3 outlet VS 2 ooutlet fuel sending unit question

[1972BackInBlackC10]

i have a 1980 gmc that im replacing the fuel tank in, the truck has dual tanks but im only replacing the pass side tank and not running the driver side, anyways i want to put a new sending unit in and theres one with 2 outlets and one with 3 outlets, what sending unit do i need if im only running the passnger side tank?

i thought that maybe the 3 outlet was used on dual tank trucks and the 2 outlet is for single tank trucks, but from what iv read im starting to think the third outlet is for a vent line? can anyone help me out here???

[ray_mcavoy]

The 2-line sending unit has a feed line (usually 3/8") to the fuel pump and a vent line (usually 5/16") that goes to either a vent cap or a charcoal vapor canister depending on what type of emissions equipment the truck has.

The 3-line sending unit has an additional return line (usually 1/4") that connects to the fuel pump. These were most common on larger engines and/or with A/C. The purpose of the return line is to help prevent vapor lock.

If your truck has a 2-line fuel pump (inlet/outlet), go with the 2-line sending unit. But if you've got the 3-line fuel pump (inlet/outlet/return), then you can go with the 3-line sending unit if you want to run a return line.

[1972BackInBlackC10]

the fuel pump on my truck has the two rubber lines that go to the back and the one metal line that runs to the carb

[ray_mcavoy]

Okay, you've got the 3-line fuel pump that is normally used with the 3-line sending unit.

[1972BackInBlackC10]

so metal line to carb is obiously the fuel line, one rubber is the inlet and the second would return line correct? and on the sending unit one will be the feed line one will be the return and the third is the vent line?

[ray_mcavoy]

Yes, that's correct. And as I mentioned earlier, each one has a different size line to help keep from getting them mixed up.

[1972BackInBlackC10]

excellent thanks ray now i can order my sending unit on mon