Hello,
A family member's 2014 VW Passat TDI (diesel) starts to vibrate in the steering wheel at 80+ MPH, but only when I am on the accelerator. If release the gas pedal, it's back to a smooth ride. Any idea what might be causing this? The dealer said it's likely due to the speed rating of the tires, but he couldn't explain why it only happens when depressing the accelerator... so speed rating seems highly unlikely to me. The master tech looked everything over and says everything is tight and looks good.
Oh and yes, I understand it's a VW...not my choice. I drive a Toyota and am very happy with it 
You probably have some sort of front end problem.
Thank you @Venator77! What would be an example of a front end problem?
Check the front end suspension parts like control arms, bushings, etc for play. The easiest way to check this is to jack up the front of the car and wiggle both front wheels up and down and side to side. If it moves, then a part has gone bad and needs replacement.
Also, your dealer is an idiot, please go somewhere else.
Sadly it's the kind of issue that requires a mechanic to get under the hood and find what's wrong.
Not the kind of issue you can diagnose remotely.
For example, on Chrysler products, it's know to be caused by wheel bearings to cause the steering wheel to shake.
The master tech looked everything over and says everything is tight and looks good
And other jokes that master tech can tell himself!
Go to a competent mechanic, and definitely NOT the dealership.
