I have an 05 avalanche. It has the 5.3 V8. I ran great, 21 mpg, the works. I then went on a road trip to Northern Idaho, which is at about 1400 ft or so. I live in a freakin 6300 ft desert.
On my way back, the truck stumbled at gas station, at idle, prior to accelerating. I then make it back home, about 1300 miles later, and it runs real rough at idle, voltage drops from the 14 mark, and blows black smoke. Engine light on, code says random misfire. Like no kidding right? But that is it. I now have put fuel inj cleaner in it, the 23 dollar stuff. That has not helped yet. I read alot ot stuff, but nothing conclusive. We took a hand held temp thermometer, and the 4 center cylinders seem to be cooler. O2 sensor? I am about to move to Idaho again, but this needs to be fixed, before rings get washed out. Please help.

 

Check for all possible vacuum leaks. Your Evap purge solenoid is located on the intake manifold and if it or the MAP sensor went bad they either or both can cause a leak. Also make sure all tune up parts are good. If #4cyl is cooler than rest then it could be wire/cap/rotor or maybe even a wire just went bad...do a compression check to make sure that all cylinders are within specs...just to rule out any head gasket issues...

Also get all the codes that might be hanging in computer memory (current and pending) sometimes the computer will store a code until it meets specific "drive cycle" conditions, ie: 3 drive cycles would be 3 start-ups run and turn-off cycles...some codes have lengthy cycles and are stored as pending codes....

 

It could be your O2 sensor. The sensor is their to rich/lean the fuel ratio and I had the same problem on my blazer with rough idle, etc. but the voltage didn't drop but rule any possible problems out and go from their.

 

Thanks for the help. I will check this asap. The vehicle has 45,000 miles..