* Faulty Spark Plugs: Even if the wires and cap are new, the spark plugs themselves could be worn, fouled, or improperly gapped. They are a critical component of the ignition system. Inspect them carefully, and replace them if necessary.
* Ignition Coil(s): The coil(s) provide the high voltage needed to fire the spark plugs. A failing coil can cause misfires and a rough running engine. Testing the coils is necessary to diagnose this.
* Distributor Issues (if applicable): The 1991 DeVille likely uses a distributor. Issues within the distributor itself, such as a worn rotor or cap (if it has a separate cap), could be causing problems despite replacing the external cap.
* Worn or Damaged Wires (Despite Replacement): While you replaced the wires, it's possible one was damaged during installation (a crack in the insulation, a pinched wire, etc.), or you received a faulty set. Carefully inspect all wires for damage.
* Ignition Control Module (ICM): This module controls the ignition system's timing and firing sequence. A faulty ICM can lead to misfires and rough running.
* Crank Sensor/Cam Sensor: These sensors tell the computer the position of the crankshaft and camshaft, crucial for proper ignition timing. A faulty sensor can cause erratic engine behavior.
* Fuel System Problems: A rough running engine isn't always an ignition problem. Issues with the fuel injectors, fuel pump, or fuel filter can also cause similar symptoms.
* Vacuum Leaks: Leaks in the intake manifold or vacuum lines can disrupt the air-fuel mixture, leading to a rough idle.
* Incorrect Wire Routing: It's crucial that the spark plug wires are connected to the correct cylinders in the correct firing order. A misrouted wire will cause misfires. Double-check the firing order in your owner's manual and ensure each wire is correctly connected.
Troubleshooting Steps:
1. Visually inspect: Carefully examine the new spark plug wires for any damage and make sure they are securely connected.
2. Check spark plugs: Inspect and replace if necessary.
3. Use a diagnostic scanner/code reader: A code reader can pinpoint specific trouble codes that might indicate the problem. This is a valuable tool for modern cars.
4. Listen for unusual sounds: A ticking or clicking noise might indicate a faulty component.
5. Professional help: If you're not comfortable performing these checks yourself, take your car to a qualified mechanic.
Without more information about the specific symptoms (e.g., misfiring at idle, only under load, specific cylinder misfire, etc.), it's difficult to provide a more definitive answer. The steps above will help you narrow down the possibilities.
