High on the Suspect List:
* Throttle Position Sensor (TPS): A faulty TPS is a very common cause of this type of problem. It tells the computer the throttle's position, and if it's sending incorrect signals, the engine can stumble, surge, and die when you lift off the gas. Test this first. You can often test the TPS voltage output with a multimeter while manipulating the throttle. A bad TPS will show erratic readings.
* Mass Airflow Sensor (MAF): The MAF sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor can cause a lean condition, leading to stalling. Clean the MAF sensor with MAF sensor cleaner (never use anything else!), and if that doesn't fix it, consider replacement.
* Crankshaft Position Sensor (CKP): This sensor tells the computer where the crankshaft is in its rotation. A bad CKP will cause misfires and stalling. It's less likely to cause the problem *only* when letting off the gas, but it's still possible.
* Ignition Control Module (ICM): While less common than the above, a failing ICM can cause intermittent misfires and stalling. This is more likely if the problem is intermittent.
* Vacuum Leaks: Check all vacuum lines for cracks, leaks, or loose connections. A significant vacuum leak can cause the engine to run poorly and stall. Pay close attention to lines connected to the fuel pressure regulator, EGR system, and the intake manifold.
Less Likely, But Possible:
* Fuel Pump Relay: Though you've addressed the fuel pressure regulator, a failing fuel pump relay could cut power intermittently.
* Wiring Harness Issues: Damaged or corroded wiring in the engine harness could lead to intermittent problems.
* Computer (PCM): While the least likely, a failing PCM can cause a wide array of problems.
Troubleshooting Steps:
1. Check for trouble codes: Use a OBD1 scanner (it's specific to that year) to read any diagnostic trouble codes (DTCs) stored in the PCM. These codes will point you in the right direction.
2. Visual Inspection: Carefully inspect all vacuum lines, wiring harnesses, and connectors for damage.
3. Test the TPS: This is the most likely culprit given your symptoms. Follow a guide specific to your vehicle for how to test it with a multimeter.
4. Clean the MAF sensor: Use MAF sensor cleaner and follow instructions carefully.
5. Check for vacuum leaks: Use a vacuum gauge or your ear (carefully and near idling engine) to listen for hissing sounds around the intake manifold and vacuum lines.
6. Test the CKP sensor: This might require an oscilloscope or a more advanced diagnostic tool.
7. Consider a professional: If you are not comfortable performing these tests, take it to a mechanic who is experienced with older GM vehicles.
Remember to always disconnect the negative battery terminal before performing any electrical work. Good luck with your Jimmy! Let me know if you have more information (like specific error codes you get, or any other symptoms) and I can offer more specific advice.
