P0300 Misfire Diagnosis for 2003 GMC Sierra 1500 5.3L with No Other Codes

P0300 indicates a multi-cylinder misfire. The significant negative short-term fuel trim on Bank 2 (-35.16%) suggests an overly rich condition, likely caused by a stuck-open fuel injector or poor fuel delivery. A fuel pressure test is recommended to verify system integrity. Check for fuel in the engine oil—this could indicate internal leakage from a faulty injector or blown head gasket. Inspect spark plugs; one may show a black, wet appearance indicating misfiring. Since this is a 2003 GMC Sierra 1500 with a 5.3L V8, fuel pressure regulator (FPR) compatibility and function are critical. Also verify that the spark plugs are firing on Bank 2—consider performing a coil pack test. Ensure you're using genuine AC Delco plugs as recommended for this model. Confirm there are no other misfire codes present.

Thanks for the detailed input—I’ve confirmed it’s a 2003 GMC Sierra 1500 4x4 with a 5.3L V8 engine. All spark plugs appear evenly worn and show no visible differences after replacement. I used AC Delco OEM plugs, which are recommended for this model. No additional diagnostic codes have been detected. The oil was changed during the fuel filter replacement, and there’s no sign of fuel contamination or unusual smoke. This makes me suspect a deeper issue such as an injector failure or sensor malfunction. I’m now focused on verifying fuel pressure and injector performance.

Check for spark at all Bank 2 plugs—this can be done without specialized tools. Look for wet, blackened plugs indicating misfiring. Then perform a fuel pressure test using a gauge to verify both running and idle conditions. The negative short-term trim on Bank 2 (-35.16%) suggests an overly rich condition, which may indicate excessive fuel delivery. Double-check that the reported value is accurate—was it indeed -35.16%? This reading is unusually high and could point to a faulty injector or sensor issue.

Thanks for the advice—I’ll perform a fuel pressure test and inspect injectors tomorrow. I’ll update with results as soon as possible.

P0300 on a multi-cylinder engine is concerning—especially without any other misfire codes. This suggests the issue may be isolated to one cylinder or system. A blown head gasket could allow coolant into combustion chambers, causing misfires and poor fuel trim readings. In some cases, especially under load (like light braking), a faulty coil pack or ignition component might cause intermittent misfiring. However, this should only be considered if the user is experienced with advanced diagnostics—otherwise, it’s best to start with basic checks first.

Here's my latest update: Fuel pressure test shows consistent readings of 48–50 psi when running and rises to 54 psi after shutdown. No significant drop. All injectors tested at 12.4–12.6 ohms—within normal range for a 5.3L engine. I repeated the test multiple times with identical results. While fuel pressure appears stable, the large negative short-term trim on Bank 2 (-35.16%) remains puzzling. This suggests either an injector that is leaking fuel or a sensor issue in the feedback loop. I’m now considering removing the vacuum line from the fuel pressure regulator (FPR) and plugging it to test if pressure increases—this may reveal a faulty FPR or poor regulation.

For a 5.3L engine, optimal fuel pump pressure is between 55–62 psi. Your readings (48–50 psi) are below this range and may explain the high fuel trim values and misfire. Even with proper injector resistance, low fuel pressure can cause the PCM to compensate by increasing fuel trim—leading to a rich condition on Bank 2. Try removing the vacuum line from the FPR and sealing it to prevent pressure loss. If pressure rises to near 55 psi or higher, this confirms an issue with the FPR or fuel pump. Note: Only use OEM-quality fuel pumps—aftermarket units may not meet performance standards and could lead to recurring misfires.