* Ignition System: Heat soak can affect the ignition coil(s) or ignition control module (ICM). When hot, these components might malfunction, leading to misfires under load (acceleration). The misfires wouldn't be as noticeable at idle because the engine isn't demanding as much power. Inspect the coil(s) and ICM for any signs of damage or overheating.
* Throttle Position Sensor (TPS): A faulty TPS, especially one affected by heat, can provide inaccurate readings to the engine control unit (ECU), leading to poor fuel delivery and/or ignition timing under load. The ECU might compensate at idle, but the problem becomes apparent when more fuel and timing are needed.
* Mass Airflow Sensor (MAF): Similar to the TPS, a failing MAF sensor can provide inaccurate air readings, leading to a lean or rich condition under load. Heat can exacerbate these issues.
* Computer Related: Though less likely given you've replaced other components, a heat-sensitive problem within the ECU itself isn't entirely ruled out. This would be a more difficult diagnosis.
* Catalytic Converter: A clogged catalytic converter would restrict exhaust flow, especially under load. The increased backpressure could cause the engine to struggle to accelerate. This isn't as likely to only manifest after prolonged operation, but is still worth considering.
* Intake Air Restriction: Check the air filter, the entire intake tract for restrictions, and the air intake ducting itself for any cracks that might allow hot air to enter and reduce the overall airflow which may cause problems.
Troubleshooting Steps:
1. Heat Soak Test: Let the engine run until it's hot. Then, carefully try to reproduce the problem while monitoring the engine's behavior with a scan tool (OBDII reader). Look for any trouble codes (DTCs) that might appear. Pay close attention to engine RPM, fuel trims, and oxygen sensor readings during acceleration attempts.
2. Visual Inspection: Carefully inspect all components mentioned above for any signs of heat damage (discoloration, melting, cracks).
3. Scan Tool Data: A scan tool is crucial. It can provide real-time data on various engine parameters and identify issues that a visual inspection might miss. Look for misfires, lean/rich conditions, TPS/MAF sensor readings, and other relevant data.
4. Check Exhaust Backpressure: While this requires more specialized tools, measuring exhaust backpressure can determine if the catalytic converter is significantly restricted.
5. Check for Vacuum Leaks: Heat can exacerbate existing vacuum leaks, leading to erratic engine behavior. Thoroughly inspect all vacuum hoses and connections.
6. Professional Diagnosis: If you're unable to diagnose the problem yourself, taking it to a qualified mechanic experienced with GM vehicles is recommended. They have access to more sophisticated diagnostic tools and can perform more thorough testing.
Given the symptoms and repairs already undertaken, it strongly suggests a heat-related issue within the engine management system. Focus your efforts on the ignition system and the sensors mentioned above. The scan tool data will be invaluable in pinpointing the exact cause.
