* Low Transmission Fluid: This is the first and easiest thing to check. Low fluid can cause all sorts of problems, including rough running and inability to engage gears properly. Check the transmission fluid level (with the engine running and warmed up, ideally) and condition (look for burnt smell or discoloration). Low fluid points to a leak, which needs attention.
* Transmission Problems: A failing transmission pump, worn clutches, or other internal transmission issues can prevent the transmission from engaging properly under load. This is a more serious and expensive problem that often requires professional diagnosis and repair.
* Torque Converter Issues: The torque converter connects the engine to the transmission. A faulty torque converter can slip or fail to transfer power efficiently, leading to rough running and stalling when put into drive.
* Engine Problems: While the problem manifests when shifting into drive, it could stem from a weak engine. A rough idle *before* shifting suggests this. Consider:
* Vacuum Leaks: These can cause erratic engine behavior.
* Ignition System Problems: Worn spark plugs, bad wires, or a failing ignition coil can cause misfires and poor performance.
* Fuel Delivery Issues: A clogged fuel filter, failing fuel pump, or problems with the fuel injectors can starve the engine of fuel, particularly under load.
* Throttle Body Issues: A dirty or malfunctioning throttle body can restrict airflow, leading to rough running.
* Computer/Sensor Problems: A faulty sensor (like the crankshaft position sensor, throttle position sensor, or mass airflow sensor) can provide incorrect information to the engine control module (ECM), leading to poor performance. A failing ECM is less likely but possible.
Troubleshooting Steps:
1. Check the Transmission Fluid: This is the absolute first step.
2. Check for Error Codes: Use an OBD-II code reader to scan for diagnostic trouble codes (DTCs). These codes can provide clues to the problem's source.
3. Inspect for Leaks: Look under the car for any visible leaks of transmission fluid or other fluids.
4. Listen Carefully: Pay close attention to the sounds the engine and transmission make while idling and when you try to put it into drive. Unusual noises can be diagnostic.
Important Note: If you're not mechanically inclined, it's best to take the vehicle to a qualified mechanic for diagnosis and repair. Attempting to fix transmission or engine problems without experience can lead to further damage and increased repair costs.
