Ran out of gas, but tank not empty

[sgrinavi]

81 K20, 5.7 - dual tanks

My gas gauge hasn't worked since owning the truck so I've been putting 12, or so, gallons in it every 120 miles. Today I ran out of gas at 110 miles. At first I thought it was the fuel pump, but when I poured a few gallons of gas in it she started right up. When I went to the gas station it only took another 8 gallons.... so assuming I added 3 gallons when I ran out then that's 11 gallons to go 110 miles. Right on.

Would that be an in-tank issue? Bad pick-up or something? Guess I should replace the sending unit while I'm at it too?

EDIT - I've never had the aux tank working, I've replaced the switch and the selector valve, when I hit the switch nothing comes out of the take (yes, there's fuel in it lol) I figure it's clogged up, maybe time for a new tank.

 

[wanderinthru]

Bottom of the pick up rotting off? Gas pick up in the tank I mean.

 

[sgrinavi]

Bottom of the pick up rotting off? Gas pick up in the tank I mean.

Gotta be.... I'll drop the tank Sunday after I burn off some gas and replace it all.

 

[CheemsK1500]

I had this same problem in my '84 K1500. There comes a point where gas tanks become so rotten that the rusted bit and other trash gets sucked into your fuel lines. There's also a little sock like filter on the end of each sending unit that rots off over time, and allows the trash to enter the system even faster.

Replace your sending units and fuel tanks, and you'll solve this problem immediately. You might be able to clean your fuel tanks, but most of the time they're too far gone when they get to this point. Luckily they cost less than $100 at most online vendors.

 

[Grit dog]

Bit of advice. Rather than playing around with it, you’ll be money ahead to spend the $400 for new tanks and fuel pickups and new hoses etc.
Pop the bed off and have at it.
And you can siphon fuel out pretty easily. Even more so with the filler hoses off.
Biggest issue is the condition of the fuel filler and vent hoses.
They are sort of difficult and expensive to procure in my experience. But that doesn’t have any impact on how you proceed, removing the bed or trying to do it all from underneath.

 

[Ron Sebastian]

Careful with the new fuel pump and sending unit you buy. I went through three of the Delphi HP 10255 Hanger Assembly's before I figured out that pump sent out fuel at like 85 PSI which blew out the fuel tank selector valve and over filled the other tank and charcoal evap canister.

 

[Grit dog]

^What makes you think he has electric fuel pumps?

 

[Ron Sebastian]

^What makes you think he has electric fuel pumps?

Don't know, just adding my two cents. If he dosen't have electric pumps, he knows to disregard my comments. Someone else reading this might though. Should I have kept my opinions to my self?

 

[CheemsK1500]

Don't know, just adding my two cents. If he dosen't have electric pumps, he knows to disregard my comments. Someone else reading this might though. Should I have kept my opinions to my self?

In your defense, a TBI truck would have electric pumps; but being an 81, I'd assume the truck in the original post is mechanical unless it was converted to TBI at some point. There are also some non-factory carburetor setups that require the use of an electric fuel pump. Either way, dirty lines, sending units and tanks will mess up both kinds of systems.

 

[sgrinavi]

To update this, I did replace both tank pickups, the switching valve (again) and the switch (for the 2nd time). I'm now able to switch between tanks, but the I still "run out" of gas at 12 gallongs on the main tank (I have not checked the aux tank). Also the new vintage USA gauge isn't working on the aux tank (shows 1/4 tank). Just love the wiring in these squares. lol.

 

[CheemsK1500]

To update this, I did replace both tank pickups, the switching valve (again) and the switch (for the 2nd time). I'm now able to switch between tanks, but the I still "run out" of gas at 12 gallongs on the main tank (I have not checked the aux tank). Also the new vintage USA gauge isn't working on the aux tank (shows 1/4 tank). Just love the wiring in these squares. lol.

Does the ohm range for the guage match the ohm range for the sending unit?

 

[sgrinavi]

Does the ohm range for the guage match the ohm range for the sending unit?

Well, I did not check the gauge, but I set the dipswitches properly. What I find odd is that the gauge reads differently depending on which tank I have selected. The "main" tank reads about right, give or take, but the aux tank... not so much. The sending units are identical although, again, I did not verify.

Edit: I say "main" and "aux" but they are identical

 

[Curt]

Maybe one tank has better grounding

 

[SquareRoot]

Math and driving style doesn't always add up. Fix the issue.

 

[Matt69olds]

Careful with the new fuel pump and sending unit you buy. I went through three of the Delphi HP 10255 Hanger Assembly's before I figured out that pump sent out fuel at like 85 PSI which blew out the fuel tank selector valve and over filled the other tank and charcoal evap canister.

What fuel pump did they send that had 85 psi?! TBI needs something like 15-20 psi, to the best of my knowledge TBI is the only fuel injection available on a square. Sounds somebody boxed the pump wrong, or gave you a pump for the wrong application