Fuel System:
* Fuel pump: A failing fuel pump can't deliver enough fuel to the engine, causing it to stall. This is a common culprit.
* Fuel filter: A clogged fuel filter restricts fuel flow, leading to similar symptoms.
* Fuel injectors: Clogged or malfunctioning injectors can prevent proper fuel delivery.
* Fuel pressure regulator: A faulty regulator might not maintain the correct fuel pressure.
Ignition System:
* Distributor cap and rotor: Worn or cracked components can cause poor spark delivery.
* Ignition coil: A weak or failing coil won't produce enough spark to ignite the fuel-air mixture.
* Ignition module: This controls the spark and can fail, leading to complete engine shutdown.
* Spark plugs and wires: Worn or damaged plugs and wires can interrupt the spark.
Electrical System:
* Alternator: If the alternator fails, the battery will drain, eventually causing the engine to die. Check the battery voltage while the engine is running.
* Battery: A weak or failing battery might not provide enough power to keep the engine running, especially under load.
* Wiring harness: Damaged or corroded wiring can interrupt power to crucial components. Look for any obvious damage, especially near the engine.
* Crankshaft Position Sensor (CKP): This sensor tells the computer the engine's rotational speed. A faulty one will cause the engine to die.
* Mass Airflow Sensor (MAF): A bad MAF sensor provides inaccurate air flow readings causing the engine to run lean or rich. This could lead to stalling.
Other Possibilities:
* Transmission Issues: While less likely to cause a complete shutdown, transmission problems could cause the engine to stall, especially if related to the torque converter.
* Overheating: An overheated engine might shut down as a safety precaution. Check your coolant level and look for signs of overheating.
* Vacuum Leaks: Significant vacuum leaks can disrupt the engine's operation and cause stalling.
Troubleshooting Steps:
1. Check the obvious: Start with the simple things: Battery voltage, coolant level, and look for any obvious signs of damage (broken wires, loose connections).
2. Listen for unusual noises: Are there any unusual clicking, grinding, or whirring sounds when the car stalls? This can give clues about the problem area.
3. Check the engine codes: Use an OBD-II scanner (though 1991 might not be fully OBD-II compliant, some scanners can still read codes). This will provide diagnostic trouble codes (DTCs) that can pinpoint potential problems.
4. Professional diagnosis: If you're not comfortable working on your car yourself, take it to a mechanic specializing in older Mustangs or fuel injection systems.
It's crucial to diagnose the problem correctly to avoid further damage or costly repairs. Don't continue driving the car if it's stalling unpredictably, as this can be dangerous.
