My dad has an 06 Envoy that has been nothing but problems for the last few years now.
First problem is actually not that big of a deal, mainly because there aren't any symptoms beyond a couple idiot lights being on in the upper left of the instrument cluster. One is the traction control symbol, and I can't remember the other one. For a while it was intermittent, sometimes they'd be on, sometimes off. Now they are on all the time. Nothing really seems to be noticeably wrong.
The second problem is worse. It only happens when it's cold out, like below 10F. When he starts it it'll run very rough, and frequently die. It becomes very hard to start. I think it even died at a traffic light once or twice. He's been told that it might be a warped intake manifold or some of the intake manifold bolts might be loose.
Third problem is when he shuts down there is a high pitched noise that comes from the engine for several seconds after the engine is off. I am almost certain that it's the intake/throttle valve closing, and it needs to be lubricated. Any idea on how to do that, and with what?