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2003 Pontiac Grand Prix 3100 V6 OBD P0303 Misfire: Diagnosis and Fix - Intake Lifter or Valve Spring Issue?

Model: 2003 Grand Prix Fault Code: P0303 Posted: 2008-02-15 20:55

My friend's 2003 Pontiac Grand Prix with a 3.1L V6 engine has triggered OBD code P0303 (cylinder #3 misfire). We've already replaced the spark plug, ignition wire, and coil pack for cylinder 3. The intake manifold was removed and all fuel injectors were inspected—confirmed proper fuel spray and voltage readings. Compression test results are normal. The spark plug on cylinder 3 is dry with no gasoline smell, and oil remains clean. The car now makes a distinct ticking noise, which we suspect may be due to a failing intake lifter or valve spring. We've also confirmed that the oxygen sensor is functioning properly, though the catalytic converter emits a rattling sound—possibly indicating internal damage. The vehicle has 80,000 miles on it. Despite replacing the fuel filter and checking fuel pressure (55–65 psi), the P0303 code persists. We've ruled out common issues like intake gasket leaks or ignition module faults based on testing with a Noid and voltmeter. Could a faulty crankshaft or camshaft sensor be causing this misfire? Given that only cylinder #3 is affected, what specific checks should we perform when evaluating the ignition module beyond visual cracks? Any insights from experienced owners or mechanics would be greatly appreciated. Thanks, Drac.

Related fault codes
P0303
Comments (9)
Anonymous 2008-02-16 09:41

The catalytic converter may be restricted—this can cause misfires and should be inspected and replaced if necessary.

Anonymous 2008-02-20 18:16

I've had the same issue: replaced a fuel injector, installed a new cat converter, but still get a persistent P0303 code. The engine idles roughly. Any other troubleshooting steps? Thanks, Drac.

Anonymous 2008-02-20 18:47

Possible causes include an intake gasket leak or ignition module failure. A cylinder leakage test and injector pulse check are recommended to confirm the root cause.

Anonymous 2008-02-20 22:35

The original intake manifold had a crack near the alternator bolt—replaced with new gaskets. I tested fuel injector wiring using a Noid and voltmeter; power readings were within specifications. Fuel pressure was stable at 55–65 psi. Tomorrow, I’ll inspect the ignition module. I’ve also heard that faulty crankshaft or camshaft sensors can cause misfires—what specific signs should I look for when testing the module beyond visible damage? Thanks for your help, Jeff—Drac.

Anonymous 2008-02-21 09:45

If a crankshaft sensor were faulty, wouldn’t it affect multiple cylinders rather than just cylinder #3?

Anonymous 2008-02-22 11:48

Jeff, I replaced the ignition module today. After resetting the system, the P0303 code returned within seconds—misfire on cylinder #3 persists. The ticking sound remains unchanged. Thanks again, Drac.

Anonymous 2008-02-22 12:59

I recall that older Grand Prix models sometimes had EGR issues or plugged intake ports—but it's unlikely to be the sole cause of a single-cylinder misfire. Was the P0303 code present before the intake repair?

Anonymous 2008-03-06 22:16

Turns out, it was a broken intake valve spring—this explains both the ticking noise and the cylinder #3 misfire. Thanks for the update, Drac.

Anonymous 2008-03-07 00:32

Thanks for sharing your resolution—it’s helpful to others facing similar issues.