1973-1991 Dual fuel tank systems theory of operation

[Dleslie212]

Originally Posted by MiniD

It is not possible to wire it so the returns go to the "wrong" tank. The tank switch switches both the supply and return with the same signal. It is possible to wire it so that the wrong tank is selected but the supply should be coupled to the return.

Of course, there are ways to screw that up but they involve re-plumbing your fuel lines. Just re-wiring can't do that.

That's what I was thinking. I can easily just cross the return lines, but it bugs me not knowing why that's happening

[Dleslie212]

I went ahead and replaced the six port valve today, just to make sure it wasn't a problem with the valve itself. As I was connecting the six lines, I double checked the paperwork/diagram and the routing of the lines to make sure everything was in the correct spot.

Right now, both my tanks are fairly full. I filled them a few weeks ago, drove maybe 20 miles, and since then has been just idling in the driveway a total of maybe 20 minutes - so both the tanks are still pretty full. I don't have a fuel gauge hooked up just yet.

When running on one tank, if given long enough, the opposite tank starts spewing gas, as if the returns were backwards but they're not. If I switch tanks when the spewing begins, it will stop spewing after a second, and after awhile start spewing from the opposite tank

The strange part is that, before I swapped the valve today, the old valve and lines had been in place for almost three years with no problems, even with both tanks completely full. Nothing ha changed between now and then except for a complete rewire of the truck, selector switch included. Tanks, senders, straps, fuel lines and regulator were all replaced at the same time about three years ago.

Anyone have any ideas?

[SunSoaked]

Originally Posted by Dleslie212

I went ahead and replaced the six port valve today, just to make sure it wasn't a problem with the valve itself. As I was connecting the six lines, I double checked the paperwork/diagram and the routing of the lines to make sure everything was in the correct spot.

Right now, both my tanks are fairly full. I filled them a few weeks ago, drove maybe 20 miles, and since then has been just idling in the driveway a total of maybe 20 minutes - so both the tanks are still pretty full. I don't have a fuel gauge hooked up just yet.

When running on one tank, if given long enough, the opposite tank starts spewing gas, as if the returns were backwards but they're not. If I switch tanks when the spewing begins, it will stop spewing after a second, and after awhile start spewing from the opposite tank

The strange part is that, before I swapped the valve today, the old valve and lines had been in place for almost three years with no problems, even with both tanks completely full. Nothing ha changed between now and then except for a complete rewire of the truck, selector switch included. Tanks, senders, straps, fuel lines and regulator were all replaced at the same time about three years ago.

Anyone have any ideas?

Do have a charcoal canister and is it connected to the vent line?

[Dleslie212]

Originally Posted by SunSoaked

Do have a charcoal canister and is it connected to the vent line?

No charcoal canister. The vents on the two tanks are teed together by the selector valve, with a single line running up to a breather under the hood. But no canister

[hatzie]

The charcoal canister isn't a big deal to plumb and it isn't a performance hit.
It's only three connections.

1. a vacuum line from the valve to a ported vacuum source,
2. a tee to the PCV valve for the purge port on the canister
3. a connection to the vents on the tanks

[Dleslie212]

Originally Posted by hatzie

The charcoal canister isn't a big deal to plumb and it isn't a performance hit.
It's only three connections.

1. a vacuum line from the valve to a ported vacuum source,
2. a tee to the PCV valve for the purge port on the canister
3. a connection to the vents on the tanks

How would a charcoal canister keep gas from spewing out of the tanks?

[Dleslie212]

I spent a bit of time after work today troubleshooting. I started the truck up running on the left tank, and removed the gas cap. After a few minutes, I can see the gas level rising up through the fill port, until eventually it starts spilling out the fill port on the left tank.

Once that happened, I removed the power wire from the selector switch that feeds the left tank, so only the right tank was hooked to the switch, and started up again. Same result

I disconnected the two vents where they tee together and plugged them off to rule out fuel somehow crossing from one tank to the other via the breather hose, and same result.

Finally, just for ****s and giggles, I swapped the return lines at the fuel selector valve, and same result.

I have no idea wtf is happening.

[Hart_Rod]

Originally Posted by Dleslie212

I spent a bit of time after work today troubleshooting. I started the truck up running on the left tank, and removed the gas cap. After a few minutes, I can see the gas level rising up through the fill port, until eventually it starts spilling out the fill port on the left tank.

Once that happened, I removed the power wire from the selector switch that feeds the left tank, so only the right tank was hooked to the switch, and started up again. Same result

I disconnected the two vents where they tee together and plugged them off to rule out fuel somehow crossing from one tank to the other via the breather hose, and same result.

Finally, just for ****s and giggles, I swapped the return lines at the fuel selector valve, and same result. That seems like the only way you could get fuel transfer from one tank to the other.

I have no idea wtf is happening.

Look at post #146. It sure seems like your return lines are crossed. That seems like the only way you could get fuel transfer from one tank to the other…

[Dleslie212]

Originally Posted by Hart_Rod

Look at post #146. It sure seems like your return lines are crossed.

I swapped them last night just to try it out, with thr exact same results

[Hart_Rod]

Originally Posted by Dleslie212

I swapped them last night just to try it out, with thr exact same results

I would completely disconnect one of the tanks feed and return line and then power up the other side. This should help you narrow it down.

[Darkrider]

Great info in here! It is nice to know I can ditch the earlier problematic switch with the simple 2-position switch from the newer trucks. I'm hoping to get my second tank operational sometime this year. I have a feeling I should pick up the switch and a new valve at minimum, then go from there. The entire 10 years I have owned the truck I am not even sure what is in the left tank if anything lol.

[Hart_Rod]

Originally Posted by Dleslie212

I swapped them last night just to try it out, with thr exact same results

Originally Posted by Hart_Rod

I would completely disconnect one of the tanks feed and return line and then power up the other side. This should help you narrow it down.

Did you try this?

[69Tom]

OK, so I still have my original stock system on my 74 C20. Actually, I need to put it back together, as it was leaking and I needed to reseal it. In any event, that's not why I'm here.

I just picked up a short bed step 74 C10. I'll be buying all new wiring for it. It did not originally have dual tanks. It will have a pretty much stock 454 in it. Because of the 454, I want dual tanks that work.

So question is, if starting from stock, what's the best system to go with? Is there someone that just sells a kit (I saw old posts containing a kit, but they're 6 years old - yikes - time flies!). Looking forward to your recommendations.

Thanks in advance.

[hatzie]

I'd install the 81-91 Pollack Motorized valve system. The solenoid valves equipped for return lines are made of unobtainium. Nobody makes them.

If you're doing fuel injection get the 87-91 16 gallon TBI tanks with the fuel pump baffles and the in-tank fuel pump & sender. Wire it the same as the 87-91 TBI setup.

The filler neck diameter differs between 73-80 & 81-91 tanks. I've forgotten whether the 73-80 tanks were smaller or larger diameter. They are definitely different.

[69Tom]

Originally Posted by hatzie

I'd install the 81-91 Pollack Motorized valve system. The solenoid valves equipped for return lines are made of unobtainium. Nobody makes them.

If you're doing fuel injection get the 87-91 16 gallon TBI tanks with the fuel pump baffles and the in-tank fuel pump & sender. Wire it the same as the 87-91 TBI setup.

The filler neck diameter differs between 73-80 & 81-91 tanks. I've forgotten whether the 73-80 tanks were smaller or larger diameter. They are definitely different.

Thanks Hatzie, appreciate the help. Nope, going pretty basic/old school with this truck. It will be carbureted. Do I pretty much have to piece the kit together for this?

With respect to filler necks, they're a pain to locate for stepsides to begin with. I ordered the stock 73-80 tanks. Was that something I should change?

[hatzie]

The only reason you'd want the TBI tanks is for fuel injection with in-tank fuel pumps.

You will want the senders with return line hookups. Not all 73-77 senders had return line provision.

[Mbeef61]

I have a question. Long story short I have a 78k10 with existing dual tanks. I am hooking up 87 tanks and senders for efi (fitech), and 97 dual in tank pumps for pressure. I have u7001 selector valve. My truck has the one wire switch (5 terminals with tan, tan/white, red/black, blue and green). I am switching to the reverse polarity dpdt switch (D7089C). I realize I will need to run a couple wires to do that.

I have read the first posts a few times. I seem to understand the wiring and switch etc. I seem to be falling under the 87 TBI section. I understand the selector valve switches between gauges and sending units.

What I don’t understand fully is how the selector valve switches between pumps. Also my fitech has a single orange wire to the fuel pump. How do I get this incorporated into the switching. My assumption is a relay, but I don’t know which or how. Can you shed any light on that?

Sorry my brain is mush as this point but thanks for the information you have provided so far.

[hatzie]

Originally Posted by Mbeef61

I have a question. Long story short I have a 78k10 with existing dual tanks. I am hooking up 87 tanks and senders for efi (fitech), and 97 dual in tank pumps for pressure. I have u7001 selector valve. My truck has the one wire switch (5 terminals with tan, tan/white, red/black, blue and green). I am switching to the reverse polarity dpdt switch (D7089C). I realize I will need to run a couple wires to do that.

I have read the first posts a few times. I seem to understand the wiring and switch etc. I seem to be falling under the 87 TBI section. I understand the selector valve switches between gauges and sending units.

What I don’t understand fully is how the selector valve switches between pumps. Also my fitech has a single orange wire to the fuel pump. How do I get this incorporated into the switching. My assumption is a relay, but I don’t know which or how. Can you shed any light on that?

Sorry my brain is mush as this point but thanks for the information you have provided so far.

The TBI tank switch is a polarity reversing switch. The power and ground from the switch to the valve shuttle motor are wyed to the fuel pump power leads.

Each fuel sender has three wires, Fuel pump power, ground, and sender.
The fuel pump gets power from the fuel pump wire when that pump is selected and ground from the ground wire always.

If you have the LH tank selected the LH fuel pump wire is hot and the RH fuel pump wire is grounded. Meaning the LH pump has + & - and the RH pump has - & -.
If you have the RH tank selected the RH fuel pump wire is hot and the LH fuel pump wire is grounded. Meaning the RH pump has + & - and the LH pump has - & -.

The DPDT switch inside the valve selects which sender to feed to the fuel gauge.

[Mbeef61]

So just forget about the orange wire with 12v from the fitech unit? And just utilize the 12v from the dash switch?

Or would running this orange fuel pump wire to the 12v at the dash switch be best to retain the fuel pump prime function?

[hatzie]

Originally Posted by Mbeef61

So just forget about the orange wire with 12v from the fitech unit? And just utilize the 12v from the dash switch?

Or would running this orange fuel pump wire to the 12v at the dash switch be best to retain the fuel pump prime function?

Power the dash switch from your ECM. That's the way the TBI system works.