* Transmission Problems: This is the most probable cause. When the car is in gear, the engine is under load even at idle. A rough idle in this situation points to issues within the transmission, such as:
* Low transmission fluid: Insufficient fluid leads to poor lubrication and increased friction, causing rough operation under load.
* Worn or damaged transmission components: Internal parts like clutches, bands, or bearings wearing out can cause rough engagement and shuddering, which translates to a rough idle when in gear.
* Torque converter problems: A faulty torque converter, which connects the engine to the transmission, can slip or bind, resulting in a rough idle under load.
* Internal leaks: Leaks within the transmission can reduce fluid levels and cause similar problems.
* Motor Mounts: While less likely to cause a rough idle *only* in gear, worn-out motor mounts can allow excessive engine movement. This movement might be more noticeable when the engine is under load from the transmission, leading to a rougher idle feel in gear.
* Other possibilities (less likely):
* Exhaust leak: While an exhaust leak usually affects idle in all situations, a significant leak might be exacerbated by the engine load in gear.
* Vacuum leak: A severe vacuum leak could affect idle smoothness, and the added load in gear might highlight it.
Troubleshooting steps:
1. Check the transmission fluid: This is the *first* step. Check the fluid level and condition. Low fluid, burnt fluid (smells burnt), or milky fluid (indicates water contamination) all point to transmission problems.
2. Listen carefully: While idling in gear, listen for any unusual noises coming from the transmission – grinding, whining, or clunking sounds are all bad signs.
3. Inspect motor mounts: Visually inspect the motor mounts for cracks, excessive wear, or looseness.
4. Professional Diagnosis: Since transmission issues are complex and can be costly to repair, it's best to have a qualified mechanic diagnose the problem. They have the tools and expertise to pinpoint the exact cause.
Ignoring this issue could lead to more significant and costly transmission damage. Don't continue driving the car extensively with this problem. Get it checked out soon.
