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73-87 Chevy _ GMC Trucks => Fuel Systems and Drivability => Topic started by: firebirdta84 on July 01, 2013, 06:08:35 am

Title: 1981 Dual Gas Tank Hose/Line Diagram Needed
Post by: firebirdta84 on July 01, 2013, 06:08:35 am

Hey guys,

I inherited a 1981 GMC Sierra 3/4 ton that I am in the process of restoring. The truck originally came with the two gas tanks. When I got the truck, only the driver's side tank was there and the lines bypassed the missing second tank and the valve. I've tried searching for a diagram with no luck. Can anyone help me out as to what I need to connect to get this thing working? Both fuel sending units have 3 ports, as does the valve:

(http://i131.photobucket.com/albums/p314/firebirdta84/2013-05-28_15-15-18_205_zpsc25781e5.jpg) (http://s131.photobucket.com/user/firebirdta84/media/2013-05-28_15-15-18_205_zpsc25781e5.jpg.html)

(http://i131.photobucket.com/albums/p314/firebirdta84/2013-05-28_15-15-29_781_zps59b96dc7.jpg) (http://s131.photobucket.com/user/firebirdta84/media/2013-05-28_15-15-29_781_zps59b96dc7.jpg.html)

Thanks
Joe

Title: Re: 1981 Dual Gas Tank Hose/Line Diagram Needed
Post by: Irish_Alley on July 03, 2013, 01:30:26 am

Quote from: bd on June 19, 2013, 01:07:09 pm

5/32" port to carburetor ported vacuum, 1/4" port to manifold vacuum (often teeing into the PCV valve plumbing), 5/16" port to fuel tanks and ~3/8" shrouded vent port to atmosphere.

http://forum.73-87chevytrucks.com/smforum/index.php/topic,26271.msg219070.html#msg219070

Title: Re: 1981 Dual Gas Tank Hose/Line Diagram Needed
Post by: bd on July 03, 2013, 10:20:10 am

The 3-port fuel tank pickups have 3/8", 5/16" and 1/4" hose nipples.  The 3/8" is supply to the fuel pump, 5/16" is vent to vapor canister (each tank vent will tee together, before running to the canister via a single steel line), and the 1/4" is fuel return from the mechanical fuel pump (the return line helps to combat vapor lock in hot climates - used on 3/4 and 1-ton chassis).  If the fuel pump doesn't have a 1/4" return nipple, you can cap the 1/4" nipples on the senders. 

The selector valve you need, 3-port vs 6-port,  will depend on whether you are using the 1/4" fuel return to the tanks.  Only the supply lines route through a 3-port selector valve.  Whereas, the supply lines and the return lines route through a 6-port valve.  Running the return through the selector valve is important to prevent the tanks from transferring fuel between them and potentially overfilling.

Regarding the valve, the paired nipples (2 on a 3-port; 4 on a 6-port) that exit the same side of the valve connect to the tanks, the single nipple or double nipples (1 on a 3-port; 2 on a 6-port) that exit the opposite side of the valve connect to the fuel pump as described above.

Title: Re: 1981 Dual Gas Tank Hose/Line Diagram Needed
Post by: firebirdta84 on July 04, 2013, 08:37:19 am

Awesome...that is exactly what I was looking for. Thanks!