Code P0300 misfire

2001 GMC SIERRA

190,000 MILES • 4.8L • V8 • 2WD • AUTOMATIC

PAULO RODRIQUEZ SALAZAR III

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I bought the car used from a friend. I did a complete tune-up with new:

Spark plugs and wires
Cylinders 4 and 6 new coils
All four O2 sensors
MAP
MAF
Alternator
Battery

The bad: The car came from up north where the bottom of the vehicle was completely rusted. There's a small gas leak near the fuel filter. I took it to a friend to have it changed out and he said no, if he does, the fuel lines may twist and break because of the severity of the rust. Could this be the problem causing random misfires (P0300)? EVAP leak in the fuel system?

I will randomly get the original code of P0304. He said to check the wiring for any cracks and/or proper grounding. How do you do that?

The truck will have problems starting. I get it to start around the 3rd time but it's rough.

Please help!!!

Jun 4, 2020 at 2:41 PM

ASEMASTER6371

CERTIFIED EXPERT
52,796 POSTS

Good afternoon,

The 300 code is most likely from low fuel pressure from a bad fuel pump.

https://www.2carpros.com/articles/engine-misfires-or-runs-rough

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

The hard start is from the leak in the fuel line.

I would replace the fuel pump, lines and the filter. There is no repairing the lines once they start leaking.

https://www.2carpros.com/articles/how-to-replace-an-electric-fuel-pump

Roy

1. Disconnect the negative battery cable.
2. Relieve the fuel system pressure. Refer to the Fuel Pressure Relief Procedure.
3. Drain the fuel tank. Refer to Fuel Tank Draining Procedure.
4. Remove the fuel tank.

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5. Remove the fuel sender assembly.

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6. Note the position of the fuel strainer (2) on the fuel sender.
7. Support the fuel sender assembly with one hand and grasp the strainer (2) with the other hand.
8. Pull the strainer off the fuel sender. Discard the strainer after inspection.
9. Inspect the strainer. Replace a contaminated strainer and clean the fuel tank.
10. Disconnect the fuel pump electrical connector (5).
11. Remove the fuel level sensor electrical connector retaining clip (6).
12. Disconnect the fuel level sensor electrical connector (7) from under the fuel sender cover.
13. Remove the fuel level sensor retaining clip (4).
14. Squeeze the locking tangs and remove the fuel level sensor (3).
15. Remove the fuel pressure sensor (1).

Images (Click to enlarge)

Jun 4, 2020 at 3:10 PM