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Discussion Starter · #1 ·
Question for the EFI guys. My 88 was originally a carbed 4.2 (it's a 4.8 EFI Vortec now). The fuel sender ground wire was broken when I bought it so the gauge did not work until I fixed the ground, and installed a new EFI sender. Delphi pump from a mid '90's GM pickup mounted on the sender frame. Used the stock Jeep wiring as the orange fuel pump feed wire was present at the bulkhead, just a matter of changing a wire around to tie in the fuel pump relay. So here is my quandary......

Everything works.....both the gauge and the pump. The gauge however is only accurate when the pump is running. If you cycle the key, the GM PCM runs the pump for 3 seconds. The gauge functions until those 3 seconds are up, then goes back to empty. If I command the pump on with the Tech 2.....the gauge functions properly until I command it off. Start the engine, the gauge is fine. For a second I thought I may have had the wires backwards, but NOTHING would work if that was the case. Fuel pressure is steady at 58-60 psi so I don't think its a ground problem.

Does the gauge in a stock Jeep function in this fashion?
 

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No. The gauge does not function in that fashion.

I believe that you honestly have it wired correctly but some how and some way your swap has created an issue where you are fighting a back feed issue and a very strange one at that. How well do you understand diodes? There is probably a simple place to put one to eliminate this issue. Start installing them to make sure current can only go in the directions that it needs to in the circuits that you have narrowed down. I would start with the gauge circuit.

Maybe, maybe not.

Grounds can be evil. You may still have a bad ground somewhere which also can cause feedback issues to stuff because grounds will find a path through other things at times and really mess them up. . I think I know your swap thread though and if so you are competent in my mind in many ways.

There is also the possibility that your sender and fuel pump somehow both got controlled by the ASD or Fuel pump relay in a previous life and your swap just brought that out.

You posed a real interesting electrical issue with enough information that makes me want to scratch my head and say we got this.
 

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Discussion Starter · #3 ·
No. The gauge does not function in that fashion.

I believe that you honestly have it wired correctly but some how and some way your swap has created an issue where you are fighting a back feed issue and a very strange one at that. How well do you understand diodes? There is probably a simple place to put one to eliminate this issue. Start installing them to make sure current can only go in the directions that it needs to in the circuits that you have narrowed down. I would start with the gauge circuit.

Maybe, maybe not.

Grounds can be evil. You may still have a bad ground somewhere which also can cause feedback issues to stuff because grounds will find a path through other things at times and really mess them up. . I think I know your swap thread though and if so you are competent in my mind in many ways.

There is also the possibility that your sender and fuel pump somehow both got controlled by the ASD or Fuel pump relay in a previous life and your swap just brought that out.

You posed a real interesting electrical issue with enough information that makes me want to scratch my head and say we got this.
Well thank you brother! Looks like I'm going to have to dig under the dash at the main ground splice (Splice D) then.....maybe I do have an underlying ground problem. That is the only spot I haven't touched. It brings all the grounds together to a single 12AWG wire at the bulkhead that I have bolted to the rear of the engine block, along with the firewall ground strap and one of the GM engine harness grounds (GM factory location). I attached the diagrams from my FSM. The fuel pump diagram is of course from a 2.5, but there was no fuel pump or ASD relay on my rig as it was a 4.2 (the MCU was wired directly to the ignition switch.....I completely removed that harness at the start of the swap). Just so happens Jeep uses the same body harness regardless of engine. We're getting snow this weekend so getting under that dash will NOT be happening lol, but I will report back when I find something.
 

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