why can we not buy just the f/p motor for a 96 jimmy?

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Give us the long name for motor?
What engine and what motor?
What is yours doing or not doing?
MT

Fuel Pump.

And yes, you CAN get just the pump (without the whole in-tank unit) if you shop around. The problem is that the design of the original tank unit has some defects. The biggest of those is the electrical connections for the fuel pump itself, which is a major reason for them failing. The replacement units eliminate the weak connectors and replace them with ONE robust connector, actually capable of carrying a sustained high current, unlike the original.

I've been there as well, replacing just the pump, and repairing the harness. Even at that, the connectors are weak.

96 jimmy 4 door 4 wd w/ 4.3 liter engine W, year code T /on outside of plastic by fuel sender #25029679-3, on very top- SFR the barcode- the 1989-13 next line 0226-3E next line 02:09 the motor is self has the Numbers on metal 5164111 0196-1 682 and below that has AC on bottom it has 7641 and also has a 3 and AC. when I test the resistance of the motor it has 2 to 3 ohms except for one spot where it jumps to 150 to 180 and if I get it just right it has open circuit. tested volts at fuse relay and at pump all check out good the replacement for this i am buying is a delphi fg0030

i bought a delphi FG0030 and the numbers on the base by the sending unit are - 25029679-1 the original was 25029679-3 SO what is the significance of the last number?

Delphi/AC delco is the way to go.
FG0030 is for the SLE,SLT,or 4dr
Specs 108 lbs full/cut off pressure.
OEM part no 25163480
Ask the parts people about the number difference.
Numbers change or get superceded and sometimes they give you the wrong one.
2 dr and 4dr and some imported ones are different pumps and part no.
But if you are not.
I would go the complete module and be done with it.
If you check around you can get a AC delco/delphi module for around 200 $ US.
MT
Good luck with it and let us know how it goes.