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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.
 

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You can eliminate balance issues because vibration goes away at higher rpm instead of getting worse. Try removing serpentine drive belt to see if vibration goes away. When new engine installed, mechanic probably used your accessories on the new engine. So eliminate accessories as suspect.

Don't forget the transmission mount also for source of vibration.

Check complete exhaust system for bolts & nuts being tight. And check all the rubber exhaust mounts. They could be transferring vibration into car at certain rpm.

My 90 daily driver has no headliner and missing some of the plastic pieces. When I come to a stop at certain rpm range car vibrates like a big bass drum. So I ask, is your interior complete?

For this next test you would need a cut off driveshaft stub to insert in back of trans. Remove driveshaft and see if vibration has gone away. You need the driveshaft stub to insert in back of trans because once driveshaft is removed, you will lose trans fluid. This test eliminates driveshaft & rear end as suspects.
 

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motor and trani mounts are brand new.
smoke test did not come up with any cracks in intake manifold.
I can feel the vibration in my legs when leaning over the front of the car when the idle has dropped lower. Also while sitting inside at a stop sign or red light. When waiting for light to turn, the idle is gradually going down. When it gets to about 750 or lower, the vibration returns.
Could be a "miss" when current is getting lower due to alternator lower output? at lower revolutions.
I had a new catylitic converter installed as well. I will check all exhaust hangars.
A tech bulletin had highlighted the power steering line and it being high pressure, might need to be changed out.

Tech guy was hoping for a computer code to point to a direction of a misfire in a particular cylinder, then perhaps isolate the coil pac that might be bad. Plugs are all new. Compression is great since the motor was rebuilt.
 
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