Fuel Sending Problem.

Cheekster96 · July 6, 2011

Howdy all. Been working on the old girl trying to get her to run again. It is a '87 V20 truck with the 350 TBI motor.

I had it up for sale last spring. I told the guy who was looking at it that it only has one tank in it so dont switch the selector valve. He did, hasnt run since.

I replaced the switch, still nothing. I turn the key and cant hear the fuel pump whine (used to).

Here are the things I have done/checked:

New selector switch on the dash (old one smelled burnt).

Swapped the fuel pump relay with the kickdown relay (same type/style). This didnt work.

Took the fuel supply line to the TBI off and cranked the motor, no gas present.

Dropped the tank, took the fuel pump/sending unit out and attached a lead from a 12V battery and pump worked fine!

Put tank back up, made sure all connections were good, ground wire was firmly attached to frame, still, nothin.

Poured some gas down the throttle body, she fired up, ran for about a second or two then died due to starvation of gas. Bummer.

I took the selector valve out (the one under the truck near the gas tank, passenger side). This is the next part to replace (pricey little bugger too).

Does this selector valve control the fuel pump in the tank? Their is only one electrical connector from the tank and it goes into a wiring harness that attaches to that selector valve.

If this doesnt work, I dont know what else to try.

I also checked all the fuses, they were fine too.

I know the pump works, like I said, hooked it up to a 12V battery and it worked fine.

Any thoughts or ideas?

Lee

jwhuber · July 7, 2011

So did the truck have two tanks at one time? I don't ever remember them having two tanks. From the sounds of it you've checked everything. I'd say it almost has to be the selector.

Does the power wire run back to the selector, and then to the pump? It's obviously not getting power to the pump so that would be the final thing to check. It could be the power supply wire somewhere along the entire run.

GMstrong · July 8, 2011

check the fuel pum relay located under the junction box on the passenger side on the firewall

Cheekster96 · July 22, 2011

I used the relay that controls the tranny kickdown and it still didnt work (same type relay).

Dunno, maybe I will pick up a new relay (exact one) and try it. It is cheaper than buying a selector valve.

I ll keep you all posted.

Lee

ChevyTech77 · July 22, 2011

The fuel pump relay on the V series trucks was a stand alone relay that was attached the firewall, just above the distributor.

And yes, the selector valve can command the pumps because you only run the pump for the tank that is selected, not both pumps all the time, right? Right.

TexasAl · July 22, 2011

Have you checked to see if there is power going to the selector valve? If there is, see if you can bypass the valve. If there is only power to the valve when the ignition is on, then you should be able to connect the power cable to the wire that goes to the fuel pump.

jblakeney · July 25, 2011

Does the pump get fire at the TANK??? And free testing a fuel pump by hooking a battery too it does not tell you anything other than the pump turns, when power is supplied......

It takes Volume and Pressure. All you tested was Volume. Never confuse the two as the same, even though it is easy to do. You can have all the volume in the world and NO PRESSURE. Your pump can run and supply volume but not have enough pressure to push the fuel through the filter and up the hill to the TBI... And the pump Run, in the manor you tested it in.

You changed the selector valve switch, swapped the relays, and tested the volume with the pump in your hand.

So, check the fuse, if it works, then go to the relay test incoming constant 12 Volt feed, then test if the output wire is fed with the key on (remember it only gets fire for a few seconds, if there is a fuel pump prime connector supply it with 12 volts and it will make the circuit hot till the wire is removed from the prime connector), if not try the trigger wire for the relay (to see if the PCM is demanding the pump to come on), if you have power leaving the relay, go to the pump connector on the frame disconnect it and test for power, if you have power remove the tank sit on ground add extra ground wire remove pump and test again directly connecting to the battery with jumper wires, if pump works or doesn't, replace pump it is the last thing, less the regulator, but if the fuel never gets there there is nothing to regulate. Also, you should also check the change over valve on the passengers frame if you have two tanks.

You can not check the fuel pump the way you have tested as it does not show pressure, that is where you need a fuel pressure tester. It takes 35 or 40 psi for a TBI unit...... Could be a bit more or less its been awhile since I had to chase down a fuel pump problem on TBI.

To test these points you need a wiring schematic. To make sure your testing the right wires.

Jbo

Wiring for the Selector Valve that will give you the wiring schematic of the take selector valve