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P0300 Fault Code on 2001 GMC 5.3L V8: Diagnosis and Solutions for Misfire at Idle and Acceleration

Model: GMC 5.3L 2001 Fault Code: P0300 Posted: 2011-04-16 21:39

I recently encountered a persistent SES light on my 2001 GMC with a 5.3L V8 engine, which blinks under heavy load—such as when climbing hills. The diagnostic trouble code (DTC) is P0300, indicating a random misfire across cylinders. Below are the key sensor readings collected at idle and approximately 2,000 RPM: - ABSLT: 0%, TPS: 10.6% - Engine Speed: 562 RPM (idle) → 2021 RPM (~2k) - Load: 2% → 7.5% - MAF (air flow): 0.73 lb/m → 2.4 lb/m - MAP pressure: 10.6"Hg → 8.3"Hg - Coolant temp: 171°F → 181°F - IAT: 99°F → 93°F - Ignition advance: 19.5° → 49° - ST FTRM1: -2.3% → 0.6% - LT FTRM1: 2.3% → -1.6% - ST FTRM2: 0% → -2.3% - LT FTRM2: 3.9% → 10.9% - Vehicle speed: 0 mph - Fuel System Status: Both systems closed (CLSD) - O2S11 voltage: 0.63V → 0.83V - ST FTRM11: -2.3% → -6.3% - O2S12 voltage: 0.635V → 0.59V - O2S21 voltage: 0.84V → 0.09V - ST FTRM21: 0% → 3.9% - O2S22 voltage: 0.655V → 0.115V During acceleration, LT FTRM2 spikes to 25%, while other values remain stable. I performed a carburetor cleaner test on the intake manifold with no noticeable RPM change when spraying around the system. However, only when I sprayed directly into a vacuum line did the engine react—indicating a possible localized vacuum leak rather than widespread intake issues. When accelerating, I hear a growling noise, which initially led me to suspect a clogged catalytic converter (CAT), especially given past rattling noises from the exhaust shield. I removed both O2 sensors to test idle stability but saw no improvement in RPM or misfire behavior. What I’m seeking is expert insight into possible root causes—especially whether fuel pressure issues, vacuum leaks, or catalytic converter failure could be responsible for this P0300 code and the erratic fuel trim readings. Any guidance would be greatly appreciated.

Related fault codes

P0300

Comments (7)

Anonymous 2011-04-17 14:10

P0300 is a very common issue on 2001 GMC 5.3L V8 engines. Have you reviewed previous threads in this forum? Did any prior repairs resolve the problem? Do you have access to a fuel pressure gauge for testing?

Anonymous 2011-04-19 17:36

I've gone through several years of discussions on this engine platform, but consistent data remains limited. So far, I’ve replaced the fuel filter. The spark plugs and wires are original equipment (OE) with about 40k miles. Fuel pressure readings: - Static (engine off): 50 psi - Idle (550 RPM), vacuum line connected to FPR: 49 psi - Idle (550 RPM), vacuum line disconnected: 60 psi - During throttle response or revving, with vacuum line attached: 56 psi - During throttle response or revving, with vacuum line disconnected: 56 psi Vacuum at the FPR line when FPR is unplugged: 15–18 inHg (idle) The data suggests a fuel pressure regulator (FPR) issue. However, I’m puzzled—why would only Bank 2 show high fuel trim values if the problem were purely related to fuel pressure? I plan to install an O2 bung adapter to measure catalytic converter backpressure next.

Anonymous 2011-04-19 19:18

Try redriving the vehicle and collecting scan data with the vacuum line removed from the fuel pressure regulator. This could indicate a low fuel pressure issue. Also, when the engine is turned off, how quickly does the fuel pressure drop? A rapid bleed-off may point to an FPR malfunction.

Anonymous 2011-04-25 21:45

I recently built an O2 bung adapter and took catalytic converter backpressure readings. Results: - Passenger side (Bank 1): Oscillated between 1–2 psi, with spikes to 3–4 psi during misfire events. - Driver side (Bank 2): Consistently held at 5–6 psi with no spikes. I suspect the driver-side converter is faulty. I also tested driving with upstream O2 sensors removed—this restored engine power and stability, suggesting a sensor-related issue or converter blockage. Would it be wise to replace both catalytic converters? Given the symptoms, an intake leak cannot be ruled out either. Thanks for the suggestion!

Anonymous 2011-04-26 05:05

I recommend replacing both catalytic converters. A reading of 5–6 psi on one side is clearly excessive and likely causing misfire; 3–4 psi is near the acceptable threshold but still borderline. What’s confusing is that Bank 1 (passenger side) appears to have more physical blockage, yet it's Bank 2 showing the highest fuel trim deviation—this suggests multiple underlying issues may be contributing to converter plugging. Have you tested for vacuum leaks? And did you perform the fuel pressure leak-down test I asked about?

Anonymous 2011-04-26 14:41

I apologize—I forgot to mention the fuel pressure leak-down test. The pressure dropped from 50 psi to 46 psi within approximately 1.5 hours after engine shutdown. Regarding vacuum leaks: I sprayed carb cleaner across the intake manifold with no noticeable effect on RPM. I also disconnected the vacuum line leading to the FPR and sprayed cleaner past the opening—no change in performance. The only time the engine reacted was when I sprayed directly into the vacuum port, indicating a localized leak rather than a broad intake issue.

Anonymous 2011-04-26 16:45

I'm interested to see your scan data after installing new catalytic converters! This suggests no significant vacuum leak. The root cause may lie in fuel pressure regulation or converter failure, with both systems potentially affected.