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2004 Pontiac Grand Prix GT P0303 Misfire: Diagnosis and Fixes for Cylinder 3

Model: 2004 Grand Prix GT Fault Code: P0303 Posted: 2013-02-19 00:20

My 2004 Pontiac Grand Prix GT with a 3800 V6 engine is showing a P0303 diagnostic trouble code, indicating a misfire in cylinder #3. The symptoms include hesitation when releasing the brakes at a complete stop, resulting in a jerking sensation during steady idle. When I removed the spark plug from cylinder #3, there was no gasoline residue on it. I performed a tune-up first—after which the check engine light returned. A compression test showed all cylinders reading between 60–65 psi, which is below manufacturer specifications. I then cleaned the fuel injector for cylinder #3 using ultra gas and fuel injector cleaner with no improvement. After replacing the entire fuel injector rail (with new AC Delco injectors), the check engine light came back on after the car cooled down and was restarted. I'm wondering if anyone else has experienced this issue or knows what steps to take next. Any insights into possible root causes—such as faulty injectors, low fuel pressure, or intake system modifications? Thanks for any advice.

Related fault codes
P0303
Comments (6)
Anonymous 2013-02-19 09:25

P0303 code — have you checked for any additional codes like P1xxx? These are manufacturer-specific and may indicate deeper issues. Your compression readings of 60–65 psi appear low; verify against the OEM specifications. Have you tested fuel pressure? Also, did you replace just the injector rail or the individual injectors as well? All parts were new AC Delco, correct? Use a noid light to check if cylinder #3's injector is receiving 12V and pulsing properly.

Anonymous 2013-02-19 14:14

The fuel injector rail I purchased came with the injectors pre-installed. It was a used unit, not new.

Anonymous 2013-02-19 14:28

This situation may reflect 'shotgunning' parts—replacing components without proper diagnosis. Let's start with simple checks: 1) Fuel pressure using a dedicated gauge; 2) Confirm cylinder #3 is receiving spark (check if it’s an AC Delco plug or if the issue lies elsewhere); 3) Verify that injector #3 has 12V and is pulsing properly. These three steps are essential to begin troubleshooting effectively. If you need step-by-step instructions, let me know.

Anonymous 2013-02-19 21:17

I'm not a mechanic, but I was present when the technician performed the compression test. I don’t know the exact brand of spark plugs, but I observed that when the plug from cylinder #3 was removed, there was no gasoline residue. The coil pack for cylinder #3 was sparking when the wire was pulled—indicating it's receiving a spark. So, I believe the issue may lie in fuel delivery rather than ignition. Initially I thought it might be a faulty injector or fuel pump, but the seller mentioned that 'intake work' was done on this vehicle—this could be a contributing factor.

Anonymous 2013-02-20 09:30

I’ll assume your technician is competent and will identify the root cause efficiently. If you need further assistance or clarification, feel free to ask.

Anonymous 2013-02-25 18:48

Many owners report issues with rocker arms, pushrods, or even loose bolts in rocker stands after intake manifold work—these mechanical problems can lead to misfires and should be considered during diagnosis.