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2002 Pontiac Montana 3.4L Engine Misfire Code P0303: Diagnosis and Solutions

Model: 2002 Pont. Montana 3.4 Fault Code: P0303 Posted: 2007-03-28 13:22

I recently installed a new crankshaft, rod, and all bearings in my 2002 Pontiac Montana 3.4L V6 engine. After driving for two weeks without issues, I began receiving a persistent misfire code P0303. Despite replacing all spark plugs, plug wires, coil pack, fuel injectors, and even the crankshaft position sensor (which had a nick), the problem persists. The engine does not exhibit dead cylinder behavior—it fires, but with irregularity—especially at idle. Compression is strong across all cylinders, and there are no noticeable noises or vibrations. I inspected the intake manifold thoroughly during disassembly and found no visible cracks or leaks. All gaskets were replaced, and fuel pressure readings remain within normal range. I'm now wondering if this could be a vacuum leak, faulty injector o-ring, or an issue with sensor calibration. The engine runs lean as indicated by fuel trim data (LT FTRM % fluctuates between -0.8% and 14.8%), but the short-term fuel trims remain within normal range (-3% to +5%). I’ve also confirmed that MAP readings are around 10.9"Hg, which is acceptable for idle conditions. I've read about a crankshaft position sensor learning procedure required after engine rebuilds—especially on 3.4L V6 engines like mine—but my Actron 9145 scanner does not support this function. Should I take it to a shop for the learn procedure? Or could there be another underlying cause such as an injector leak or faulty signal from the #3 cylinder? Any insights, real-world fixes, or diagnostic steps would be greatly appreciated—especially for owners of 2002 Pontiac Montana 3.4L models with P0303 misfire codes.

Related fault codes

P0303

Comments (9)

Anonymous 2007-04-29 17:15

I have the same issue on my 1999 Pontiac Montana 3.4L V6 with a 4T65E transmission. The P0303 code appears consistently—especially when accelerating or after a few minutes at idle. This is a remanufactured engine that runs smoothly otherwise, but the misfire persists. I've replaced all spark plugs, wires, injectors (professionally reconditioned), and installed a new fuel filter. Fuel looks clean, and coil tests passed according to GM guidelines. I’ve ruled out the crankshaft position sensor learning procedure because my code appears at idle around 800 RPM as well as under load. The only odd reading is LT FTRM % fluctuating from -0.8% to 14.8%. This is very puzzling—no obvious leaks or mechanical issues. Fuel pressure, battery charge, and vacuum systems appear normal.

Anonymous 2007-04-30 16:38

This isn't exactly the same situation, but I have a few suggestions: Have you double-checked all wiring connections, vacuum lines, and harnesses during reassembly? When the misfire occurs, check your fuel trim readings and share them. Regarding the MAP sensor reading—your scan tool shows 11"Hg, but this is likely incorrect. Most scan tools display pressure as a difference (inverted). To get accurate data: turn off the engine, key on, read vacuum level directly from the scanner—it should be around 32"Hg. Start the engine and take another reading. Subtract the second value from the first. A result of ~21"Hg is normal. If you measured only 11"Hg using a gauge or scan tool, that indicates a significant vacuum leak—causing the engine to run lean—and fuel trim will compensate by enriching. 11"Hg at idle is far too low and points strongly to a leak.

Anonymous 2007-05-02 08:20

The 11"Hg reading was from the MAP sensor on my scan tool, so that’s within acceptable range. I’ve read that after an engine rebuild, a crankshaft position sensor 'learn' procedure is required to synchronize the 3x and 24x sensors—especially for 3.4L V6 engines like mine. However, my Actron 9145 scanner doesn’t support this procedure. Should I take it to a professional shop to perform this calibration step?

Anonymous 2007-05-02 15:59

Agreed—this seems like the most plausible next step.

Anonymous 2007-05-02 22:18

I cleared the codes and started cold. Here’s the freeze-frame data when P0303 appeared: TPS% = 0 RPM = 1012 CALC LOAD = 5.1 MAF (LB/M) = 1.09 MAP ("Hg) = 10.9 COOLANT (°F) = 73 LT FTRM1 (%) = 5.5 ST FTRM1 (%) = -3.1 0 MPH CLOSED LOOP The engine is running lean, and the system is compensating with fuel trim. Could old fuel—despite looking clear—be causing this? I’ll check other symptoms listed in lean-running scenarios. Any additional ideas?

Anonymous 2007-05-03 08:25

Double-check that intake gaskets are properly torqued. Even after removing the intake, these gaskets can still leak if over-torqued (crushing) or under-torqued (allowing vacuum leaks). They’re sensitive to torque specifications—this could be a subtle but critical issue.

Anonymous 2007-05-03 14:15

To the original poster: Your fuel trim data shows normal behavior. The short-term reading of -3% means the PCM is actively reducing fuel delivery—a standard, healthy response to oxygen sensor feedback. Fluctuations between negative and positive single digits are normal. Values above double digits indicate a problem that needs attention. Your long-term fuel trim at +5% is acceptable—only a slight lean trend. In one of your posts you mentioned a long-term reading reaching 14%, which could suggest a lean condition—but the freeze-frame data shows no alarming values. The engine appears to be compensating properly.

Anonymous 2007-05-05 10:16

Thanks for the detailed analysis. Since my last update, I had the ECU flashed with updates related to P0300/P0303 handling. I also drained the fuel tank and refilled with fresh gas. The latest freeze-frame data shows both long-term and short-term fuel trims at -0.8%, MAP at 10.9"Hg in closed loop, and coolant temperature at 95°F. If there were a vacuum leak, wouldn’t we see large fuel enrichment (positive trim values)? I’m now leaning toward either the crankshaft sensor learn procedure (which I can get done Monday) or a faulty #3 injector o-ring or signal issue. Even so, such issues should show up in fuel trim data. I’ll wait until Monday to test if the learn procedure resolves it.

Anonymous 2007-05-05 18:11

If your fuel trims remain within normal ranges during both cold and hot engine conditions—especially without large fluctuations—you likely don’t have a gasket leak. Check fuel trim at idle (when gaskets are contracted) and after warm-up (when they expand). If readings stay stable, then gasket issues are unlikely. For P0303 codes on 2002 Pontiac Montana 3.4L engines, the next logical step is to perform a crankshaft position sensor configuration or learn procedure.