Fuel Problems(Kinda)
 

Well I have an 88 n/a but the motor is a 1990 n/a with 88 ecu and 86 wiring harness with no OMP and no emissions at all no bac no nothing everything has been bypassed, But my question is why would my fuel idiot light not work and also could this also be why my fuel gauge is off by almost quarter of a tank?

My guess is it's a problem somewhere in the ECU itself (although I am not sure if S4 and S5 ECU's were much different), or the cold solder joints on the idiot cluster itself. 99.9% of the time, those are your issue for those lights. As for the fuel level, you're going to want to check your pump assembly in the hatch, and see if that is giving you problems reading the level.

Side note: Get yourself another BAC, it was there for good reason, and should be kept.

Fuel tanks are different size between S5 and S4 and thus the fuel level sensors are different so maybe you have a mixed set as opposed to a matched set between tank and fuel level sensor.

Alright, I'll try to locate another BAC and I will check the pump assembly and wiring tomarrow is there a place I can check the wiring without taking out the carpet?

its a S4 cluster with s4 pump and s4 tank so I don't think its that. But could be.

Then perhaps the float sensing device has seen better days.

could be Its been ran premix for the past couple years prior to me buying it

I doubt it.

why do you doubt it its had 4 motors in it and all have had the OMP removed and blocked off besides original motor

Satch is saying that the Premix is not the culprit.
The sender could be Kaput though seeing it is over 20 years old.

Both the fuel gauge and the low fuel lamp are using variable grounds (just like the oil pressure and water temp gauges, actually)...most likely culprit is going to be the sending units or the bulkhead connector at the tank.

If you disconnect the 4 wire plug at the tank, with the key to ON, you should get +12v at both the fuel and low level wires.
If yes, it's your sending units in the tank.

If no, start working back to the dash and find out why.

Also, as long as you're there, make sure the 4th wire in the connector, which is the ground for both sensors AND the fuel pump, has good continuity to the chassis.

The BAC and ECU have absolutely nothing to do with it.

EDIT: It's been a while since I've been in that area of my car...there may be a ground wire connected to the bulkhead plate, separate from the connector itself (although I'm pretty sure it joins the fuel pump ground in the harness)...make sure that's clean/tight as well.