I have a 97' chevy silverado 2500 deisel, It's started running real bad, it won't idle properly and it puts out a load of black smoke. If I try to drive it, it will cough and splutter and won't go over about 25mph. Every so often it will start revving up real hard on it's own. I'm on a budget and would like to fix it myself so if anyone has any ideas I would appreciate it.