8

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?

Peter Mortensen
  • 193
  • 4
Timothy Scherer
  • 839
  • 8
  • 15

2 Answers2

11

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.

Pᴀᴜʟsᴛᴇʀ2
  • 165,084
  • 32
  • 259
  • 508
6

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.

HandyHowie
  • 25,681
  • 3
  • 40
  • 76