High-Probability Causes:
* Ignition System: This is often the culprit in misfires. Check these components:
* Spark Plugs and Wires: Worn, fouled, or damaged spark plugs and wires are common causes of misfires. Replace them – they're relatively inexpensive.
* Ignition Coil(s): The DeVille likely has multiple ignition coils (one for each cylinder or a group of cylinders). A failing coil will prevent a cylinder from firing correctly. Testing is necessary to pinpoint a faulty coil.
* Distributor Cap and Rotor (if applicable): While less common on later model vehicles like this, some variations might still utilize a distributor. Check for cracks, corrosion, or wear.
* Ignition Control Module (ICM): This module controls the ignition system. A failing ICM can lead to intermittent or widespread misfires.
* Fuel System: Problems here can also lead to misfires:
* Fuel Injectors: Clogged or leaking injectors can prevent the proper amount of fuel from reaching the cylinder. A fuel injector cleaner might help temporarily, but often replacement is needed.
* Fuel Filter: A clogged fuel filter restricts fuel flow, leading to a lean condition and misfires. This is a relatively cheap and easy part to replace.
* Fuel Pump: A weak or failing fuel pump might not deliver sufficient fuel pressure.
* Sensors: Various sensors monitor engine conditions and send information to the computer. A faulty sensor can cause the computer to miscalculate fuel delivery or ignition timing:
* Mass Airflow Sensor (MAF): Measures the amount of air entering the engine. A faulty MAF sensor can cause incorrect fuel mixture, leading to misfires.
* Crankshaft Position Sensor (CKP): This sensor tells the computer the crankshaft's position. A faulty CKP will disrupt ignition timing.
* Throttle Position Sensor (TPS): Tells the computer the throttle position. A faulty TPS might cause incorrect fuel delivery.
Less-Probable (but still possible) Causes:
* Vacuum Leaks: Leaks in vacuum lines can disrupt engine performance and cause misfires.
* Compression Issues: Low compression in one or more cylinders suggests a more serious problem, such as worn piston rings, valve problems, or a blown head gasket. A compression test is needed to diagnose this.
* Engine Control Module (ECM): While less likely, a faulty ECM can cause misfires. This is a more expensive repair.
Troubleshooting Steps:
1. Start with the easy and inexpensive items: Spark plugs, wires, and fuel filter. Replace them first.
2. Check for trouble codes: Use an OBD-II scanner to read any diagnostic trouble codes (DTCs) stored by the car's computer. These codes can provide valuable clues.
3. Visual Inspection: Carefully inspect all components of the ignition and fuel systems for visible damage or signs of wear.
4. Professional Diagnosis: If you're not comfortable working on your car yourself, take it to a qualified mechanic. They have the tools and expertise to diagnose the problem accurately and efficiently.
Important Note: The 1998 DeVille has a Northstar engine, known for its complexity. Working on it requires specific knowledge and tools. If you're not experienced, it's best to leave the diagnosis and repair to a professional to avoid further damage. A misfire, if left unchecked, can lead to significant engine damage.
