Possible Culprits:
* Vacuum Leaks: A significant vacuum leak can cause a rich condition. The engine's computer (PCM) may be adding more fuel to compensate for what it perceives as a lean condition (due to the leak), even though the overall mixture is rich. The power surge when you let off the gas is consistent with a vacuum leak affecting the throttle body or other vacuum-controlled components.
* EGR Valve: The Exhaust Gas Recirculation (EGR) valve might be stuck open or malfunctioning. This would introduce exhaust gases into the intake manifold, potentially leading to a rich condition and rough running. A stuck-open EGR can cause a power surge when you let off the gas because it reduces the intake charge.
* Mass Airflow Sensor (MAF) Sensor: A dirty or faulty MAF sensor provides inaccurate readings of the incoming air. This inaccurate information can cause the PCM to inject the wrong amount of fuel, resulting in a rich mixture. A faulty MAF can cause a variety of strange running conditions.
* Throttle Position Sensor (TPS): Similar to the MAF sensor, a faulty TPS provides inaccurate information about the throttle position to the PCM. This can lead to incorrect fuel delivery.
* Oxygen Sensors (O2 Sensors): These sensors monitor the exhaust gas composition and provide feedback to the PCM about the air-fuel mixture. Faulty O2 sensors can cause inaccurate readings and a rich condition. A slow or sluggish O2 sensor is particularly likely to cause these symptoms.
* Fuel Injectors: If one or more fuel injectors are sticking open or delivering too much fuel, this will obviously cause a rich condition.
* Fuel Pressure Regulator: A faulty fuel pressure regulator could be delivering too much fuel pressure to the system.
* PCM (Powertrain Control Module): In rare cases, a faulty PCM itself could be the culprit. This is less likely than the other components but is a possibility if other components have been ruled out.
Troubleshooting Steps:
1. Check for Vacuum Leaks: Visually inspect all vacuum lines and connections for cracks, holes, or loose connections. Use a vacuum gauge to check for proper vacuum at idle.
2. Inspect the EGR Valve: Check for proper operation of the EGR valve. You might be able to manually test it (look up a specific procedure for your engine).
3. Clean or Replace the MAF Sensor: Use MAF sensor cleaner to clean the sensor element (be very careful!). If cleaning doesn't help, replacement is necessary.
4. Check/Replace the TPS: Testing the TPS often requires a multimeter and a bit of technical knowledge.
5. Check/Replace the O2 Sensors: Often these need to be replaced after a certain mileage, but a faulty one can cause these symptoms.
6. Check Fuel Pressure: This needs a fuel pressure gauge to measure if fuel pressure is within specifications.
7. Scan for Diagnostic Trouble Codes (DTCs): Use an OBD-II scanner to read any trouble codes stored in the PCM. These codes can provide valuable clues about the problem.
Important Note: Working on fuel systems and engine components can be dangerous. If you're not comfortable performing these repairs yourself, take your truck to a qualified mechanic. Improperly addressing a rich condition can damage your catalytic converter.
