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2001 Chevrolet Suburban 1500 5.3L Engine P0300 Random Misfire: Fuel Pressure and ECM Reprogramming Issues

Model: Suburban 1500 Fault Code: P0300 Posted: 2008-07-27 19:15

My 2001 Chevrolet Suburban 1500 with a 5.3L Vortec engine has suddenly developed a persistent random misfire, indicated by fault code P0300. The vehicle previously ran smoothly at 154,000 miles and was performing well after a recent ECM reprogramming for performance and 33-inch tires using a Superchips Model 2715 MAX MicroTuner. However, the issue began exactly 10 miles after refueling with 91-octane Sunoco gasoline. After the refill, I noticed severe rattling at 4500 RPM in first gear—potentially indicative of pre-ignition or cylinder misfire. The problem only occurred during testing of the performance tuning settings and disappeared when I reverted to stock ECM programming. Despite this, the engine now runs very roughly, with multiple random misfires, and fails to operate smoothly. I have already replaced the fuel pump (NAPA brand, approximately 20,000 miles ago) and confirmed fuel pressure readings: 40 psi when the ignition is on, and 50 psi while running. The fuel filter sample showed no visible sediment or water contamination. I also recently installed new spark plugs and wires around 35,000 miles ago. The vehicle will now crank but runs extremely rough with the check engine light illuminated. No other diagnostic codes are present—only P0300 is stored in the ECM. The local gas station reports no issues with fuel quality or delivery from their pumps. I am seeking advice on potential causes related to the ECM reprogramming, fuel pressure regulation, and whether the 5.3L Vortec engine may be experiencing injection unit failure or fuel pressure drop due to a leaking regulator. Any insights into diagnosing random misfires in this specific model with a history of tuning modifications would be greatly appreciated. R/Mike Crosby

Related fault codes
P0300
Comments (6)
Anonymous 2008-07-28 05:20

The fuel pressure appears too low for a 5.3L Vortec engine—should typically be around 60 psi while running, with minimal drop when the key is off. A leaking fuel pressure regulator can cause inconsistent delivery and lead to random misfires, especially under load or at higher RPMs. I've observed similar symptoms where cylinders intermittently fire during a leak-down test before failing to ignite again.

Anonymous 2008-07-28 15:36

Could you clarify what specific modifications were made during the ECM reprogramming? Did the tuning alter fuel delivery timing or octane sensitivity settings? Was this change applied only after refueling with 91-octane gas, and did the issue manifest immediately upon testing these settings?

Anonymous 2008-07-28 16:41

I used a Superchips Model 2715 MAX MicroTuner to reprogram the ECM. The modifications included adjusted transmission shift points and reduced fuel pressure settings—both brought closer to stock levels. The 'Towing/Performance' profile recommends using 91-octane, which is why I filled up with that grade. The vehicle ran fine during routine trips to and from the gas station. However, after attempting to test the performance tuning, the engine began misfiring severely. I immediately reverted to stock settings, but the problem persists. For all practical purposes, the car now runs like cold steel—cranking but very rough with multiple random misfires.

Anonymous 2008-07-28 17:41

It would be helpful to know which cylinders are affected. Does your scan tool provide cylinder-specific data? If not, a more advanced diagnostic device may be needed to identify individual cylinder misfire patterns or detect intermittent ignition issues.

Anonymous 2008-07-28 18:07

My scan tool only reports P0300—no individual cylinder codes. The fault is labeled as 'random multiple misfires.' While my current device doesn't provide detailed cylinder analysis, I suspect a deeper issue with fuel delivery or ignition timing. Fuel pressure yesterday was 40 psi with the key on and 50 psi while running. Today, the engine cranks but runs very roughly with the check engine light on. The gas station has no complaints about fuel quality. I’m now planning to visit them again for further testing.

Anonymous 2008-07-29 08:38

For a 5.3L Vortec engine, fuel pressure should remain stable at approximately 60 psi during operation and not drop significantly when the ignition is off. A leaking regulator can cause inconsistent fuel delivery, leading to random misfires—especially in the injection unit located under the intake plenum. I’ve personally observed this behavior where cylinders fire intermittently during a fuel pressure test but fail to ignite consistently afterward.