High-Probability Causes:
* Fuel System:
* Fuel Pump Relay: A failing relay might not be supplying power to the fuel pump consistently, especially when the engine is under load and then idles down. Check for power at the relay and at the pump itself when the engine is running and when it's turned off.
* Fuel Pump: The fuel pump itself could be weak or failing, unable to maintain sufficient pressure at idle. A pressure test is crucial here.
* Fuel Filter: A clogged fuel filter restricts fuel flow, causing problems particularly at low engine speeds. This is a cheap and easy part to replace.
* Fuel Pressure Regulator: This regulates fuel pressure. A faulty regulator can lead to inconsistent fuel delivery, causing stalling.
* Injectors: Clogged or malfunctioning fuel injectors could lead to poor fuel delivery at idle. This is less likely but possible.
* Ignition System:
* Idle Air Control (IAC) Valve: This valve regulates air flow at idle. A dirty or faulty IAC valve can disrupt the air/fuel mixture, leading to stalling. Cleaning it is often the first step.
* Distributor Cap and Rotor: Wear and tear on these components can cause misfires and stalling, particularly at lower RPMs. They are cheap to replace.
* Ignition Coil: A weak coil can struggle to provide the necessary spark at idle.
* Other Potential Causes:
* Crankshaft Position Sensor (CKP): This sensor tells the engine's computer the crankshaft's position. A faulty sensor can lead to erratic engine behavior and stalling.
* Mass Air Flow (MAF) Sensor: The MAF measures the amount of air entering the engine. A dirty or faulty MAF sensor will disrupt the air/fuel mixture.
* Vacuum Leaks: Leaks in vacuum lines can disrupt the proper functioning of many engine components. Carefully inspect all vacuum lines.
* Throttle Position Sensor (TPS): A faulty TPS provides inaccurate information to the engine control unit (ECU) about throttle position.
* Engine Temperature Sensor: An inaccurate reading could cause the ECU to use the wrong fuel mixture.
Troubleshooting Steps:
1. Check for Error Codes: If your Volvo has an OBD-I system (likely), you might be able to get diagnostic trouble codes (DTCs) read by a mechanic or using a scanner.
2. Visual Inspection: Inspect all visible vacuum lines, hoses, and wiring for damage or leaks.
3. Start with the Easy Stuff: Replace the fuel filter and clean the IAC valve. These are inexpensive and relatively easy to do.
4. Systematic Testing: Test the fuel pressure, check for spark, and inspect the distributor cap and rotor. These steps require more tools and knowledge.
5. Professional Help: If you're not comfortable working on your car's fuel and ignition systems, take it to a qualified mechanic specializing in older Volvos. They will have the tools and experience to diagnose the problem efficiently.
The age of the car means multiple components might be failing simultaneously. A systematic approach and careful testing are key to pinpointing the cause. Remember safety first—work on the car in a well-ventilated area and disconnect the battery negative terminal before working on any electrical components.
