1. Transmission Issues:
* Low Transmission Fluid: Check the transmission fluid level. It should be checked with the engine running and warmed up (following the procedure in your owner's manual). Low fluid can cause all sorts of problems, including stalling. If low, add the correct type and amount of fluid specified in your owner's manual. If it's already full and the fluid is burnt (smells bad or is dark brown/black), the transmission might need a more serious repair or replacement.
* Torque Converter Issues: A faulty torque converter can prevent the engine from properly transferring power to the transmission. This often manifests as stalling when shifting into gear. This is a more complex issue requiring professional diagnosis.
* Internal Transmission Problems: Worn clutch packs, broken gears, or other internal issues within the transmission are possibilities, but less likely to present *only* as stalling when shifting. More symptoms (e.g., slipping, hard shifting, unusual noises) would usually accompany internal transmission problems.
2. Engine Issues:
* Idle Air Control (IAC) Valve: This valve regulates the air flow at idle. A dirty or malfunctioning IAC valve can cause the engine to stall, especially under load (like when shifting into gear). Cleaning the IAC valve (carefully, following online instructions specific to your engine) is a relatively easy fix, but replacement might be needed.
* Throttle Position Sensor (TPS): The TPS tells the engine's computer the position of the throttle. A faulty TPS can cause erratic idle and stalling when put into gear. A replacement TPS might be necessary, and proper adjustment or recalibration might be required.
* Vacuum Leaks: A leak in the vacuum system can disrupt the engine's air-fuel mixture. Check all vacuum hoses for cracks, splits, or loose connections.
* Ignition System Problems: While less likely to cause *only* this symptom, a weak spark or faulty ignition components (distributor cap, rotor, spark plug wires, spark plugs) could contribute. Inspect these components and replace as needed.
* Fuel Delivery Problems: Although less common, a fuel pump issue or clogged fuel filter could restrict fuel flow, causing stalling under load.
3. Other Potential Causes:
* Neutral Safety Switch: This switch prevents the car from starting unless it's in Park or Neutral. A faulty switch might not be registering the gear properly, leading to stalling.
* Catalytic Converter: A severely clogged catalytic converter can restrict exhaust flow, leading to stalling. This is less likely to be the only symptom, however.
Troubleshooting Steps:
1. Check Transmission Fluid: This is the easiest and quickest check to do.
2. Inspect Vacuum Hoses: Look for any obvious cracks or leaks.
3. Clean the IAC Valve: This is a relatively inexpensive and easy DIY fix.
4. Check the TPS: If you have a multimeter, you might be able to check the voltage output of the TPS, but replacement is often simpler.
5. Check for Diagnostic Trouble Codes (DTCs): Use an OBD-II scanner (if your car has an OBD-II port – less likely for a 1991 model, though possible) to retrieve any trouble codes stored by the engine's computer. A scanner for older vehicles might be necessary.
Important Considerations:
* Safety First: Never work under a running vehicle. Use jack stands securely if you need to get under the car.
* Consult a Repair Manual: A repair manual specific to your 1991 Chevy Cavalier Z24 will provide detailed diagrams and instructions for troubleshooting and repair.
* Professional Help: If you're not comfortable tackling these repairs yourself, take your car to a qualified mechanic. Incorrect diagnosis or repairs can lead to further damage and increased costs.
This problem requires a systematic approach. Start with the simpler checks (fluid levels, vacuum hoses) and work your way toward the more complex repairs. Good luck!
