* EGR System: The Exhaust Gas Recirculation system is a prime suspect. When hot, the EGR valve can stick open or malfunction, allowing excessive exhaust gases back into the intake, leading to a lean condition (though you say it's running rich, a stuck-open EGR can initially create a lean condition that the computer tries to compensate for by adding more fuel, leading to a rich condition). A faulty EGR temperature sensor can also cause issues. Heat makes these problems worse.
* Intake Air Leaks: A vacuum leak, particularly one that worsens when the engine is hot due to expanding materials, can cause erratic idle and a rich mixture. Inspect all vacuum hoses and connections, paying close attention to those near the intake manifold. Cracks in the intake manifold itself are also possible.
* Mass Airflow Sensor (MAF) Sensor: The MAF sensor measures the amount of air entering the engine. If it's failing, particularly as it heats up, it could provide inaccurate readings, leading to a rich fuel mixture. Heat can exacerbate a failing sensor.
* Fuel Injectors: While less likely to be the sole cause of the symptoms, sticking or leaking fuel injectors can contribute, especially as they warm up. They could be leaking more fuel when hot, leading to a rich condition.
* Throttle Position Sensor (TPS): An inaccurate TPS reading can disrupt the air/fuel mixture, especially at idle.
* Computer/PCM Issues: While less common, a failing PCM (Powertrain Control Module) could be misinterpreting sensor readings or failing to adjust properly as the engine heats up.
* Oxygen Sensor(s): A faulty oxygen sensor provides inaccurate feedback to the PCM about the air/fuel mixture. It may not be able to compensate correctly when hot.
Troubleshooting Steps:
1. Check Engine Light Codes: Obtain the diagnostic trouble codes (DTCs) from the PCM using an OBD-II scanner. This is the *most crucial* first step. The codes will pinpoint specific problem areas.
2. Inspect Vacuum Lines: Carefully examine all vacuum lines for cracks, leaks, or loose connections. Pay extra attention to those that become brittle with heat.
3. Visual Inspection: Look for any obvious signs of damage or leaks around the intake manifold, fuel injectors, and other engine components.
4. Test EGR System: This often involves a vacuum test and checking the EGR valve for proper operation. A mechanic might need special tools for this.
5. Test MAF Sensor: A MAF sensor test can be done with a multimeter or a more specialized scan tool.
6. Test TPS Sensor: This involves checking its voltage output with a multimeter.
7. Fuel Injector Cleaning/Testing: A fuel injector cleaning might resolve the issue if it's due to clogged injectors. More rigorous testing may be needed to determine if they're leaking.
Because multiple issues can combine to create this kind of problem, it’s strongly recommended to start with retrieving the diagnostic trouble codes. This will help to narrow down the possibilities significantly and prevent unnecessary repairs. A mechanic specializing in older vehicles will likely be able to diagnose and fix the problem efficiently.
