Drivetrain Related:
* Torque Converter Issues: A failing torque converter can cause vibrations and rough idling, especially when the transmission is engaged. Slipping or shuddering during acceleration would further support this.
* Transmission Problems: Internal transmission problems, like worn clutches or low fluid, can put extra strain on the engine, leading to rough idling in drive. This is often accompanied by other transmission symptoms like slipping gears or hard shifting.
* Motor Mounts: Worn-out motor mounts allow excessive engine movement, which can translate into vibrations felt as a rough idle, especially under load (like when in drive).
* Driveshaft Issues: While less likely to be the primary cause of a rough idle, a damaged driveshaft or universal joints could transmit vibrations to the engine and chassis, making the idle feel rougher when in drive.
Engine Related (but exacerbated in drive):
* Vacuum Leaks: A vacuum leak will affect engine performance at all times, but the added load of the transmission in drive might make the rough idle more noticeable.
* Ignition System Problems: Weak spark plugs, failing ignition coils, or a faulty distributor (if equipped) can cause misfires, leading to a rough idle, especially under load.
* Fuel Delivery Issues: Problems with fuel injectors, fuel pump, or fuel filter can cause inconsistent fuel delivery, leading to a rough idle. This might be more apparent under load.
* PCV Valve: A faulty PCV valve (Positive Crankcase Ventilation) can lead to excessive crankcase pressure, affecting engine performance and causing rough idling.
Diagnostic Steps:
1. Check Engine Light: Is the check engine light on? A diagnostic code scan from an auto parts store or mechanic will pinpoint potential issues.
2. Fluid Levels: Check the transmission fluid level and condition. Low or dirty fluid is a major suspect.
3. Listen Carefully: Pay close attention to the sounds the engine is making while idling in drive and neutral. Unusual noises can help pinpoint the source of the problem.
4. Visual Inspection: Look for any obvious signs of damage to motor mounts, driveshaft, or other components.
5. Professional Diagnosis: If you're not comfortable performing these checks or the problem persists, take your Cougar to a qualified mechanic for a proper diagnosis and repair.
The fact that the rough idle is *only* apparent in drive strongly suggests a drivetrain-related issue, particularly the torque converter or transmission. However, it's crucial to address all possibilities to ensure a proper fix.
