Explain dual tank operation

[TrybalRage]

Hey everyone. I have a 1985 C10 with dual tanks, I've owned it for over a year but only ever used the driver's side tank. I verified that the switch worked, seeing that the other tank was empty, and just went back to the driver side tank.

I have a long road trip coming up, so I decided to test out driving with the other side - I put $10 of gas in, flipped the switch over, and it read a fuel level. Drove around like that for a couple of days.

Yesterday I filled up the passenger side, and then flipped the switch to the driver side tank - I was going to leave it full for my trip. The gauge correctly showed (Driver)half full/ (Pass) full when I flipped it back and forth. Kept it on the driver's side, and went home.

This morning, fired up the truck, and the tank immediately read Full. This is the driver's side, that just yesterday was showing half. Flipped the switch to passenger, and it now reads only 3/4 full.

WTF?

Where I park, there is a slight lean towards the drivers side. Is there some kind of crossover tube that balances out the levels? I wasn't aware of one, but I've never really looked or used the tank, so I'm a little puzzled.

Now I'm worried that the fuel level isn't reading correctly, and again with this long trip I'm not thrilled that I'll basically be guessing how much gas I have.

Any insight? Thanks in advance.

[chrismoore701]

I have a 79 and there is a switch that draws the gas from the specific tank selected. As well as changes the tank it is reading on the gauge. There is no cross over that could cause the gas to switch sides. Sounds like your gauges maybe a little out of whack. Mine is working now, but when the gauges didnt work before, I would fill both tanks and when the one with the bad gauge would cause the engine to stutter, I knew it was time to switch really fast before the truck died. good luck

[SCOTI]

More than likely once you drive it a little the gauge will read different again & possibly back to where it was. There is no factory cross-over/balance tube between the tanks.

On my dually, the Pass side tank always reads higher than full once I start driving & then drops down relatively quickly once I'm nearing 1/2 tank volume. Since my truck only gets about 8mpg, I simply fill the tanks & then switch around the 150mile mark (20gal tanks).

[Firebirdjones]

My 79 has dual tanks, love it for the long trips. I haven't had trouble with my gauges reading fuel levels when switching but you will find that as you drive the levels may change. When I switch sitting still, I usually at least go down the road 2-3 miles and watch the gauge, they settle in..

[Dead Parrot]

If your fuel system has returns from the pump to the tank, it is possible a PO modified the returns to both to to one side. Maybe the PO had trouble filling a particular tank at his favorite gas station. Or the switch valve isn't switching the returns. Or he replaced a 6 connection valve with a 3 connection valve and just used a T to connect the returns.

It is common for the senders in each tank to read different. Full on my driver's side tank is just past the F mark, E still leaves about 50 miles. Full on the passenger side is about 10* beyond F, E is be ready to switch tanks.

[masterlock]

I agree with the above thoughts. The PO of my K20 had the return for both tanks running into the passengers side tank due to a leak in the drivers side tank. If you put gas in the DS it would return into the PS and over flow. Replaced the tanks and fixed the plumbing now all is well.

[Tom]

Originally Posted by TrybalRage

Now I'm worried that the fuel level isn't reading correctly, and again with this long trip I'm not thrilled that I'll basically be guessing how much gas I have.

Dude, you have two giant gas tanks. Run one till its out, then switch to the next and fill up at the next station. What exactly is there to worry about?

[tucsonjwt]

The dual tanks on my 83 C20 will be erratic sometimes. When switching tanks, it sometimes takes five or ten minutes to register correctly. I don't know why that happens, but I suspect that the contacts on the switch itself are not making good contact (either corrosion on the spade contacts or degraded wiring.)

I would think that as long as you are actually getting fuel from both tanks at some point, then the switching valve and other apparatus below the truck are working OK, and I would start with the switch itself.

If you have not already done so, I would crawl under the truck and make sure you have good fuel lines (both hard lines and rubber hoses) and there is no evidence of leaking. Depending on how energetic you are, I think it is always a good idea to replace the rubber lines on any newly acquired old truck.

For research, I would fill both tanks full, and observe which tank is selected. Then drive long enough to mostly drain that tank and refill that tank. Did the tank fill up with as much fuel as you would expect? If so, then that tank is reading correctly. Select the other tank and try the same experiment.

[RaceProMarketing]

I agree with Tom. I had a '79 when I was in college and neither tank registered. I'd fill them both up. Run off the left until it sputtered, then run off the right until I got to a station or my destination.

This truck had a massive Olds engine in it from the PO, so it got like 7 MPG; still I never had any trouble or ran out of gas, even when I would drive home from school.

Whew, that was in 1992. What I'd give to have that truck back!

[wilkin250r]

Chevy had two different fuel selector vavles within the 73-87 body styles. The early version was a single wire, 6 ports, with the ground connection being the frame body. Applying 12V to that single wire would activate a solenoid that switched the tanks, and removing power would switch the tanks back.

The later version was still 6 ports, but it had SIX wires connected to it, and instead of a solenoid, it used a small motor. This meant that removing power didn't "switch back", it stayed at the same point until you actually REVERSED the polarity of the power, and then it would switch the valve. Your 84 should this later style.

[68post]

I had a '78 that would read different the next day and I believe I checked the gauge and the same amount was different in the other tank. I think it siphoned from the other tank overnight somehow, unless this is impossible -I could imagine this could happen if there were some sort of pressure imbalance ?