* Incorrectly Installed Spark Plugs: This is the most common culprit. Even a slightly misaligned plug can cause a misfire, leading to shaking. Check that:
* Gap is correct: The gap between the spark plug electrodes needs to be within the manufacturer's specification (check your owner's manual). A gap that's too wide or too narrow will prevent proper ignition.
* Plugs are tightened properly: Over-tightening can damage the threads, while under-tightening can lead to misfires. Use a torque wrench to ensure you're tightening them to the correct specification.
* Plugs are seated correctly: Make sure the spark plugs are fully seated in the cylinder head.
* Correct plugs were used: Double-check that you installed the correct type and heat range of spark plugs for your engine.
* Incorrectly Installed Spark Plug Wires: Similar to the plugs, incorrectly routed or attached wires can cause misfires. Ensure:
* Wires are securely attached: The wires should be firmly snapped onto both the spark plug and the distributor cap (if applicable, some Grand Prix models use coil packs). A loose connection will prevent a proper spark.
* Wires are in the correct order: The firing order of the cylinders is crucial. A mis-wired system will cause significant misfires. Consult your owner's manual for the correct firing order. Getting this wrong is a major cause of shaking.
* Wires are not damaged: Inspect the wires for any cracks, fraying, or other damage that could disrupt the electrical signal.
* Damaged or Failing Ignition Components: The problem might not be the new parts themselves, but pre-existing issues exacerbated by the process of changing them. This could include:
* Distributor cap and rotor (if applicable): Inspect these for cracks, wear, or carbon buildup.
* Ignition coil (or coil packs): A failing coil can cause misfires in one or more cylinders.
* Crankshaft position sensor: This sensor tells the computer the engine's rotational position. A faulty sensor can lead to misfires.
* Other Issues: While less likely, the shaking could be unrelated to the spark plug and wire change. Consider:
* Vacuum leaks: A leak in the intake system can disrupt engine performance.
* Engine mounts: Worn engine mounts can allow excessive engine vibration.
Troubleshooting Steps:
1. Double-check all connections: Carefully inspect all spark plug and wire connections.
2. Verify firing order: Ensure the wires are in the correct order.
3. Check spark plug gap: Use a feeler gauge to ensure the gap is correct.
4. Visually inspect the distributor cap and rotor (if applicable): Check for cracks or damage.
5. Listen for misfires: With the engine running, listen carefully for any unusual sounds that indicate misfires (a popping or sputtering sound).
If you've checked all of the above and the shaking persists, it's best to consult a qualified mechanic. They have the tools and expertise to diagnose and repair more complex ignition system issues or other potential problems.
