* Torque Converter Issues: A failing torque converter is a prime suspect. The torque converter transmits power from the engine to the transmission. If it's slipping badly or locked up improperly, it can't handle the increased load when you put it in gear, causing the engine to stall. This is a very common cause of this symptom.
* Transmission Problems: While less likely to cause immediate stalling (more likely to cause slipping or rough shifting), a major internal transmission problem could create an excessive load the engine can't overcome. This is less probable than a torque converter issue in this scenario.
* Low/Dirty Transmission Fluid: Low transmission fluid will cause the transmission to struggle and place excess strain on the engine. Dirty fluid reduces lubrication and performance. Check the fluid level and condition – it should be bright red and not dark or burnt smelling.
* Weak or Failing Engine Components: Though less likely given the specifics (idles fine), consider:
* Ignition System: A weak coil, failing distributor cap/rotor, or faulty ignition wires could cause the engine to lose power under load.
* Fuel System: A clogged fuel filter or fuel pump issues could restrict fuel delivery, especially under stress.
* Vacuum Leaks: Significant vacuum leaks can severely impact engine performance, especially under load.
* Alternator Issues: While not directly causing the stall, a failing alternator might not provide sufficient power when the engine is under load, leading to a sudden loss of power. Check the alternator's output voltage.
Troubleshooting Steps:
1. Check Transmission Fluid: This is the easiest and cheapest step. Check the level and condition of the transmission fluid. If low, add the correct type (consult your owner's manual). If it's dark, burnt, or smells bad, it needs to be changed immediately.
2. Listen Carefully: When the engine stalls, listen closely. Is there a grinding noise from the transmission? This could indicate internal transmission problems.
3. Check for Vacuum Leaks: Inspect all vacuum lines for cracks or disconnections. A spray bottle with a soapy water mixture can help identify leaks.
4. Test the Alternator: Have the alternator tested at an auto parts store.
5. Professional Diagnosis: If the above steps don't reveal the cause, you'll likely need a professional mechanic to diagnose the problem. They have tools and expertise to test the torque converter, fuel system, and other components under load. This is especially important considering the torque converter is a prime suspect.
The fact that the engine idles fine strongly suggests the problem only manifests under load, making torque converter failure or a severe transmission issue the most plausible explanations. Start with the transmission fluid check, then consider a professional diagnosis if the problem persists.
