Stumbling idle when stopping.

[firebird1999us]

I had a similar issue but it turned out the alternator that I had recently replaced was not tightened down enough. I had cranked down the bottom bolt pretty tight but it was still not as tight as it needed to be. My mechanic got it on there tight and the car stopped a good bit of what I felt was mechanical vibration. I felt like I was in danger of overtigtening but I guess it had a bit to go.

My car has 350k+ on it... we also discovered through the whole process that the vibration I thought was an idle issue was actually the motor/transmission mounts failing. I was only getting a vibration when the car was in gear and I had the AC on at a stoplight when the car was under load. Now that we've pinpointed the mounts failing I can feel that the vibration is coming from the direction of the transmission and lower motor mounts.

There's also an Idle Control Valve that can be problematic too... I believe it's located under the throttle body? Those are fairly delicate and expensive to replace per some of the info I've read so be careful if you try to clean it!