* Faulty Temperature Sensor: While you may have replaced some sensors, the crucial one here is the engine coolant temperature (ECT) sensor. This sensor tells the computer the engine's temperature, which influences fuel mixture and other parameters. A faulty ECT sensor, especially one that gives incorrect readings at higher temperatures, could lead to stalling. This is a very high probability given your symptoms.
* Ignition Control Module (ICM): This is a common point of failure in older Ford vehicles, especially under heat stress. Even if it's seemingly functioning at cold temperatures, the ICM might overheat and fail once the engine reaches operating temperature.
* Wiring Harness Issues: Heat can cause wires to become brittle and break, short-circuit, or develop high resistance. Look for any signs of damage, chafing, or melting in the wiring harness, especially near the engine and exhaust manifold. A heat-related short in a critical circuit could cause the stall.
* Distributor (if applicable): While you mention replacing the pickup coil, the distributor itself could have internal issues exacerbated by heat. This is less likely if you have a completely electronic ignition system without a distributor.
* Radiator Fan: If your radiator fan isn't working efficiently, the engine could overheat, leading to the stalling issue. Ensure it's cycling on and off correctly. Overheating can indirectly affect various sensors and modules.
* Fuel Pump Relay (Heat Soak): Even though you replaced the relay, the socket or wiring might be causing the relay to fail when the engine gets hot. Check for good clean connections, and consider replacing the socket.
Troubleshooting Steps:
1. Check the ECT sensor: This is the most likely culprit. Try to get a reading from the sensor using a multimeter while the engine is hot. Compare it to specifications in your repair manual. If it doesn't fall within the range, replace it.
2. Inspect the wiring harness: Look carefully for any signs of damage or wear near heat sources.
3. Test the ICM (Ignition Control Module): This requires more specialized testing equipment or a known good ICM for comparison.
4. Verify radiator fan function: Ensure the fan activates when the engine reaches a certain temperature.
5. Check for fuel pressure: If you can, check the fuel pressure at the fuel rail while the engine is hot. A drop in pressure when hot could indicate a heat-related fuel pump issue (even with a new pump), though less likely given you've replaced the system.
If you're not comfortable tackling these yourself, take your Bronco to a qualified mechanic experienced with older Ford vehicles. They have the tools and expertise to diagnose the problem more accurately. It's crucial to mention all the parts you've already replaced to save time and money.
