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I have a 2000 FIAT Siena 1.4el with the FIAT Lampredi Monoalbero engine, and the car is 22 years old now still with its original engine mounts.

The whole car, and also the steering wheel, both shake and vibrate a lot at idle, but then don’t shake anymore when I give the engine some gas and increase the rpms. I looked online and people say it’s due to bad motor mounts.

Why would bad motor mounts only shake the car when at idle, but not when at higher rpms? Shouldn't it vibrate even more at higher rpms? Why does it smoothen out when we increase the rpms?

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In some cases (not all), when the engine torques over it takes up the slack which is there when the engine is idling. Since the slack is taken up, the engine becomes smoother. If it truly is the motor mounts, you'd most likely feel a "thump" as the engine torques from idle into power where it's taking up that slack, and then again as the engine is released from the torque.

That you feel a vibration at idle which goes away during acceleration, this isn't indicative of a bad motor mount. There are other things which might cause this as well. A lot of running issues are hidden after the engine speed picks up, such as if one cylinder is not putting out as much power as another. The unevenness of the engine running is masked when the RPMs go up and could give you the same feel as you describe. IOW, don't go on the assumption you have bad motor mounts. Verify it before you plan to replace parts.

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At idle, you will be hitting the resonant frequency of the engine and transmission that are supported by the mounts. As the engine revs increase, the frequency of the moving parts in the engine will move away from the resonant frequency and the vibration will stop. With the engine securely fastened to the body via the rubber mounts, the body will add more mass and will dampen the resonance at the idle speed.

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    The collapse of the Tacoma Narrows Bridge was more complicated than simple resonance. It was due to aeroelastic flutter, which would happen whenever the wind got above a certain speed. (Cars also occasionally experience flutter, usually when part of the bodywork comes loose.)
    – Mark
    Commented Nov 8, 2022 at 3:12
  • A resonant frequency in the usable RPM range (and especially corresponding to idle RPM) sounds like an extremely bad motor design. A lead engineer could literally go to jail for designing such an engine on purpose. Of course, bad motor mounts could change the resonant frequency, but to get a resonance on idle RPM you'd have to forget to fasten the motor altogether.
    – Dmitry Grigoryev
    Commented Nov 9, 2022 at 9:48
  • @DmitryGrigoryev I was assuming that the engine mount had failed. Happy to delete this if you are saying it is impossible.
    – HandyHowie
    Commented Nov 9, 2022 at 10:06
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    @HandyHowie No, resonance cannot of course be excluded with failed engine mounts. I'm just not convinced it's the root cause, and a bit surprised how people can easily calculate resonance frequencies of such poorly specified mechanical systems in their heads. I must admit I struggle to give a number.
    – Dmitry Grigoryev
    Commented Nov 9, 2022 at 11:25

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