The car shakes past 75 mph, but only when accelerating or maintaining speed. Coasting stops the vibration. There's also an issue where the car will stop receiving power from the engine randomly. It will do it during acceleration, maintaining speed, and cruise control. Sometimes giving it more gas will cause the engine to rev up but no power to be given to the wheels, but sometimes the engine does not rev up. Usually this lasts 2-3 seconds.
My car has 198000 miles.
My car has an automatic transmission.
Faulty motor mounts are a possibility but suspension faults, such as worn struts, can also cause shaking at high speed. If the problem is suspension related, it may occur during acceleration versus coasting because during acceleration the suspension geometry (running angles) accentuate, or make apparent, the suspension fault (typically a loose or worm part). Failure to accelerate, when engine RPM rises, can be due to slippage in the transmission and that can be due to faults in the torque converter, clutch pack or the hydraulic controls (e.g., inadequate line pressure). With regard to the shaking, there is also a possibility that such is due to an engine management/controls fault, that is once engine speed passes a certain RPM it is the engine itself that is operating poorly and vibrating under load.
The bottom line is you should request a vehicle vibration diagnostic. It is possible to use remote chassis ears (wireless microphones with an in-cabin receiver) to narrow the origin of the vibration and, if it gets really complicated, there are frequency domain analyzers that can take the noise from the vibration and decompose it to the most likely mechanical sources. My guess is, if you request the diagnostic, it will probably not get that complicated because, from what you are describing the vibration is marked, and consequently to a trained mechanic the likely origins will be obvious. If you have additional concerns or a follow-up on this, don’t hesitate to re-contact YourMechanic.
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