06 Silverado type x 4.3

[Josh]

Check fuel pressure and see if it will hold pressure. Running rich on the 4.3 is usually a fuel pressure regulator or injector problem. I would guess you have a leaking injector or regulator causing a rich condition that isn't severe enough to trip a check engine light.

It could be a oxygen sensor but usually it will throw a code. Start with fuel pressure and go from there. If you have an ignition issue it would likely cut out and try to backfire under a load. They are also full of crap that because it has no codes it means the scan tool is worthless. It should provide a valuable amount of info for misfire counters, fuel trims etc.....

Josh