have a rad sticker it was gone, thought the can was too
.
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[Sodak]

I bought mine at the local chevy dealer and i just ran mine up the back of the bed header panel.to mid point that is.

[ray_mcavoy]

Originally Posted by motornut

...the 78 dosn't have a rad sticker it was gone, thought the can was too.
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That's actually another difference I've noticed between the light duty & heavy duty emissions setups. Light duty ones usually had the emissions sticker located on the radiator support while the heavy duty ones usually had the emissions sticker on the air cleaner lid.