by The Roadie » Sun Feb 17, 2013 2:12 pm
The erratic idle that you can see the RPM drop on the tach, and the voltmeter dip, was probably taken care of by the throttle body cleaning.
In most of our minds, "sputtering" means a misfire. In case you have the OTHER major odd cause of vibration, you might have bad/collapsed motor mounts. This issue has been discussed a LOT on gmtnation.com (not so much here), but it's a 10 Hz rotational vibration that can't be seen on the tach, goes away in neutral and comes back with e vengeance in drive, and is often (not not always) worse when cold instead of warmed up. Observe the driver's side intake manifold plastic with somebody else in the driver's seat, and see if the engine rotates more when it's in DRIVE than NEUTRAL. If so - motor mounts. It's a magic cure that I needed and was highly skeptical of for a long time. Engineering-wise, I now understand the reason to be a rotational resonant vibration due to the coupled mass of the engine/tranny/driveline/diff gears/axles that is different in neutral.