Ok, so I’ve got a 03 LB7 duramax, with 402,000 miles. it has a kinda rough idle, like it’s missing on a cylinder, with a constant power surge. when you give it some gas, either in park, or going down the road, it clears up and runs great, but it black smokes like crazy. I’m talking about cars behind me pulling out of the way if you get into it. It’s got plenty of power. Could it be an injector?