fuel tank question

[hatzie]

Originally Posted by tex_waco_usmc

I have recently done the same swap. 86 to 87 drivetrain. I used one brand new tank for an 87 as one had a rust hole in it. The other was from the 86, when it gets to below half a tank it will just die for no reason running down the road. I am about to put a brand new tank in on the passenger side to correct the issue.

I used the 86 tank, put the 87 sender, fuel pump and complete wiring harness. It functions but it will piss you off when it shuts off on you going down the road. It always fires right back up once you bring it to a complete stop but as soon as the slosh starts happening it dies again!

First comment I've heard about any unpleasant effects that the 73-86 no baffle tanks have on the TBI in-tank pump operation...
It's not a cruddy sock because this is a new sender.

If it's cutting out just running flat on a stretch of road it's probably not the missing tank baffles... I'd try something cheaper that will probably fix the problem before spending your dough on a new tank...

Change out your dash switch with a new $14 AC Delco D7089C.

This sounds odd until you think about how the fuel pump and tank switching circuit works.
The fuel pumps' operating ground is the short pigtail to the frame shared with the gauge sender.
The dash switch is a polarity reversing DC switch. It's only hot to one fuel pump via one half of the switch. The other half of the switch is feeding a second ground to the resting fuel pump.
When you flip the switch the wire that was ground becomes hot and and the wire that was hot becomes ground. This reverses the DC motor in the valve moving the shuttle to the opposite tank. This also feeds hot to the previously resting fuel pump completing the circuit thereby turning it on and double grounds the un-selected pump breaking its' circuit thereby turning it off.

Whichever pump is at rest has two grounds so that half of the switch is not carrying any current to the resting tank. When the good tank is selected the weak half of the switch is not carrying any current.

Then you switch over and kick in the weak tank. But it's full. So you chug merrily along and the fuel pump is fat dumb and happy...
As the fuel level in the tank, on the weak side of the switch, drops... the fuel pump has to do more work and the current draw across those weak contacts rises. When you reach around 1/2 tank the current draw reaches the point that the weak switch contacts will no longer carry it and they intermittently arc dropping the pump current below its' cutout threshold. Dead truck.

If it's only when you're on curves or changing speed then it points more toward the missing baffles... But. The fact that it doesn't recover till you bring it to a complete halt with around 1/2 tank on that side is odd at best. It should buck and stumble as the pickup sock is covered and uncovered. It should not cut all the way out without recovering.