79 k20 losing fuel somewhere

[79k2001]

I have a 79 k20 and my problem is with the duel fuel tanks, I filled both of them up and was using the passenger side first, well as that side neared empty I switched over the the left tank (drivers side) I did a double take when the gauge said only half full, I thought it must not be reading correctly so I went to a gas station to check and it did in fact take ten gallons of fuel, so my question is, where did it go, it's not leaking anywhere, nobody but me has driven the truck so I know it's only been on the rh tank, is it possible that it's using both tanks? Also is there a better method than turning the truck around to fill the other tank, it's quite annoying.

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[1987 GMC Jimmy]

I think it's possible if the switch valve is stuck by something lodged in it. I'd also double check that your return lines and vent lines look good and aren't crossed or anything if you have them, but I'd suspect the valve first and foremost. I wish I knew of a way to make the fueling setup better (aside from switching to a Suburban fuel tank) like a crossover kit or something. As far as I know, people just live with it.

 

[smoothandlow84]

Do you smell fuel outside? My last square developed a crack in the passenger side saddle tank from the frame mounted clamp rubbing/expansion. I would lose a few gallons after a fillup from evaporation.

 

[chengny]

I have a 79 k20 and my problem is with the duel fuel tanks, I filled both of them up and was using the passenger side first, well as that side neared empty I switched over the the left tank (drivers side) I did a double take when the gauge said only half full, I thought it must not be reading correctly so I went to a gas station to check and it did in fact take ten gallons of fuel, so my question is, where did it go, it's not leaking anywhere, nobody but me has driven the truck so I know it's only been on the rh tank, is it possible that it's using both tanks? Also is there a better method than turning the truck around to fill the other tank, it's quite annoying.

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X2 the switching valve being the problem. A brief description of operation:

The tank select system you have is of the earlier design - the switching valve is controlled by a one wire circuit.

The valve:

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When the dash mounted tank select switch is in the LH position, power is supplied to the solenoid coil and the valve is held in a position that allows fuel flow from the LH (auxiliary) tank.

When the dash switch is in the RH position, no power is supplied to the solenoid. Without power to the solenoid, an internal spring moves the valve to the default position and fuel is then drawn from the RH (production) tank.

So, if you had the dash switch set to the RH tank - and fuel was also apparently being pulled from the LH tank - it would appear that the valve is not stroking completely over to the RH position.

Since the RH tank is the default (no power) position - and is the valve is moved to that position by spring pressure - an intermittent wiring connection can pretty much be ruled out.

A couple of questions:

1. Is this the first time you've noticed this (pulling gas from both tanks)?

2. How long were you running only on the RH tank? Did this all happen on one fill up of both tanks or did you fill them both initially and then - as the RH tank went down - you kept re-filling it?

 

[79k2001]

Well, I don't think that there is a crack in it since I never smell fuel or fumes although it is possible. This is the first time I noticed this, I just bought the truck and the owner I bought it from was just using the rh tank, since it's the default, but he did say they both worked. I filled them both up at the same time, after I filled them I flipped the switch to see if the gauge was now full on both instead of empty on lh and it was, then I only used the right hand tank until empty. So maybe the solenoid is sticking from a lack of use?

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[chengny]

Well, I don't think that there is a crack in it since I never smell fuel or fumes although it is possible. This is the first time I noticed this, I just bought the truck and the owner I bought it from was just using the rh tank, since it's the default, but he did say they both worked. I filled them both up at the same time, after I filled them I flipped the switch to see if the gauge was now full on both instead of empty on lh and it was, then I only used the right hand tank until empty. So maybe the solenoid is sticking from a lack of use?

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Yeah, I would try switching back and forth a few times and see if it you can get it to stroke full L&R.

 

[75gmck25]

Also check to see if your fuel pump has a return line, and where that line runs. Some pumps have it and some don't, depending on the emissions setup.

If you have a 3 port solenoid, like in the picture, it can only switch the fuel supply line from one tank to the other. On my truck they had used a 3 port switch for supply, but then connected the fuel return line only to the right tank. That meant that when I was pulling from the left tank it was returning fuel to the right tank. My right tank would get more full as I ran off the left.

If you have a fuel pump with a return line, you need a six port solenoid that can switch both lines (supply and return). These are hard to find, and I had to get one from the junkyard.

Bruce

 

[Shilaeli hugger]

I bet its a leak on the top of the motor

Check.around ur tank brackets. Moms 86 had that stupid recall shield on it and it collected dirt n gravel. Slowly rubbed a hole in the tank and rotted.out the strap.