Fuel sending unit questions

[jpayne79k20]

I have a 79 k20 with a single tank on passenger side. I replaced my fuel and sending unit. I replaced the 2 line sending unit with a new 2 line sending unit. I did fine a third line on my frame which is for the vent. So the person that had it before switch to a two line sending unit. My question with setup work or will the tank not vent through cap. I already had the two line in not seeing the third line on the frame. So if I have to change it I will.

[hatzie]

If you have a vented cap it'll work fine.

[jpayne79k20]

I have the old style that is not a threaded. I'm pretty sure it is not vented.

[hatzie]

If there's no vapor line I believe you need a vented cap...

That being said. I was unaware there were light trucks built after 1974 with no vapor line. GM made em with no return line but I thought they all had vent lines. I also reserve the right to be full of hooey...
Are you positive the two lines are feed and return and not feed and vapor?

[jpayne79k20]

I have a supply, return and vent line on frame. The vent line goes to a black charcoal canister. So im pretty sure who had this truck before me screwed up.

[tucsonjwt]

3/4 trucks were considered heavy duty for emissions purposes and had a short hose off the third nipple with a one way vent cap - same as the vent cap on the differential vent. On my 83 C20, I ran the vent lines back to the rear bumper area so I would not smell gas vapors in the cab. 1/2 tons all had charcoal canisters, as far as I know.

[hatzie]

When there's a return line from the fuel pump there are three hose connections on the sender. Vapor, Return, and Supply.
The 2 hose senders are Vapor and Supply. I don't believe GM used vented gas caps on these trucks.

[jpayne79k20]

So only having two lines is incorrect. Since my cap cannot vent.

[hatzie]

Originally Posted by tucsonjwt

3/4 trucks were considered heavy duty for emissions purposes and had a short hose off the third nipple with a one way vent cap - same as the vent cap on the differential vent. On my 83 C20, I ran the vent lines back to the rear bumper area so I would not smell gas vapors in the cab. 1/2 tons all had charcoal canisters, as far as I know.

A simple basic charcoal canister is very easy to plumb up. It gets rid of the raw fuel smell completely, so you can park it inside, without affecting performance or fuel mileage.
I bet the emissions drones would have a fit if you added it without permission.

Ignore the bowl vent line in this diagram. The purge valve control hose is on ported vacuum and the purge hose feeds to the PCV vacuum hose. If you want to be real fancy put in a thermo-vacuum valve in between the purge valve and the ported vacuum.

Originally Posted by jpayne79k20

So only having two lines is incorrect. Since my cap cannot vent.

I believe so. If you have three hoses then you probably need the three nipple sender.

[tucsonjwt]

On the negative side, a charcoal canister will eventually plug up and need replacement. Gas smell is an issue, but cars were parked in garages long before charcoal canisters existed. One poster here just ran a vent line to the air cleaner and let the carburetor burn the gas vapors.

[hatzie]

Originally Posted by tucsonjwt

On the negative side, a charcoal canister will eventually plug up and need replacement. Gas smell is an issue, but cars were parked in garages long before charcoal canisters existed. One poster here just ran a vent line to the air cleaner and let the carburetor burn the gas vapors.

Yup. TANSTAAFL. If it has parts then they may eventually need replacing.
It's a convenience feature. Mitigating the petrol fumes is more for keeping peace with SWMBO...

[jpayne79k20]

Thank you everyone for your help. I'll see what I can come up with tomorrow.