Ignition System:
* Worn spark plugs or wires: This is a common cause of misfires, leading to jerking. Old spark plugs lose their ability to ignite the fuel-air mixture efficiently.
* Faulty ignition coil: The coil provides the high voltage needed for the spark plugs. A failing coil can cause intermittent misfires.
* Distributor cap and rotor (if equipped): These components distribute the high voltage to the spark plugs. Cracks or corrosion can disrupt the spark.
* Ignition control module (ICM): This electronic component controls the ignition timing. A failing ICM can cause erratic engine behavior.
Fuel System:
* Clogged fuel filter: A dirty filter restricts fuel flow, leading to inconsistent engine performance.
* Fuel pump issues: A weak or failing fuel pump can't supply enough fuel at the necessary pressure.
* Fuel injectors: Clogged or malfunctioning injectors can deliver an inconsistent amount of fuel to the cylinders.
Transmission:
* Transmission problems (automatic): If it's an automatic, a slipping transmission, worn clutch packs, or low transmission fluid can cause jerking. This is especially likely if the jerking is more pronounced during acceleration or shifting.
Other Possibilities:
* Vacuum leaks: Leaks in the vacuum system can disrupt the engine's timing and fuel delivery.
* Oxygen sensor (O2 sensor): A faulty O2 sensor provides inaccurate information to the engine computer, leading to poor fuel mixture and jerking.
* Mass airflow sensor (MAF sensor): A dirty or faulty MAF sensor can cause the engine to receive incorrect readings about the amount of air entering the engine.
* Engine mounts: Worn engine mounts can cause the engine to move excessively, leading to vibrations and jerking.
Troubleshooting Steps:
1. Check engine light: If the check engine light is on, have the codes read by an OBD-II scanner (though 1991 models might not fully support this). The codes will point to potential problems.
2. Listen to the engine: Pay attention to unusual noises like knocking, ticking, or whining.
3. Check fluids: Ensure the engine oil, coolant, and transmission fluid (if automatic) are at the correct levels and condition.
4. Inspect visually: Look for obvious signs of damage, leaks, or loose connections under the hood.
Recommendation:
Because there are so many potential causes, it's best to have a qualified mechanic diagnose the problem. Attempting to fix it yourself without the proper knowledge and tools could lead to further damage or injury. The mechanic can perform tests and pinpoint the source of the jerking.
