* Torque Converter Issues: The torque converter is a fluid coupling between the engine and transmission. If it's slipping, it won't properly transfer power, leading to a rough idle or stalling in Drive. This could be due to low fluid, worn clutches, or a failing converter.
* Transmission Problems: Problems within the transmission itself, such as worn gears, low fluid, or internal damage, can cause a significant increase in load on the engine. This additional load might be too much for the engine to handle at idle.
* Vacuum Leaks: A vacuum leak can affect idle speed. The severity of the leak might only become apparent under load (in Drive).
* Throttle Position Sensor (TPS): A faulty TPS might provide incorrect information to the engine control unit (ECU), leading to poor idle, especially under load.
* Idle Air Control (IAC) Valve: This valve regulates airflow at idle. If it's dirty or malfunctioning, it may not compensate for the additional load in Drive.
* Computer/ECU Issues: While less common, a problem with the ECU itself could be misinterpreting sensor data, leading to inappropriate fuel delivery or ignition timing under load.
* Ignition System Problems: Weak spark, especially under load, could cause the engine to struggle in Drive.
* Fuel Delivery Problems: Issues in the fuel pump, fuel filter, or fuel injectors can also cause the engine to struggle under load. It might run fine at idle in Park because the demand is low, but struggle when more fuel is needed in Drive.
Troubleshooting Steps (in order of likely cause & ease of diagnosis):
1. Check the transmission fluid: Is it low? Is it dirty or burnt? This is the easiest and cheapest thing to check first.
2. Listen carefully for unusual noises: Any grinding, whining, or other strange sounds from the transmission area point to internal problems.
3. Check for vacuum leaks: Visually inspect vacuum lines and connections for cracks or loose fittings. Use a vacuum gauge to check vacuum pressure at idle.
4. Check the throttle position sensor: This often requires a multimeter and a bit of mechanical knowledge.
5. Check the IAC valve: It might be dirty; cleaning it can sometimes solve the problem.
Important Note: Unless you have significant mechanical experience, diagnosing and repairing transmission issues or ECU problems is best left to a qualified mechanic. Improper diagnosis and repair can lead to further damage and expensive repairs.
