* Ignition System Problems (Heat Soak): This is a prime suspect. High temperatures can cause components like ignition coils or the ignition control module (ICM) to fail intermittently. When they cool down, they might function again temporarily. The heat soak affects the components even after the engine is shut off.
* Fuel Delivery Issues (Vapor Lock): The fuel pump or fuel lines might be failing under the strain of high temperatures. Vapor lock occurs when fuel vaporizes in the lines, preventing the engine from receiving liquid fuel. This is more common with older fuel systems.
* Sensor Failure (Temperature-Sensitive): Various sensors (like the crankshaft position sensor, coolant temperature sensor, or mass airflow sensor) could be malfunctioning due to the heat. A faulty sensor might provide inaccurate data to the engine control module (ECM), leading to misfires or stalling. These often fail intermittently as they heat up.
* Cooling System Problems (Overheating): While you mention it runs *until* the heat of the day, a partially failing cooling system (radiator, water pump, thermostat, etc.) could be allowing the engine to overheat subtly. This could indirectly cause other components to fail due to excessive heat. Check your coolant level and look for signs of leaks.
* Electrical Connections: Heat can cause expansion and poor connection at electrical terminals and connectors. This could interrupt the flow of electricity to crucial components. Look for any burnt or corroded connections, especially around the ignition system and sensors.
Troubleshooting Steps:
1. Check Coolant Levels: Ensure the coolant level is correct and there are no leaks in the cooling system. A pressure test might be necessary to rule out a pressure leak.
2. Inspect Ignition System: Visually inspect the ignition coil(s), wires, and connectors for any signs of damage or overheating (discoloration, melting, etc.). Consider replacing the ignition coil(s) as a first step since this is often a culprit.
3. Examine Fuel System: Check fuel pressure and look for any signs of fuel leaks. Consider replacing fuel filter and check fuel lines for any cracks or weakening.
4. Diagnose Sensors: A scan tool (OBD-II reader) can help diagnose any sensor problems. Pay close attention to readings from the coolant temperature sensor, mass airflow sensor, and crankshaft position sensor, especially when the engine is hot.
5. Check Electrical Connections: Inspect all electrical connections related to the ignition system, fuel system, and engine sensors. Tighten any loose connections and clean any corroded terminals.
Important Note: Since this involves potential safety hazards related to overheating and engine failure, it's advisable to have a qualified mechanic diagnose and repair the problem. Attempting complex repairs without the proper knowledge and tools could worsen the situation. This is especially important with a large engine like the 8.1L.
