* Crankshaft Position Sensor (CKP): This sensor tells the computer the engine's rotational position. A failing CKP, especially one affected by cold temperatures, can cause the engine to crank but not fire. It's a common culprit in this type of problem.
* Cam Position Sensor (CMP): Similar to the CKP, but for the camshaft. A faulty CMP will prevent proper ignition timing.
* Ignition Coil(s) or Ignition Module: These components are responsible for delivering the high voltage spark to the spark plugs. If they are weak or failing, they might struggle to ignite the fuel, especially in cold conditions.
* Fuel Delivery Problems:
* Fuel Pump: While less likely if it runs sometimes, a weak fuel pump may struggle to provide enough pressure when cold.
* Fuel Filter: A clogged fuel filter restricts fuel flow, potentially causing starting issues, especially noticeable when cold.
* Fuel Injectors: Clogged or faulty injectors could prevent proper fuel delivery.
* Throttle Position Sensor (TPS): A faulty TPS can send incorrect signals to the computer about the throttle position, preventing the engine from starting or running smoothly.
* Mass Airflow Sensor (MAF): A dirty or faulty MAF sensor provides inaccurate readings about the air intake, potentially leading to poor fuel mixture and starting issues.
* Battery Cables/Connections: Even with a 12.5V reading, corroded or loose battery terminals or cables can cause a poor connection and result in insufficient current during cranking. Clean and tighten these.
* Alternator: Though you're focusing on starting, a weak alternator might not be charging the battery fully overnight, leading to the lower voltage reading. Test the alternator's output while the engine is running.
Troubleshooting Steps:
1. Check Battery Cables and Connections: Clean and tighten all battery terminals and cable ends. Ensure there's good contact.
2. Test the Alternator: Have an auto parts store test your alternator's charging output.
3. Check for Diagnostic Trouble Codes (DTCs): Use an OBD-II scanner to retrieve any stored trouble codes from the engine's computer. These codes can pinpoint the problem area.
4. Check Fuel Pressure: This requires a fuel pressure gauge and some mechanical skill. Low fuel pressure is a strong indicator of a fuel pump or filter issue.
5. Inspect the CKP and CMP Sensors: Look for visible damage or corrosion. They might need replacement.
6. Test the Ignition Coil(s) and Ignition Module: A mechanic or auto electrician can usually perform these tests.
It's difficult to pinpoint the exact cause without more information or diagnostics. Starting with the simpler checks (cables, alternator, DTCs) is a good first step. If these don't reveal the problem, a mechanic specializing in older vehicles might be needed to further diagnose the electrical and fuel systems.
