Fuel System:
* Fuel Pump: A failing fuel pump might not provide consistent pressure. It could be weak, intermittently failing, or have a failing relay. Listen for the whirring sound of the pump when the key is turned to the "on" position (before cranking). No sound suggests a pump or relay problem.
* Fuel Filter: A clogged fuel filter restricts fuel flow, leading to intermittent starting. These are relatively inexpensive and easy to replace.
* Fuel Injectors: Clogged or failing injectors might not deliver fuel consistently. This is more difficult to diagnose without specialized tools.
* Fuel Pressure Regulator: A faulty regulator can cause inconsistent fuel pressure.
Ignition System:
* Ignition Coil: A failing coil can cause weak or intermittent sparking.
* Distributor Cap and Rotor: Corrosion or wear in these components can disrupt the spark delivery to the plugs.
* Ignition Switch: The switch itself could be failing, preventing power from reaching the ignition system.
* Spark Plugs and Wires: Worn or fouled spark plugs and damaged wires prevent reliable ignition.
* Crank Angle Sensor (CAS): This sensor tells the ECU the position of the crankshaft, essential for proper ignition timing. A failing sensor can cause intermittent starting issues. (More likely on later fuel injected versions but possible)
Electrical System:
* Battery: A weak or failing battery can struggle to crank the engine, especially in cold weather. Test the battery voltage and have it load tested.
* Battery Cables and Connections: Corrosion on the battery terminals or loose connections can impede current flow. Clean the terminals thoroughly.
* Starter Motor: A failing starter can sometimes crank the engine weakly or fail entirely.
* ECU (Engine Control Unit): While less common, a malfunctioning ECU can disrupt various systems, leading to starting problems.
Other Potential Causes:
* Temperature Sensor: A faulty temperature sensor provides inaccurate data to the ECU, impacting fuel delivery and ignition timing.
* Alarm System (if equipped): An issue with the alarm system could be preventing the car from starting.
Troubleshooting Steps:
1. Check the basics: Battery voltage, clean terminals, inspect spark plugs and wires.
2. Listen for the fuel pump: Does it prime when you turn the key?
3. Check for obvious signs of corrosion: Anywhere in the electrical system.
4. Test the fuel pressure: This requires a fuel pressure gauge.
5. Have the ignition system inspected: A mechanic can test the coil, distributor cap and rotor, and spark delivery.
Because it's intermittent, pinpointing the cause might require some detective work. Start with the easier and cheaper checks (battery, cables, fuel filter) before moving to more complex components. A reliable mechanic with experience in older VWs would be invaluable in diagnosing this problem. The year and model are crucial for specific troubleshooting, so double-check you've got the exact details right.
