Dual tank problem: appears to switch side but only seems to draw off left tank - The 1947 - Present Chevrolet & GMC Truck Message Board Network

olblue81 · 10-03-2023, 08:08 PM

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?

**kwmech** · 10-03-2023, 09:01 PM

Quote:

Originally Posted by olblue81

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

olblue81 · 10-03-2023, 09:05 PM

Quote:

Originally Posted by kwmech

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.

**hatzie** · 10-04-2023, 07:23 PM

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

10-03-2023, 08:08 PM	#1
olblue81 Registered User Join Date: Jul 2005 Location: Saskatchewan Posts: 189	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? __________________ 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.

10-04-2023, 07:23 PM	#4
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 4,977	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 __________________ 1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials... 1973-1991 GM Light Truck Service, Unit/Overhaul, Wiring Manuals, Parts Books and Some 1968-1999 GM Service Training textbooks in PDF 1988-1998 GMT400 CK service manual project 1988-1993 GMT400 CK parts books. 1967-1972 GM Light, Medium, & Heavy Truck Service Manuals in PDF Please include at least the year and model in your threads. It'll be easier to answer your questions. And please let us know if and how your repairs were successful.

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