Fuel sending unit: 3 line vs 2 line

[KP Jon]

what's the difference (Other than the number of lines), or purpose, of the three line sending unit vs the 2 line sending unit on an '83? I'm switching from a straight six to a V8 but want to make sure what I put back in the tank (Driver's side small tank) is correct for the V8.

Thanks!

[74CustomK20]

Really, the 2 line is just supply and return. The 3 line is supply return and vent to the charcoal canister. If you have a 3 but are removing all at stuff you can just the line.

[KP Jon]

Originally Posted by 74CustomK20

Really, the 2 line is just supply and return. The 3 line is supply return and vent to the charcoal canister. If you have a 3 but are removing all at stuff you can just the line.

Thanks!

[cadillac_al]

I would always use the 3 line so I could have the vent. Sometimes these tanks are finicky to fill when the vent gets pinched or plugged. If you don't want to run the vent to the charcoal canister you can use a differential vent like the 70's trucks did.

[74CustomK20]

That's what I did. I originally had a 3 line sender but deleted the emissions stuff. Used an axle vent cap and spliced like a 6" piece of line to sending unit. No problems since.

[KP Jon]

Thanks- I'll have to look at where to vent it.

[tucsonjwt]

My 83 C20 has the factory dual tank set up and it has 3 nipple sending units. The third nipple just has a short piece of fuel line hose capped with the same one way vent cap as the differential vent cap, as was stated above. You will get some gas vapor odor near the cab with this set up, so I ran my vent hoses to the back of the truck and terminated them just above the spare tire near the rear bumper. Now the gas smell is far enough away from the cab that I don't smell it when driving.

[LostMy65]

Subscribed for future reference.

[hatzie]

The 1970's charcoal EVAP system is very simple and it has zero damaging effects on mileage or performance. It's not worth bypassing it.

If you run without it you'll get gasoline odor from evaporation. This can actually get noticeable inside a garage.

If you don't have the simple two or three hose EVAP can then find one for a 1970-81 F-Body or 1970's Corvette.

The vent hose goes to the Vent connection on the EVAP charcoal can.
The Purge hose is teed into the PCV valve hose.
The purge valve vacuum actuator is teed into the vacuum advance hose or some other ported vacuum pip on the carburetor.
If the can has a float bowl connection put a cap on it. The float bowl on the Carter Thermoquad, Holley, Rochester 2G, and Rochester Quadrajet vents directly into the clean side of the air cleaner.