* Transmission Issues: This is the most probable cause. A problem with the automatic transmission (if you have one) could be causing increased drag on the engine at idle when in gear. This could be due to:
* Low transmission fluid: Check your transmission fluid level and condition. Low fluid or fluid that's burnt or dirty will cause problems.
* Torque converter issues: A failing torque converter can cause increased drag at idle.
* Internal transmission problems: More serious internal issues within the transmission could be the culprit, requiring a professional diagnosis.
* Driveshaft/Differential Problems: While less likely, binding in the driveshaft or differential could cause increased resistance on the engine at idle when in gear. This is more likely if you hear unusual noises as well.
* Vacuum Leaks (less likely): While vacuum leaks usually affect idle in both neutral and gear, a very specific leak *might* only show up under load, but this is less probable given the smooth running at higher RPMs.
* Engine Mounts (less likely): Worn engine mounts could lead to increased vibration, but usually wouldn't cause a significant difference between neutral and gear.
Troubleshooting Steps:
1. Check Transmission Fluid: This is the first thing to do. Check the level and condition of your automatic transmission fluid. If it's low, add the correct type of fluid. If it's dark, burnt, or smells bad, it needs to be changed or flushed.
2. Listen Carefully: Pay close attention to any unusual noises coming from the transmission or drivetrain when idling in gear. Grinding, whining, or clunking sounds point to internal transmission problems.
3. Test Drive (carefully): Pay attention to how the transmission shifts. Any slipping, delayed shifting, or harsh shifts could indicate transmission problems.
4. Professional Diagnosis: If you've checked the transmission fluid and found no obvious problems, or if you hear unusual noises, take your Xterra to a trusted mechanic specializing in automatic transmissions or a Nissan specialist. They have the tools and expertise to diagnose the problem accurately.
Don't ignore this problem. Continuing to drive with a rough idle in gear could cause further damage to your transmission.
