Fuel/Air Mixture & Idle Control:
* Idle Air Control (IAC) Valve: This is a very common cause. The IAC regulates air flow at idle. A dirty, sticking, or failing IAC valve can cause high idle and stalling. Cleaning it (carefully) is often a first step. Replacement is relatively inexpensive.
* Throttle Position Sensor (TPS): The TPS tells the computer the throttle position. A faulty TPS can lead to incorrect fuel delivery, causing high idle and poor drivability. Testing is usually necessary to diagnose a bad TPS.
* Mass Air Flow (MAF) Sensor (if equipped): While less likely on a TBI system (which uses a MAP sensor), a dirty or failing MAF sensor can affect the air/fuel mixture. Cleaning it might help, but replacement is often required if faulty.
* MAP Sensor (Manifold Absolute Pressure Sensor): This sensor measures intake manifold pressure. A faulty MAP sensor provides incorrect data to the computer, leading to poor fuel delivery.
* Vacuum Leaks: Leaks in the vacuum lines can disrupt the idle and overall engine performance. Inspect all vacuum lines for cracks, holes, or loose connections. Pay close attention to the lines leading to the brake booster, PCV valve, and other vacuum-operated components.
* Throttle Body: A dirty throttle body can restrict airflow, contributing to idling issues. Cleaning it thoroughly with throttle body cleaner is a good maintenance step.
Transmission Related:
* Torque Converter Clutch: If the torque converter isn't engaging properly, the engine might struggle when you put it in gear. This can manifest as stalling. This is more of a mechanical issue that requires a transmission inspection.
* Transmission Issues: Internal transmission problems could also cause stalling, especially if the transmission is slipping.
Other Possibilities:
* Distributor (if equipped): While less likely on a TBI system with electronic ignition, issues with the distributor (if it's not entirely electronic) could affect timing and idle speed.
* Clogged Fuel Filter: A severely clogged fuel filter can restrict fuel flow, leading to poor performance.
Troubleshooting Steps:
1. Check for Vacuum Leaks: Visually inspect all vacuum lines. Use a vacuum gauge (if you have one) to check manifold vacuum at idle.
2. Clean the Throttle Body and IAC Valve: This is the easiest and cheapest thing to try first.
3. Check the TPS: You'll likely need a multimeter to test the TPS voltage. A simple test is to move the throttle plate and see if the voltage changes smoothly and within specifications (consult a repair manual).
4. Check the MAP Sensor: This often requires a multimeter and checking the sensor voltage against specifications in a repair manual.
5. Inspect the Fuel Filter: Consider replacing it as part of routine maintenance.
6. Check Engine Codes (if equipped): A trouble code reader can pinpoint potential problems. An OBD-I scanner (suitable for your vehicle's year) would be needed.
Important Note: Without proper diagnostics, it's difficult to pinpoint the exact cause. Consulting a repair manual specific to your 1988 Chevy 350 TBI pickup is highly recommended. A mechanic specializing in older vehicles would also be helpful. Attempting repairs without sufficient knowledge can lead to further damage.
