Q: Vehicle shakes during acceleration

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I have a 2004 Toyota Sienna with 150k miles on it. It starts to shake at 35mph+ (under seats, not steering wheel) only during acceleration. I'm on my 3rd mechanic including the dealership. The tires have been balanced twice. They've checked cv joints/tie rods/drive shafts, and brakes. Tires are new. Stabilizers have been replaced. I was thinking tranny but no noise/grinding/slipping. What else could this be? I have to drive cross country with kids to get home. The mechanics are stumped.

My car has 150000 miles.
My car has an automatic transmission.

Among the remaining possibilities are defective motor mounts, struts, and engine operating condition (the engine needs to be looked at with a scope under load and a compression test might be helpful, too). If there is no problem with the motor mounts, the remainder of the suspension checks out, and the engine checks out (lots to check there), then the vibration must stem from an imbalance in a rotating component, such as the torque converter or the crankshaft. If the crankshaft, vibration can occur if the harmonic balancer is defective. Based on what you are describing, the vibration is marked enough that the origin should be readily pinpointed and consequently I suggest you request a vehicle vibration diagnostic. If you have further questions or concerns, please do not hesitate to re-contact YourMechanic as we are always here to help you, particularly if you just need more information to make a wise, informed decision.

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