Q: Car starts n idles fine. When I put it in gear it stalls. I'm thinking transmission fluid problems? Also it "CLANKS" when shifting

The torque converter and torque converter lock up solenoid should be evaluated. Another possibility is an internal hydraulic leak in transmission seals causing pressure to apply both reverse and forward gears at the same time, lock up the transmission and stall the engine. Pressure testing (external gauges) can be used to see if such a leak is caused by a leaking seal or a leaking reverse or forward solenoid/valve.

Another possibility relates to engine controls. Test the throttle position sensor, idle control valve as well as the manifold absolute pressure (MAP) sensor. The MAP sensor detects engine load and generates a signal that is proportional to the amount of vacuum in the intake manifold. The ECU uses this information to adjust ignition timing and fuel enrichment relative to the amount of power needed. When the engine is working hard, intake vacuum drops as the throttle opens wide and the engine takes in more air, which requires more fuel to keep the air/fuel ratio in balance. When the computer reads a heavy load signal from the MAP sensor, it adjusts the fuel mixture to slightly more rich than normal so the engine can produce more power. The computer will then retard (back off) ignition timing slightly to prevent detonation (spark knock) that can damage the engine and decrease performance. If the MAP sensor is faulty, this foregoing sequence may explain why the engine dies as you put the vehicle into gear as a load is put on the motor.

YourMechanic offers a stalling diagnostic which covers both transmission and engine related faults. If you request that service a certified Mechanic, dispatched by YourMechanic right to your location, will get this problem resolved for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you