Tell me what this is and prove it.

[SquareRoot]

Mechanical fuel pumps with 3 ports. In, out and a return. The return is a 1/4 barb. Is it a return or a vent? Google search gives conflicting info. Asking because im converting to return style system with 87 tanks and EFI. With the stock 3/8" feed lines I need at least 5/16th return line. HOWEVER, the 5/16th line on my 85 is the VENT line, NOT the return line. To add to the confusion the return line on my new 87 sending units is indeed the return line. So it looks like I'll need to use the factory "Vent-5/16th" line as the return line and the factory " Return line-1/4 inch as the vent line.

Ya'll following? Prove me wrong or clarify on the OEM 3rd port function on the mechanical pumps.

 

[PrairieDrifter]

Every tank system, will have a vent, whether it's a hose running to a charcoal canister or to the atmosphere with a vent valve. However not every tank system will have a return, my 84 has a vent line and it also has a return line. Can confirm it's a return line, as fuel runs back through this line when the truck is shut off after running. I believe the supply is 3/8, the vent is 1/4, and the return is 5/16.

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[SquareRoot]

It's odd to me the 1/4" vent comes off the mechanical fuel pump? Unless it changed in 85, my 1/4 goes to the pump, 5/16th to the canister.

 

[PrairieDrifter]

85 was a year that quite a few things changed. A lot of sites like to say 81-84, then 85-87/91. So it is possible it changed. I will have to confirm also, I can check tomorrow. I just remembered I have an 85 lol, I can check that.

Ive never heard of a pump having a vent line, if it has the third port I've always known it to be a return. Then the vent always runs to a charcoal canister or the atmosphere.

 

[75gmck25]

On my ‘75 the supply line is 3/8”, vent is 5/16” and return is 1/4”. This often does not meet the requirements of newer High flow pumps that want a larger return line.

The stock fuel pump has no vent line; just supply in from tank, return to tank, and supply to carburetor.

 

[Grumpy]

I am not able to locate one around here, look for the Emissions Decal on either the upper Radiator Cover or on the Air Filter Cover.
Almost everything you need to know about the hose connections for that particular vehicle will be printed on it.

 

[AuroraGirl]

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my truck has no evap emissions and the return goes back to the tank if you follow it (smaller line) the bigger one is the supply. it vents at the tank on sender with a small axle vent thing.