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Old 10-03-2023, 08:08 PM   #1
olblue81
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Dual tank problem: appears to switch side but only seems to draw off left tank

1981 GMC longbed 2wd. Just got home from a 2500 mile trip to Cruise to the Woods in Iowa. The switch and valve have worked fine for years and for 1800 of those miles. I was running on the left tank and getting low, so I filled up both tanks and selected the right tank. I drove 1.5 hours and noticed the fuel gauge had not dropped. I switched to the left tank and saw that it was down below half. I switched back to the right and gauge goes up. My next fuel stop I see that the right tank still selected, still shows full. I filled the left tank and it took 14 gallons yet I had selected the right tank. Tried topping off right tank and it was completely full. I drove 3 hours with the right tank selected and gauge never dropped. Switched to left and sure enough, it's just above empty. I got home stopping every 2 hours to fill the left tank and quit trying to switch.

I haven't got any tools out yet but I did turn the key on/engine off, tried the switch and I can hear the motor in the selector valve move. Is it possible for the valve to fail internally so it continues to only draw from the left tank despite the motor moving?
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1981 GMC Sierra Classic LWB
355- original 2 bolt block, 10.2:1, vortec heads, Comp XE268 cam, built by self in 1992
700R4-2000 stall with lockup, shift kit, Corvette servo
3.07 open diff- I need a 3.73 posi!

2nd owner since 1986 - 388,000 km and counting. 100k by the first owner and the rest by me.
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Old 10-03-2023, 09:01 PM   #2
kwmech
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Re: Dual tank problem: appears to switch side but only seems to draw off left tank

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Originally Posted by olblue81 View Post
1981 GMC longbed 2wd. .......


Is it possible for the valve to fail internally so it continues to only draw from the left tank despite the motor moving?
Is the motor actually moving or is it just making noise? I would pull the lines off the valve and using a hose, select one of the ports and see if you can blow through the valve then switch to the other and try to do the same
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Old 10-03-2023, 09:05 PM   #3
olblue81
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Re: Dual tank problem: appears to switch side but only seems to draw off left tank

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Originally Posted by kwmech View Post
Is the motor actually moving or is it just making noise? I would pull the lines off the valve and using a hose, select one of the ports and see if you can blow through the valve then switch to the other and try to do the same
It makes noise, don't know if it actually moves anything inside the valve yet.
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Stu-

1981 GMC Sierra Classic LWB
355- original 2 bolt block, 10.2:1, vortec heads, Comp XE268 cam, built by self in 1992
700R4-2000 stall with lockup, shift kit, Corvette servo
3.07 open diff- I need a 3.73 posi!

2nd owner since 1986 - 388,000 km and counting. 100k by the first owner and the rest by me.
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Old 10-04-2023, 07:23 PM   #4
hatzie
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Re: Dual tank problem: appears to switch side but only seems to draw off left tank

A lot of the times it's the switch.
Sounds like you may actually need a Pollack valve.
I would sanity check the power at the valve with a meter to be sure

The part you need is Pollack 42-159
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