* Low/Reverse Shared Components: Automatic transmissions often share some internal components between the low gear and reverse gear. A worn or failing clutch pack in this area is a common culprit for slipping in low gears, especially noticeable after a stop when the transmission is re-engaging.
* Heat and Pressure: Coming to a stop allows the transmission fluid to settle and potentially reduces pressure temporarily. When you shift to Park and back into Drive, you're potentially re-establishing pressure and allowing the clutch pack to temporarily engage better. This is a *temporary* fix, not a solution.
* Fluid Level and Condition: Low transmission fluid or fluid that's contaminated (burnt, dirty) can also contribute to slipping. The fluid is crucial for lubrication and hydraulic pressure.
* Other Potential, Less Likely Causes: While less probable given your description, other possibilities include:
* Solenoid Problems: Electrical solenoids control the transmission's hydraulic circuits. A faulty solenoid could prevent proper clutch engagement.
* Valve Body Issues: Problems within the valve body, which regulates fluid flow, can also cause shifting problems.
* Torque Converter Issues: Though less likely to manifest only after stops, a failing torque converter could contribute.
What to do:
1. Check the transmission fluid: This is the first and most important step. Check the fluid level with the engine running and at operating temperature. Note the fluid's color and smell. Dark, burnt-smelling fluid is a very bad sign.
2. Professional Inspection: The temporary fix highlights the issue but doesn't resolve the underlying problem. Take your Lumina to a trusted transmission specialist or mechanic. They have the tools and expertise to diagnose the problem properly. Simply hoping the fluid is the problem is risky. Ignoring this can lead to catastrophic transmission failure.
Don't keep driving the car like this! Continuing to drive with a slipping transmission will cause further damage and likely lead to a much more expensive repair later. The temporary fix only masks the problem. Get it checked out soon.
