Fuel System:
* Clogged Fuel Filter: A dirty fuel filter restricts fuel flow, leading to poor performance, especially under load. This is one of the easiest and cheapest things to check first.
* Faulty Fuel Pump: A weak or failing fuel pump may not deliver enough fuel pressure, resulting in hesitation and bogging. You might hear a buzzing sound from the tank if the pump is struggling.
* Clogged Fuel Injectors: Clogged injectors can prevent proper fuel delivery to the cylinders. This often leads to rough running and stuttering. A fuel injector cleaner might help temporarily, but a professional cleaning or replacement might be necessary.
* Bad Fuel Pressure Regulator: This regulator maintains consistent fuel pressure. A faulty one can cause inconsistent fuel delivery.
* Low Fuel: Sounds obvious, but a low fuel level can sometimes lead to fuel starvation and similar symptoms.
Ignition System:
* Worn Spark Plugs or Wires: These are fundamental to combustion. Worn plugs or damaged wires can cause misfires, leading to poor performance and stuttering.
* Distributor Cap and Rotor: Cracks or corrosion in the distributor cap and rotor can interrupt the spark delivery.
* Ignition Coil: A failing ignition coil won't provide sufficient spark to ignite the air/fuel mixture properly.
* Ignition Module (ECM): The Electronic Control Module plays a vital role in the ignition system. A faulty ECM can lead to numerous drivability issues.
Air Intake System:
* Dirty Air Filter: A severely clogged air filter restricts airflow, leading to a lean fuel mixture and poor performance.
* Vacuum Leaks: Leaks in the vacuum lines can disrupt the air/fuel mixture and cause erratic engine behavior.
Other Potential Causes:
* Oxygen Sensor (O2 Sensor): This sensor monitors the exhaust gases and helps the engine computer adjust the air/fuel mixture. A faulty O2 sensor can lead to a rich or lean condition.
* Mass Airflow Sensor (MAF Sensor): This sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor will provide inaccurate readings, leading to poor performance.
* Throttle Position Sensor (TPS): A malfunctioning TPS provides incorrect information to the engine computer about the throttle position, resulting in erratic engine operation.
* Catalytic Converter: A clogged catalytic converter can restrict exhaust flow, leading to reduced engine power and potentially causing stuttering.
Troubleshooting Steps:
1. Start with the easiest and cheapest: Check the fuel filter and air filter. Replace them if necessary.
2. Check for vacuum leaks: Visually inspect vacuum lines for cracks or loose connections.
3. Inspect spark plugs and wires: Check for wear, damage, or corrosion. Replace if needed.
4. Consider a fuel system cleaner: This may temporarily resolve issues with clogged injectors.
5. Use a code reader (OBD-I): A code reader can help identify diagnostic trouble codes (DTCs) stored by the vehicle's computer, pointing to potential issues. Note that 1991 vehicles don't have OBD-II, so the codes and reader will be different.
If you're not comfortable working on your vehicle, take it to a qualified mechanic for diagnosis and repair. The symptoms you describe could indicate multiple problems, and a professional can pinpoint the root cause effectively.
