Originally Posted by kkspeed
The weird thing about my vibration issue is that it is intermittent. Sometimes it goes away when the engine warm and runs real smooth. That is why I thought it was in the EGR system.
Secondly, the engine idles differently when the heater is set to defrost with the heat on.
I suspect the AC turns on automatically when the defroster is turned on to prevent the windows from fogging up.
I know that's wierd, but the material used in the motor mounts is a soft (hydraulic) substance. Does a great job of isolating and dampening vibration. Even if the engine is running poorly, the mounts in good condition, will stop all that vibration.
I believe as there are changes in its temperature, it probably expands a little leading to the vibration to go away. Also, as the engine changes its mixture, it'll change just how much vibration the mounts have to dampen. Mine would start up and vibration would be felt until warmed up AND then shut off and restarted.
Keep your focus on the mounts for now. If the RH mount had failed, it could easily have led to excessive stress on the other 2 mounts and a failure on either or both.
And yes, if your vehicle is equipped with A/C and you turn the air to Defrost, the A/C compressor will turn on. Even if you hit the A/C button on the panel to turn off the A/C light, I believe the A/C compressor still runs.