I recently bought a 2001 Jimmy SLE with 140k. It runs very will but idles extremely low. I also noticed that at times when the tank get's low, the trucks seems to lose power when accelerating. Earlier today I was getting some items out of the back and it suddenly went to a high idle and spit a little black smoke out the tailpipe, and slowly went back down to a regular idle. I filled it up last week, drove 82 miles and the fuel light came on. I took 10.7 gallons of fuel. This is around 7.6 MPG. Sometimes the check engine light is on and then some days it isn't. Something has to be wrong.