* Idle Air Control (IAC) Valve: This is a very common cause. The IAC valve regulates air flow at idle. If it's dirty, sticking, or malfunctioning, it can cause erratic idle speeds, stalling, and rough running. Cleaning it is a relatively easy fix, but replacement might be necessary.
* Mass Air Flow (MAF) Sensor: The MAF sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor will send incorrect information to the computer, leading to a poor fuel mixture and rough running, including stalling. Cleaning it (carefully!) is worth a try, but replacement is often needed if cleaning doesn't help.
* Throttle Position Sensor (TPS): The TPS tells the computer the throttle's position. A faulty TPS can cause erratic idle, stalling, and poor drivability.
* Vacuum Leaks: A leak in the vacuum system can cause erratic idle and stalling. These leaks can be anywhere in the intake manifold, vacuum hoses, or even a cracked PCV valve. Finding these leaks requires a thorough visual inspection of all hoses and connections.
* Dirty or clogged fuel injectors: If the injectors aren't spraying fuel properly, the engine may stall or run roughly, particularly at idle. A fuel injector cleaner might help, but professional cleaning or replacement might be needed.
* Crankshaft Position Sensor (CKP) or Cam Position Sensor (CMP): These sensors tell the computer where the engine's crankshaft and camshaft are in their rotation. A faulty sensor can cause misfires, rough running, and stalling.
* Ignition System Problems: Weak spark plugs, bad ignition wires, or a failing ignition coil can cause misfires and stalling.
* Computer (PCM) Issues: While less common, a problem with the Powertrain Control Module (PCM) could be the root cause. This is a more involved and expensive diagnosis and repair.
Troubleshooting Steps:
1. Check for trouble codes: Use an OBD-II scanner to read any diagnostic trouble codes (DTCs) stored in the car's computer. This is the most important first step. The codes will point you in the right direction.
2. Visual Inspection: Carefully inspect all vacuum hoses and connections for cracks, leaks, or loose fittings.
3. Clean the IAC and MAF sensors: This is a relatively simple and inexpensive first step. Look up specific instructions for your 1999 Grand Prix GT, as the location and cleaning procedures vary slightly between models.
4. Check the spark plugs and wires: Look for cracks, corrosion, or excessive wear.
5. If you're not comfortable working on your car yourself, take it to a qualified mechanic. They have the tools and expertise to properly diagnose the problem.
It's crucial to address this issue promptly. Continued driving with these symptoms could cause further damage to your engine. Remember to always disconnect the negative battery terminal before doing any work on the electrical system.
