* Incorrect gap: The most likely reason. Spark plugs need to have the correct gap between the electrodes. If the gap is too wide or too narrow, the spark won't jump reliably, leading to misfires (missing). You should check the gap with a feeler gauge and adjust it according to the manufacturer's specifications found in your car's owner's manual.
* Incorrect plug type: Did you install the correct type of spark plug? Using the wrong heat range, thread size, or reach can cause problems. Double-check your owner's manual or a reliable parts catalog to confirm you have the right plugs.
* Damaged spark plug wires (or coils): A damaged or loose spark plug wire can prevent a good spark from reaching the plug. Inspect the wires for cracks, fraying, or loose connections. Similarly, problems with the ignition coil(s) (which supply voltage to the plugs) are common sources of misfires and can manifest even after a plug change.
* Loose or damaged plug installation: Did you tighten the plugs sufficiently? Over-tightening can damage the threads, and under-tightening can cause misfires due to poor contact. They should be tightened to the manufacturer's specified torque (again, check your manual). Cross-threading is also a possibility if you weren't careful during installation.
* Other ignition system problems: The problem might lie elsewhere in the ignition system – the ignition control module, distributor cap and rotor (if applicable), or crankshaft position sensor.
* Fuel delivery issues: A problem with the fuel injectors, fuel pump, or fuel filter could also lead to misfires.
* Air intake problems: A dirty air filter or problems with the mass airflow sensor can affect the air/fuel mixture, resulting in a rough running engine.
What to do:
1. Recheck your work: Verify you have the correct spark plugs installed, that the gap is correct, and that they are properly tightened.
2. Inspect the spark plug wires and ignition coils: Look for any visible damage or loose connections.
3. Check for error codes: Use an OBD-II scanner to read any diagnostic trouble codes (DTCs) stored by the car's computer. These codes can pinpoint the problem area.
4. Consult a mechanic: If you can't identify the problem, it's best to take your car to a qualified mechanic for diagnosis and repair. Continuing to drive with a misfiring engine can cause further damage.
Don't dismiss the possibility that the problem existed *before* you changed the plugs, and the plug change simply revealed a pre-existing issue.
