[bluedog]

I forgot:


Sure it minimises vibration to increase Idle. But that also causes more fuel consumption.
Apart from that, the Engine Control unit should increase idle automatically as soon as there is a gear lever position other than P or N selected. Of course the resistance of the transmission with still standing vehicle works against that, so the idle rpm is lowered again, and you might not notice the so called fast-idle of the engine.


Simply, the engine has to work more in idle if in any drive-position with brakes engaged, than in N or P. As the wheels do not turn and thus the force of the fast-idling engine is dissipated in the torque converter in the forms of heat and, smaller part, mechanical energy, that causes vibration. And since the car body is lightweight, that causes a resonance and lets you feel those vibration quite closely.


The engine mount dampers are vibration blockers to the engines vibration (more or less effective). When that engine is connected to the transmission, there is an other path opened for those vibrations via transmission mounts. And, the engine load is increased, as explained above... Somewhere, there is a limit to the vibration dampers... and they are not getting more effective as the system is ageing and gets some thear and wear.