selector valve and running out of fuel question

[74k20]

So while driving my '74k20..it just dies.  Pull over and NOT getting any gas.  Tow home and change out the older fuel pump.  NOPE…still no fuel to carb.  So I blow into the fuel line to the tank and blow ONLY air.  ? gage shows gas, not much but some???  YES I tried to switch tanks with NO help and/or difference.  So I go ahead and put in 5 gal of gas in the “empty” tank.  Low and behold it fires up.

So will/can my selector valve switch over once a tank runs out?  As-in…if there is only air, will it NOT switch over to the other tank.  Seems I remember waaaaay in the past my gpa telling me to never run a tank out or it won’t switch over.  Is this correct?

thanks,

MJ

[Jason S]

It sounds like you simply ran out of gas. Were both tanks empty?

Do you have the electric selector valve (rocker switch on dash)? 

Presuming you have the electric selector valve:
If the selector valve is in good shape (not worn or having a sticking point)- it should switch between the fuel ports regardless of air. Running a tank out of gas can allow fine trash from the bottom of the tank to get into the valve and then carburetor.

Also, like any other electrical item, the selector valve won't operate well without good clean connections to power and ground.

[74k20]

yes I've got the factory electric selector on the dash.  I knew this tank was low, but NOT on empty.  So something has changed with that side sending unit.  Other tank was almost full, but the valve wouldn't switch over.  I tried blowing in the line (back to the tank), and it blew only air (not bubbly as if thru fuel) and also listened with both fuel caps off.  Had kid listen in tank for air, while I blew.  I switched the valve/tank to the almost full, and was still only blowing out the "empty" tank.  As if the valve was not switching.  Gauge did go to almost full upon switching, but the air was only coming out he empty side.  I cleaned up the switch a couple years ago, and it now looks fine.  once I put fuel in the tank and got it running, I switched the tank to the almost full and it seems to have switch this time.  As-if it only worked if I didn't let it run dry.  Then I remembered maybe my gpa told me this waaaaaaay in the past about not letting it run dry.  It stumped me tho.

[Jason S]

It's possible that Grandpa knew the quirks of his old truck...

Maybe you have a persnickety worn valve that needs fluid in it to complete operation?

I've used the Borg-Warner replacement style fuel selector valves in quite a few different trucks and they've always worked great.

BTW, way back when, my Dad's '76 GMC had a bout of issues with the factory tank selector valve. It was intermittent operation, sometimes sticking on one tank. But it'd happen only occasionally. It was resolved when he traded the '76 GMC in on an '87 GMC...

[78BIG-TEN]

The origan 3 line valves were bad about sticking, replaced many of them

[Captkaos]

Sometimes it is the switch that causes them not to switch over.  I posted a way to pull them apart and clean them in the tech section.

[Irish_Alley]

i was messing with my 91 the other day. must of bumped the switch but i couldnt start it, no fuel. messed with the wires at the switch and it gets fuel and starts. but the selector valve has a default tank it should switch to. so if the valves not getting power via the switch or is broken it will only pull from one side and return to the same side. so by the sounds of it the valve itself is could be good and the valve still reads from the correct tank. but the switch isnt giving the valve the correct power to switch tanks.



this kind of makes sens of why you could only blow air threw the empty tank no mater what side you picked