* Alternator/Charging System: This is the most likely suspect given the nighttime worsening. The alternator charges the battery, and if it's failing, it might produce adequate power during the day when less electrical load is placed on it (headlights, blower motor, etc. are off). At night, with increased electrical demand, the failing alternator might struggle, leading to lower voltage, causing erratic engine operation. The engine's computer relies on a stable voltage; low voltage can lead to misfires and rough running. Get your alternator and battery voltage tested.
* Ignition System: While less likely to be solely night-dependent, a failing ignition component (distributor cap, rotor arm, ignition coil, spark plug wires, or even a failing crank sensor) could cause misfires. Nighttime heat differences can sometimes exacerbate problems with worn parts, but this is less common than alternator issues.
* Temperature-Sensitive Sensor: Some engine sensors (like the coolant temperature sensor or mass airflow sensor) might be failing and are more affected by the cooler nighttime temperatures. This would affect the engine's fuel/air mixture calculation, leading to a rough idle or running.
* Vacuum Leaks: A vacuum leak can cause erratic running, and while not typically night-dependent, it's a possible issue to consider.
* Fuel Delivery Problems: A fuel pump that's weakening with age could struggle to keep up with demand, leading to rough running. It's less likely to be dramatically worse at night but is still a possibility.
Troubleshooting Steps:
1. Check the Battery Voltage: Test the battery voltage both when the engine is off and when it's running. A low voltage while running points to an alternator problem.
2. Inspect the Ignition System: Visually inspect the distributor cap, rotor arm, spark plug wires for cracks, wear, or damage.
3. Scan for Diagnostic Trouble Codes (DTCs): If you have access to an OBD-II scanner (most auto parts stores can do this for a small fee), scan for any trouble codes. This can give you specific clues to the problem. Note that older vehicles like the EB Falcon may not be fully OBD-II compliant, but some codes might still be available.
4. Listen for Unusual Noises: Pay attention to any unusual noises coming from the engine bay – clicking, whining, etc. – that might indicate a failing component.
Given the nighttime correlation, I strongly suggest focusing on the alternator and charging system first. A simple alternator test at an auto parts store is a very inexpensive and quick way to rule this out or confirm it as the problem. Don't drive the car excessively while it's running rough, as further damage is possible.
