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2004 GMC Z71 Avalanche 5.3L Rough Idle with P0300 and P0171 - Vacuum Leak or Fuel Issue?

Model: 2004 Z71 Avalanche Fault Code: P0300 Posted: 2009-01-23 22:56

My 2004 GMC Z71 Avalanche with a 5.3L V8 is experiencing a rough idle, and the OBD2 diagnostic system shows both P0300 (random misfire) and P0171 (system too lean). The P0171 code remains pending, though I've only driven short distances (under 5 miles) since the issue started. I recently inspected all vacuum hoses—no visible cracks or disconnections. I also cleaned the MAF sensor with no improvement. However, when I quickly press and release the accelerator pedal, I hear a distinct 'sucking' sound, which suggests a possible vacuum leak. The noise seems to originate near the throttle body area, but all associated hoses appear intact. I'm considering whether it might be the intake manifold gasket. If that were the case, would I expect to see both P0171 and P0174 (lean condition on both banks)? Or is a vacuum leak more likely to cause only one bank to misfire? Any suggestions for diagnosing this issue—especially how to locate the exact source of the vacuum leak or determine if it's related to fuel contamination or sensor failure?

Related fault codes

P0300

Comments (8)

Anonymous 2009-01-24 07:19

It's possible only one bank is currently leaking, with the other side potentially developing a leak later. Intake gaskets are a common suspect in this type of issue—worth checking thoroughly.

Anonymous 2009-01-29 07:29

I replaced both intake gaskets on my 5.3L Z71, and after just a short drive, the P0300 code returned with a rough idle. This suggests that while intake gasket failure is possible, it may not be the root cause—there could be another underlying issue.

Anonymous 2009-01-29 09:46

After monitoring sensor data with my scan tool, I believe the problem lies with the upstream oxygen sensor (O2S11). It reads 0.0 volts in open loop and shows no movement—this indicates either a wiring fault or a faulty sensor. Here are the readings: - O2S11: 0.0 Volts - ST FTRM11: 50.7% - LT FTRM1: 25% - O2S12: 0.955 Volts I suspect a short to ground in the sensor or wiring, as the expected bias voltage of 450mV is not being detected.

Anonymous 2009-01-29 18:07

Oxygen sensors don't provide usable data during open loop operation—the heater must fully warm up before closed-loop control begins. A reading of 0 volts in closed loop suggests a short to ground, not just a weak signal. To confirm, unplug the sensor and observe idle behavior. If the engine runs smoother, it's likely faulty. Fuel trim values show severe lean conditions—this could stem from vacuum leaks or dirty injectors. Also, check which cylinders are showing misfire counts; if all are on bank 1, that may point to a different issue.

Anonymous 2009-01-29 18:38

The readings were consistent in closed loop as well. Even though open-loop data isn't used for fuel trim, the sensor should not register zero volts under normal conditions. I agree—0V likely indicates a short to ground or wiring issue. The intermittent fluctuation suggests a weak signal, but it's still concerning. I plan to inspect the wiring first before replacing the sensor. Also, I'm unsure which cylinders are misfiring since my tool doesn't display detailed cylinder-specific misfire counts.

Anonymous 2009-01-29 19:20

A simple test is to swap the upstream oxygen sensors between banks—observe if the readings follow the other side. If one sensor stays at zero volts, it's likely faulty or shorted.

Anonymous 2009-01-29 19:48

I considered swapping the sensors, but I'm unsure about access—while the suspected bad sensor is easily reachable, the other one is more difficult to remove. I might try this test soon—it could be a quick way to confirm if it's a sensor issue or something else.

Anonymous 2009-03-01 15:38

Hi everyone—first post! I recently had P0300 on my 5.3L GMC Sierra and discovered that water was collecting in the passenger-side fuel rail due to its angled position. It only surfaced when the engine warmed up or during hard braking. After draining the rail and cleaning it out, the misfire disappeared. I added isopropyl alcohol and fuel stabilizer to prevent recurrence. This may be a helpful tip for others with similar symptoms—especially if they suspect fuel contamination.