Engine Vibration at idle
2000 Pontiac Firebird, 3800 V6, has engine vibration at idle-750 RPM. Apply gas, motor smoothes out. Come to a stop at a light, idle comes down, vibration returns. Had vacuum checked, fine. Good oil pressure. Car had 115k miles on it(bought used), decided to do a motor rebuild. Got an exchange motor-rebuilt and installed. At same time, had transmission rebuilt with fresh torque converter and flexplate.
Totally different motor has same issue. Changed EGR with "used" tubes, GM no longer offers EGR tubes that connect to EGR.
So, intake manifold is the same and induction system. I was thinking the IAC-- Idle Air Valve could be it. Is there anythink else I am overlooking? Dealership does not want to get involved because of another motor installed. They say it could be harmonic balancer-changed on earlier motor and used on this motor-no effect. Torque converter-replaced, etc. No check engine codes/lights coming up on dash.