* Starter Solenoid: This is the most likely culprit. Heat causes the solenoid's internal contacts to expand and potentially stick or create poor contact. When cold, everything is tighter, allowing the connection to be made, but the heat expands the parts, hindering the current flow. This results in a slow crank or no crank at all.
* Starter Motor Brushes: The brushes wear down over time, and the heat can exacerbate the issue. Worn brushes may not make good contact with the commutator, especially when expanded by heat.
* Battery Cables and Connections: Heat can cause corrosion and looseness in battery cables and connections, increasing resistance. This is less likely to completely prevent starting, but it could contribute to the slow cranking when warm. Check for corrosion and ensure tight connections.
* Starter Motor itself: The starter motor windings might have internal shorts or weakened windings. These problems often become more apparent when the motor heats up due to increased resistance and potential for failure.
* High Resistance in Wiring: A high-resistance connection anywhere in the starting circuit (between the battery, solenoid, and starter) can cause this symptom. Heat increases this resistance, further hindering the current flow.
Troubleshooting Steps:
1. Check Battery and Cables: Ensure the battery is fully charged and the cables are clean, tight, and free of corrosion. A voltage drop test across the battery terminals and at the starter connection while cranking can help identify a cable issue.
2. Tap the Solenoid: With the ignition key in the "start" position, gently tap the starter solenoid with a hammer or wrench. If it starts, the solenoid is almost certainly the problem.
3. Check Starter Voltage: Have someone crank the engine while you measure the voltage at the starter motor's terminals. A significant voltage drop (more than a volt or two) indicates a problem in the wiring or the starter itself.
4. Inspect Starter: If the solenoid is still suspected, it might be necessary to remove and inspect/rebuild or replace the starter. A mechanic can bench test the starter to isolate a faulty solenoid or motor.
5. Consider the Engine: While less likely, issues within the engine (like extremely high compression or a seized bearing) could put excessive strain on the starter motor, making it harder to turn when warm and potentially contributing to early failure.
In summary, the most probable causes are a faulty starter solenoid and/or worn starter motor brushes. Focusing on these areas first is the most efficient way to diagnose and resolve the issue. If you are not comfortable working on your car's electrical system, it's best to take it to a qualified mechanic.
