After starting my 2004 Chevy Trailblazer, the engine will vibrate in Park and Neutral. When I shift from Park to Reverse the vibration increases. Shifting from Neutral to Drive is less intense than in Reverse. Turning on the A/C greatly increases the intensity of the vibration. When the A/C compressor turns on, the vibration increases, when the compressor turns off the vibration decreases. I’ve had a mechanic test the engine and transmission mounts for wear and damage. Mounts are good. Any advice would be helpful. Thank you, Ed Ransom.
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What makes your mounts "good"? They are 15 years old, and at the very least, the rubber dampness are likely dry rotted and not doing much of a job dampening anything. The reason things happen your air conditioning compressor engaged and then not when disengaged has to do with the harmonics of the engine at different speeds and loads. If there is an engine performance issue at all, it will be accentuated with the air conditioning compressor engaging.
That said, I would look both at engine performance components, and also look even closer at the engine and transmission mounts. As a side note -- you have another question about a clunk - mount issue can do that also, as food for thought.
