fuel problem, pump or switch

[jonny]

I have an 87 with duel tanks. I had been driving an when I stopped an filled up both tanks I couldn't get started again. Out of dumb luck I just tried switching tanks an fired right up. While running if I flip switch dies instantly. So do I start with pump, transfer switch or cab switch? Or is there anyway to test?

[DnStClr]

A couple of things you can do-
1. To see if the fuel pump is dead-When you turn the key on without cranking the engine, can you hear the fuel pump turn on for a couple of seconds? You should be able to hear either pump energize, depending on which tank is chosen.
2. To see if the tank selector valve is working-  Again, without cranking the engine- with key on. Can you hear the valve move ( it's under the passenger side of the truck) as you switch tanks?
3. To test the cab switch- there's an electrical connector under the hood, on the firewall next to the steering column that carries the signal from the switch to the switching valve. Switch on again, take the connector apart and you can use a test light to check for voltage as you switch positions on the switch and move your test light probe from one pin to the other pin in the connector. I'll try to copy the procedure from the GM Repair manual with a picture and post them for ya.

[DnStClr]

Maybe this diagram can be of help.It turned out fuzzy.  Hopefully a moderator can assist...
 

[77C20 Desert Rat]

Your situation of switching tanks would indicate the pump is fine and the selector is fine.

 If your selector was bad it would still pull from one tank it just wouldn't switch to the other tank yours indicates it does the switching by either running or not...  If your pump was bad it wouldn't run good at all on either tank.

My internet diagnosis is a obstruction  on one tank..  its easy to drop the tank remove sock use a steel filter just outside the tank.

Also what I do with any old school in tank pickup is after taking off the sock is I bend the pickup closer to the bottom of the tank I try to get it to near touching  without the o`ring in place on the pickup then when the o`ring is installed on final assy. your pickup will be at least the o`ring thickness off the botton of the tank so you don't have any rubbing and wear through and you will be able to use more of your tank of gas before running out

To rule out the selector valve is easy to do just plumb the bad tank directly to the fuel pump bypassing selector all together you just need some bulk fuel line

[Irish_Alley]

one thing im not sure of, but arnt tbis electric in tank pumps? so it would have 2 pumps one in each tank? if you use the switch and it changes if its getting fuel or not, it tells me the switch or the valve isnt the problem. the valve work like this if no power it switches to a default tank(passengers), if you give it power it will switch to the other(drivers).

if your switch isn’t working then it switches to the default tank(passengers, i think) and would pull from that side no matter what. now if you flip the switch to energize the valve to the drivers side and if the switch in the dash works but the valve is broken it should still pull from the default tank. or what ever side its stuck on.

Now lets say the switch works and you valve works, then the only thing left is the pump.  So its either a pump or bad connection or even a broken/cracked rubber line. you can tap/beat on the in-op tank strap and this might free up the pump

[DnStClr]

Yes, you're right Irish- an 87 tbi has a pump ,filter sock,and sending unit in each tank.

[jonny]

I appreciate everyone's input. Now if ya can make it stop raining an snowing everyday I'm going to get testing your methods. Thanks

[1979C20]

X2 on individual electric pumps in each tank. Sounds like one died, flip ti whichever tank the truck will not start on and turn the key on. If you dont hear a buzzing sound from that tank its bad. Unless there are two seperate fuel pump relays, than the relay for that tank could be bad. Those are on the firewall at the top to the left of the air cleaner if you are looking at the motor from the front of the truck.

[77C20 Desert Rat]

Yeah my bad on not realizing the cut off date on the carb to EFI.  In tank socks look good on paper and seem to have a logical use but almost always cause a problem deep in the life of a truck or car

[DnStClr]

There's also a fuel filter under the passenger side door, attached to the frame. The fuel from either tank goes thru this filter on its way to the throttle body injectors. This problem doesn't sound like a filter problem tho, of course I've been wrong plenty of times   

[1979C20]

Speaking of filter socks, are those really necessary? I just had a fuel pump issue and it turned out the filter sock started deteriorating and clogged the screen on the pump inlet.

[Captkaos]

the sock filter prevents large particles from entering the pickup tube and blocking it completely.  You need it.

[Irish_Alley]

i know from the part stores they say it will void the warranty if you dont buy a sock with a new pump. but if your sock deteriorated it was in there for sometime. and it prob stopped some other junk from getting into the pump, so it payed for itself imo

[1979C20]

The filter sock was replaced with the pump 3 years ago. But, none the less, it is replaced and all good.