Fuel System:
* Idle Air Control (IAC) Valve: This valve regulates air flow at idle. A dirty or malfunctioning IAC valve is a very common cause of stalling at idle. It needs to be cleaned or replaced.
* Fuel Pump: A weak fuel pump might not provide enough pressure at idle, leading to stalling. Listen for a humming sound from the fuel pump when the key is turned to the "on" position (but not started). A weak or failing pump will often make a weaker or erratic sound.
* Fuel Filter: A clogged fuel filter restricts fuel flow, causing similar problems to a weak fuel pump. This is a relatively cheap and easy part to replace.
* Fuel Injectors: Clogged or leaking injectors can cause inconsistent fuel delivery. This is a more involved diagnosis and repair.
* Throttle Position Sensor (TPS): A faulty TPS provides inaccurate information to the computer about the throttle position, affecting idle control.
Ignition System:
* Distributor Cap and Rotor: Worn or cracked components can lead to poor spark delivery. Inspect for cracks, corrosion, or excessive wear.
* Spark Plugs and Wires: Worn spark plugs or damaged wires can cause misfires, particularly at idle. Inspect for fouling, gaps that are too large or small, and cracks/damage to wires.
* Ignition Coil: A failing ignition coil can cause weak or intermittent spark.
Vacuum System:
* Vacuum Leaks: Leaks in vacuum hoses can disrupt the operation of various components, including the IAC valve and other emissions control systems. Inspect all vacuum hoses for cracks, holes, or loose connections. Pay particular attention to hoses connected to the brake booster and PCV valve.
* EGR Valve: A sticking or malfunctioning EGR (Exhaust Gas Recirculation) valve can cause idle problems.
Other Potential Issues:
* Mass Air Flow (MAF) Sensor: While less likely to cause stalling at idle specifically, a faulty MAF sensor can disrupt the air/fuel mixture.
* Computer/ECU Problems: Though less common, a faulty engine control unit (ECU) can cause various problems, including stalling. This is usually a last resort diagnosis.
* Transmission Issues: In some rare cases, transmission problems could contribute to stalling. This is less likely if the stalling happens consistently regardless of gear.
Troubleshooting Steps:
1. Check the obvious: Start with the easiest and cheapest things first: Check the spark plugs, wires, distributor cap and rotor (if applicable), and fuel filter. Replace any worn or damaged parts.
2. Listen for unusual noises: Pay attention to any unusual sounds coming from the engine bay, particularly the fuel pump.
3. Inspect vacuum hoses: Carefully check all vacuum hoses for cracks, leaks or loose connections.
4. Clean the IAC valve: This is often a quick and easy fix that yields significant results. Consult a repair manual for proper cleaning procedures.
5. Use a code reader: If you have access to an OBD-II code reader (though your 1991 car may not be OBD-II compliant, it might have a diagnostic port that can reveal trouble codes), check for diagnostic trouble codes (DTCs) that can help pinpoint the problem. You may need a specialized code reader for older vehicles.
Important Note: Without a proper diagnostic procedure, it's difficult to pinpoint the exact cause. If you are not comfortable working on your car yourself, take it to a qualified mechanic for diagnosis and repair. Attempting repairs without the necessary knowledge could worsen the problem.
