78 Chevy Dual tank replacement

[mcast]

Have a question regarding 78 chevy dual tanks, I have searched and searched and find limited or different information. So I figured I would give this forum a shot. If this is the wrong area to ask my apologies.
I have replaced both tanks on 78 chevy with new sending units. Old sending units had 3-outlet on driver side and 2-outlet on passenger side with a 3-port selector switch. Previous person that worked on it years ago cut lines and ran line directly from passenger side to carb bypassing 2nd tank.

I have replaced selector valve with a new 3 port and replaced sending units with same ones that were stock (3outlet,2 outlet). I appears my fuel pump also has 2 outlets

My problem is trying to reroute the fuel lines. My question, can I just run driver side (3outlet) supply line to selector valve, passenger side supply to selector valve and selector valve to carb?

The return line from the carb, can I just run it to passenger side tank? Can connect vent lines to a T and vent to rear of vehicle.

Any suggestions or diagrams would be helpful..

Thank!

 

[4WDKC]

Have a question regarding 78 chevy dual tanks, I have searched and searched and find limited or different information. So I figured I would give this forum a shot. If this is the wrong area to ask my apologies.
I have replaced both tanks on 78 chevy with new sending units. Old sending units had 3-outlet on driver side and 2-outlet on passenger side with a 3-port selector switch. Previous person that worked on it years ago cut lines and ran line directly from passenger side to carb bypassing 2nd tank.

I have replaced selector valve with a new 3 port and replaced sending units with same ones that were stock (3outlet,2 outlet). I appears my fuel pump also has 2 outlets

My problem is trying to reroute the fuel lines. My question, can I just run driver side (3outlet) supply line to selector valve, passenger side supply to selector valve and selector valve to carb?

The return line from the carb, can I just run it to passenger side tank? Can connect vent lines to a T and vent to rear of vehicle.

Any suggestions or diagrams would be helpful..

Thank!

Welcome,

If you route the return line to ps tank only it will fill the ps side when using ds tank. The valve should route the return fuel to the correct tank. You may have gotten an early valve that was not designed for return fuel system, is the only thing I can figure out.

 

[mcast]

The selector valve was the stock one as far as I can tell, do you know if it would it be possible to just cap off the return line. I read somewhere along the way someone did that. Your thoughts on that.

 

[Charlie]

 

[4WDKC]

The selector valve was the stock one as far as I can tell, do you know if it would it be possible to just cap off the return line. I read somewhere along the way someone did that. Your thoughts on that.

If you cap the return line you run the risk of vapor locking the truck as the fuel would heat up under the hood.

 

[dvdswan]

Welcome to the site. My 78 had the 3 port switching valve. one from each tank and one to the engine. My fuel pump was a 2 port which was one from the switching valve and one to the carb. No return lines.

 

[mcast]

dvdswan were your sending units only 1 outlet on each?

 

[C10MixMaster]

Have a question regarding 78 chevy dual tanks, I have searched and searched and find limited or different information. So I figured I would give this forum a shot. If this is the wrong area to ask my apologies.
I have replaced both tanks on 78 chevy with new sending units. Old sending units had 3-outlet on driver side and 2-outlet on passenger side with a 3-port selector switch. Previous person that worked on it years ago cut lines and ran line directly from passenger side to carb bypassing 2nd tank.

I have replaced selector valve with a new 3 port and replaced sending units with same ones that were stock (3outlet,2 outlet). I appears my fuel pump also has 2 outlets

My problem is trying to reroute the fuel lines. My question, can I just run driver side (3outlet) supply line to selector valve, passenger side supply to selector valve and selector valve to carb?

The return line from the carb, can I just run it to passenger side tank? Can connect vent lines to a T and vent to rear of vehicle.

Any suggestions or diagrams would be helpful..

Thank!

You have mixed and matched parts. NO you cant run a return line to 1 tank.

The early systems had tanks with 2 lines, 1 for fuel, 1 for vent. From there the fuel lines went to a three line switching valve 1 line from each tank and 1 to the fuel pump. the vent lines "T" together and connect to the vapor canister. the fuel pump on this system is simple in and out

Later systems had tanks with 3 lines, 1 for fuel, 1 fro fuel return, 1 for vent. From there the fuel lines went to a 6 line switching valve 1 line from each tank and 1 to the fuel pump the return lines also go thought the switching valve 1 from the fuel pump return a 1 to each tanks return line . the vent lines "T" together and connect to the vapor canister. the fuel pump on this system has an aditional return line to prevent vapor lock.

if you chose the later system you will need to rewire it, a different tank switch , tank valve and a second sending unit with 3 connections. If you keep the early system you can just plug the return line on the one tank. you might be able to plug the return line on the pump but just to be safe replace it with a 2 line pump.

 

[dvdswan]

dvdswan were your sending units only 1 outlet on each?

Yes, it was a single port outlet. I bought them at NAPA about 2 years ago. You could get a 2 port and put a short hose on it and plug it or install a longer hose that would be couple inches higher than the tank with a breather valve on it like the differentials have. The return line is about an inch long inside the tank so unless you tilt the truck hard or flip it, it won't leak.

 

[Frankenchevy]

It would be best to replace the two port sender with the correct 3 port sender. That way each tank has supply to the valve, return from the valve and vents.

 

[C10MixMaster]

It would be best to replace the two port sender with the correct 3 port sender. That way each tank has supply to the valve, return from the valve and vents.

only problem is the wiring is different between 3 and 6 port switching valves. if i read his post rite it was originally the earlier style.

 

[C10MixMaster]

Yes, it was a single port outlet. I bought them at NAPA about 2 years ago. You could get a 2 port and put a short hose on it and plug it or install a longer hose that would be couple inches higher than the tank with a breather valve on it like the differentials have. The return line is about an inch long inside the tank so unless you tilt the truck hard or flip it, it won't leak.

single port sending units would be from before evap canisters, when the gas caps were the vent.

 

[Frankenchevy]

only problem is the wiring is different between 3 and 6 port switching valves. if i read his post rite it was originally the earlier style.

Yep, you’re right. I missed that part!

OP, can you clarify what fuel pump you have? Does it have two or three nipples? If just two, you could simply cap the middle sized(5/16”) port on the 3 port sender. If you have all other components that are not return style, that may be the most simple fix.

Last thing you’d want is a fuel pump return circumventing the selector going to only one tank. That’s asking for an overfill situation.

 

[75gmck25]

There are different versions of the six port valves. I have an old one ('81?) that works the same as the 3 port valve, and it only has one 12 volt connection post on it. When activated it will switch both supply and return fuel lines. The vent lines are just T'd together and run up the the evaporative canister in the engine compartment. It works fine with my OEM 3 port sending units and 3 port fuel pump (stock for my '75 ).

Most trucks with six port valves seem to have a newer version that has multiple terminals in a line and a larger plug. I'm not sure how this valve and switch works. The old single-post six port Pollack valve seems to be no longer available.

Bruce