Transmission Related Issues (Most Likely):
* Low Transmission Fluid: This is the first thing to check. Low fluid can cause all sorts of shifting problems, including harsh shifts and surging. Check the fluid level (with the engine running and warm) and its condition. It should be bright red and clean; dark, burnt fluid indicates problems.
* Worn or Damaged Transmission Components: Worn clutch packs, a failing torque converter, or problems with the valve body (which controls fluid flow) are all common causes of rough shifting and surging. These require professional diagnosis and likely a transmission rebuild or replacement.
* Transmission Solenoid Issues: The solenoids electrically control the flow of transmission fluid. A faulty solenoid can cause erratic shifting.
* Shift Linkage Problems: Though less likely in a 1998 Grand Prix with an automatic transmission, a problem with the linkage that controls the gear selection could contribute to rough shifting.
Engine Related Issues (Less Likely, but possible):
* Vacuum Leaks: A vacuum leak can affect the engine's performance and cause surging. This is less likely to directly cause the second gear issue, but could exacerbate it.
* Throttle Position Sensor (TPS): A faulty TPS provides inaccurate information to the engine control unit (ECU), leading to inconsistent engine performance and possible surging.
* Mass Airflow Sensor (MAF): Similar to the TPS, a faulty MAF sensor can lead to poor engine performance and surging.
Other Possibilities:
* Transmission Computer/ECU Problems: A malfunctioning transmission control module (TCM) or ECU could be sending incorrect signals to the transmission.
Troubleshooting Steps:
1. Check the Transmission Fluid: This is the easiest and cheapest step. If low or dirty, address it immediately. A fluid change might help temporarily, but won't solve internal transmission problems.
2. Get a Diagnostic Scan: Take the car to a mechanic or auto parts store that can perform a diagnostic scan. This will read trouble codes from the car's computer, providing clues to the problem's source.
3. Professional Inspection: If the scan doesn't reveal a clear culprit or if the transmission fluid is low or dirty, a professional transmission mechanic should inspect the vehicle. They'll have the expertise to diagnose the more complex issues.
Important Note: Continuing to drive the car with these symptoms could cause further damage to the transmission, potentially leading to a much more expensive repair. It's best to address the problem promptly.
