2004 Grand AM GT 3.4L P0300 Misfire Code After Replacing Head Gasket and All Major Components
Hi, My 2004 Grand AM GT with a 3.4L engine is showing a persistent P0300 random misfire code, along with intermittent misfires on cylinder #4. I initially suspected a leaking head gasket based on coolant loss and rough idle symptoms, but after replacing several key components, the issue remains. Parts replaced include: - Head gaskets (both sides) - Spark plugs (all cylinders) - Fuel pressure regulator - Ignition coil pack (installed on cylinder #2 during prior work) - Spark plug wires - Thermostat - LIM and UIM gaskets - Suspected fuel injector on cylinder #2 After completing a full drive cycle, the P0300 code reappears. While I no longer experience coolant loss or rough idle at startup, the misfire persists. My current suspicions are focused on either a vacuum leak or clogged fuel injectors. However, without access to an OBD2 scanner to check fuel trims (FTs) or individual cylinder performance data, I'm looking for guidance on next steps. Any advice on diagnosing these issues in this specific model would be greatly appreciated.
Did you use AC Delco spark plugs? P0300 is commonly triggered by incorrect or low-quality spark plugs. Check fuel pressure using a gauge and monitor for leaks during operation. Try the vacuum leak spray test to identify any intake system leaks. Regarding clogged injectors — if you don't have an OBD2 scanner to review fuel trims, try using a bottle of SeaFoam or Techron fuel system cleaner. These can help clear injector passages and are worth trying as a next step.