vibration at idle with load on engine...tough problem to diagnose
 

This is a follow up post to a previous. My 99 Tahoe started producing a vibration at idle when in drive. With the engine belt removed, the vibration is greatly reduced to normal levels, point to an accessory. I have a 200 AMP after market alternator in there that has never given me problems before and it still might not be a problem, but when it is disconnected from the 12AMP input, the vibration is also greatly reduced.
In general, it seems like when the load on then engine reaches a certain level at idle, it vibrates significatnly. If I hold down the brake and give it just a touch of gas to bring the idle in drive to about 600rpm, the vibration is also reduced greatly.
A tech at Chevy tried playing with a throttle body screw to increase the idle rpms and in neutral it's up around 700, but dips right back down to 500 in drive. I'm surprised to see such a difference all of the sudden between neutral and drive. It used to just stay around 500rpm at idle no matter what. What's going on here? What am I missing. I'm thinking that maybe my truck just started idling more slowly. I know there is an idle control motor. Shouldn't that maintain the idle at a more predictable rpm that doesn't swing so much between neutral and drive? Is the computer taking over in drive and keeping it a 500rpm? I'm not sure how to proceed to correct the problem or why it even started all of the sudden. I would like it if the rpm would just stay around 600rpm idle no matter what gear it was in if it would get rid of the vibration.

It's been my experience that on computer controlled engines, the idle remains stable whether in P, R, N, D, or L.

If the alternator is under a heavy load it will pull down the engine.. and it sounds like that from your description... might be worth having your battery(s)
checked or even the alternator.... or looking at any accessories that might be running...
Are you using any reducing pulleys?

I don't think I'm using any reducing pulleys since I'm not even sure what that is. The alternator is definitely putting a heavier load than a stock alternator. I'm thinking that it's causing the engine to idle too low, but like the moderator said in the first response, it seems like the computer should adjust for the load and keep the idle the same. The alternator was checked and it is producing about 20 AMPs at idle and goes up to 80 AMPs with a load. The battery is an Optima batter about 5 years old, but I had it checked at Kragen about 6 months ago and it the meter said it was ok. Maybe it's worth replacing the battery though.
My only other hope is to change the motor mounts even though visually they are ok and the motor doesn't buckle too much when applying gas with the brake on. Maybe they are just hardened and are not absorbing any of the vibration of the engine. I don't know how much vibration they absorb when new.

Reducing pulleys or performance pulleys are sometimes used to change the amount of load on the engine to help increase horse power...(supposedly)

motor mounts and transmission mounts that are torn can induce some vibrations
something else to consider is the harmonic balancer...

poor idle could be many items related to a tune up, a vacuum leak, dirty throttle body plate... I would avoid just adjusting the throttle stop screw as they may just mask the real issue by bumping up the RPM's

throttle body adjustment screw
 

Yeah, I sought out a service advisor I used to work with at a Chevy dealership that closed down. I told him that the GM tech adjusted that screw and he said that they really should not have done that because it's factory set and once it's been changed, it's impossible to say if there is a problem with the throttle body.
My old service advisor was saying that he wasn't sure if the vibration was normal or not and I didn't know what to say, but this morning I was reminded when I started the car that this whole time it's been smooth as glass when the engine is cold and the vibration only starts when the engine is warm. That's why the more I look at the problem, the more I think it's related to idle speed.

Must have something to do w/ when the ECM goes from "open loop" (cold engine) to "closed loop" (warm engine). Kind of like when a carburetor car choke opens up and the mixture goes from rich to normal. A rich mixture will cover up some sins.

I would imagine that the ECM would make the fuel mixture less rich gradually, not all at once right? The vibration comes on gradually, not all at once. I hadn't really thought about the fuel ratio being different from cool to warm so thanks for the pointer. I'm trying to see if GM will send an engineer to look at the problem since the dealer seems at a loss to fix the problem. We keep going to the fact that it is load dependent, but if the engine can run smooth with a full load while cold, then it should be able to do it warm, at least from my point of view.

might be worth checking out the O2 sensors...

I know this is an old thread but i hope this might help others. I have a 95 silverado that developed the same vibration issue and it turned out to be the battery. Had the battery checked at several different places and they all said it checked fine. Was later told by a shadetree that a battery can develop issues and still check good, though it may have difficulty accepting a charge from the alternator.