1. Idle Air Control (IAC) Valve: This valve regulates air flow at idle. A dirty or malfunctioning IAC can cause the engine to stall under increased load (like engaging the transmission).
* How to check: Visually inspect the IAC for dirt or grime. A more thorough check might involve cleaning it with carb cleaner (follow the instructions carefully), or replacing it if cleaning doesn't solve the problem. A multimeter can be used to test its electrical operation, but this requires a service manual to know the correct readings.
2. Transmission Issues (Less Likely, but Important): While stalling upon gear engagement often points to engine problems, transmission issues can't be ruled out.
* How to check: This is harder to self-diagnose. Listen for unusual noises when shifting. A slipping transmission or low fluid level can cause stalling. Check your transmission fluid level (using the dipstick when the engine is running and warm) and condition. Low fluid or burnt-smelling fluid suggests a problem. This is best left to a mechanic for definitive diagnosis.
3. Throttle Position Sensor (TPS): The TPS tells the computer the throttle's position. A faulty TPS can lead to erratic idling and stalling.
* How to check: This typically requires a multimeter and a service manual to check the voltage output at different throttle positions. Replacement is often the solution if faulty.
4. Mass Air Flow (MAF) Sensor: The MAF sensor measures the amount of air entering the engine. A dirty or malfunctioning MAF sensor can cause incorrect fuel delivery, leading to stalling.
* How to check: Clean the MAF sensor carefully with MAF sensor cleaner (NEVER use anything else!). A faulty sensor needs replacing.
5. Vacuum Leaks: Small vacuum leaks can disrupt the engine's air/fuel mixture, leading to stalling.
* How to check: Visually inspect all vacuum lines for cracks or disconnections. You might need a vacuum gauge to pinpoint leaks more effectively. Pay special attention to lines connected to the brake booster and intake manifold.
6. Engine Speed Sensor (Crankshaft Position Sensor): This sensor tells the computer the engine's speed. A faulty sensor can cause erratic running or stalling.
* How to check: This requires a multimeter and a service manual for testing. Replacement is usually necessary if faulty.
Troubleshooting Steps:
1. Check engine oil and coolant levels. Low levels can affect engine performance.
2. Check the battery terminals and cables. Corrosion can reduce power.
3. Start with the easy checks: Inspect the IAC valve and clean it. Check for vacuum leaks.
4. Consult a repair manual: A service manual specific to your 1993 Pontiac Grand Am will provide detailed diagnostic procedures, wiring diagrams, and specifications.
5. Consider a professional mechanic: If you're uncomfortable working on your car or if the problem persists after checking the common causes, take it to a mechanic for diagnosis and repair.
Important Note: Safety first! Always disconnect the negative battery terminal before working on any electrical components. If you are not comfortable working on your car's electrical or engine systems, it's best to have a qualified mechanic diagnose and repair the issue.
