Fuel gauge acting erratically
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You may want to try checking the fuel pump relay. The relay wires should have ground on two wires, power on one wire and switched power on another wire. To activate the fuel pump your ECU actually connects one of the ground wires to complete the circuit and send power to the fuel pump. If the wires all have power running through them, then it is likely you have a faulty relay. The wire coming from the fuel pump is the other ground wire. I would recommend having an expert from YourMechanic come to your location to diagnose your wiring as this can be very difficult and time consuming without the proper wiring diagram.
Below is partial list of steps, copied and pasted directly from the service manual:
1. Disconnect battery and put protective material in front of the charge air cooler (CAC) or damage to the CAC may occur.
2. Remove air cleaner assembly.
3. Disconnect the oil drain tube from the crankcase vent oil separator.
4. Loosen the air cleaner outlet tube clamp and detach the air cleaner outlet tube from the turbocharger.
5. Remove the bolts, the crankcase vent oil separator and the air cleaner outlet pipe as an assembly.
6. Remove and discard the crankcase vent oil separator press-in-place gasket. (To re-install, tighten to 13 Nm (115 lb-in).
7. Disconnect the PCM electrical connector (BE SURE BATTERY IS DISCONNECTED FIRST!) and retainer. Disconnect the in-line electrical connector and position the engine wiring harness on the engine.
8. Remove the pushnuts and the glow plug module heat shield.
9. Disconnect the high-pressure fuel injection pump electrical connector and detach the retainer from the glow plug module bracket. Disconnect the glow plug module and the exhaust gas recirculation temperature (EGRT) sensor electrical connectors, detach the wiring retainer and position the wiring harnesses aside.
10. Remove the nut and position the ground strap aside. To install, tighten to 13 Nm (115 lb-in).
11. Remove the engine wiring harness bolt. To install, tighten to 13 Nm (115 lb-in).
12. Remove the heater supply tube nut. To install, tighten to 13 Nm (115 lb-in).
13. Remove the bolts and the glow plug module bracket. To install, tighten to 13 Nm (115 lb-in).
14. Remove the nut and position the transmission fluid indicator tube aside. To install, tighten to 8 Nm (71 lb-in).
Warning: Do NOT disconnect the glow plug electrical connector before dislodging the seal from the valve cover or the wiring harness may be damaged.
15. Using an appropriate tool, dislodge the glow plug wiring harness seals from the valve cover.
16. Disconnect the glow plug electrical connectors by pulling on the glow plug wiring harness tee above the seal. Remove the glow plug wiring harness.
17. Remove the nut and position the crankcase ventilation drain tube aside.
18. Disconnect the wiring harness retainer from the valve cover stud.
19. Disconnect the wiring harness retainer from the generator bracket.
20. Disconnect the A/C pressure switch electrical connector. Position the harness aside.
Warning: Do not bend or flex the heater supply tube or damage to the tube may occur.
21. Remove the stud bolts, bolts and the valve cover. Remove and discard the valve cover gasket. To install, tighten to 9 Nm (80 lb-in).
Warning: Contact with exposed fuel injector wiring, if energized, may result in electric shock.
Use care when working on or around energized fuel injector wiring. Fuel injector wiring supplies HIGH VOLTAGE to operate the fuel injectors of course, you disconnected the battery though.
22. Disconnect the fuel rail pressure (FRP) sensor electrical connector.
Warning: Fuel injection equipment is manufactured to VERY precise tolerances and fine clearances (think space shuttle). To prevent fuel system damage, it is essential that absolute cleanliness is observed when working with these components. Always install fuel system caps on any open orifices or tubes.
23. Remove the FRP sensor. To install, tighten the FRP sensor in 2 stages.
Stage 1: Tighten to 7 Nm (62 lb-in).
Stage 2: Tighten an additional 40 degrees.
24. Bleed the high-pressure fuel system.
If you feel like this job may be too overwhelming or run into any issues, consider YourMechanic to get it done. One of our mobile technicians can come to your home and replace the fuel pressure sensor at your own convenience.
It sounds like you may have a clogged fuel injector or potentially a faulty injector that is fried internally. I would suggest testing them if you have access to a multi meter. Disconnect the injector from the electrical connector and set your multi meter to Ohms mode and measure the resistance of the injector. After testing all injectors, compare the resistance. They should be the same or very close. If they are not, they will likely need to be replaced.