* Motor Mounts: Worn motor mounts allow excessive engine movement, leading to vibrations that are felt most strongly under acceleration. This is a very common cause of this type of shaking.
* Transmission Mounts: Similar to motor mounts, worn transmission mounts can cause vibrations to transfer to the chassis, especially noticeable under load (acceleration).
* Driveshaft/Axles: Worn CV joints in the driveshafts or axles will create vibrations, often more pronounced under acceleration and when turning. A worn center support bearing (if applicable) for the driveshaft could also contribute.
* Clutch Issues: While less likely to cause a shaking *while* accelerating (more likely during engagement), a worn clutch could contribute to the problem if it's slipping slightly under load. This slipping would cause the engine to rev higher without a corresponding increase in speed, which *might* feel like being in too high a gear.
* Torque Converter (Automatic Transmission): You specified manual, but if it's somehow an automatic, a failing torque converter could cause shuddering during acceleration.
* Ignition System Issues: A misfire in one or more cylinders can cause a rough running condition and shaking, particularly under load. This is less likely to feel exactly like being in too high a gear, but it's worth considering.
* Engine Problems: Internal engine problems like worn piston rings, connecting rod issues, or valve train problems could contribute to vibrations felt throughout the car. However, these are usually accompanied by other symptoms like loss of power, unusual noises, or smoke.
Troubleshooting Steps:
1. Check Engine Mounts and Transmission Mounts: Visually inspect these for cracks, tears, or excessive wear. A mechanic can easily assess their condition.
2. Listen for Unusual Noises: Pay close attention to any noises from the drivetrain (clunking, whining, clicking) while accelerating.
3. Inspect Driveshafts and Axles: Look for any visible damage or signs of leaks (CV joint boots). A mechanic might be able to assess the condition of CV joints by feeling for play or listening for clicking noises.
4. Clutch Test (Manual Transmission): If you suspect the clutch, try performing a clutch engagement test. A slipping clutch will show a discrepancy between engine RPM and car speed.
5. Check for Misfires: A mechanic can perform a diagnostic scan to check for misfire codes.
It's highly recommended to have a qualified mechanic diagnose the problem. Trying to pinpoint the exact cause without proper tools and experience can be difficult and potentially lead to further damage. The age of the car also makes thorough inspection important, as multiple components might be showing wear simultaneously.
