My 2002 Duramax runs fine and doesnt smoke at idle. On the highway it has low power and smokes under any load. If I try to pass somebody or climb a hill it hits passing gear and stays there and smokes like crazy. The fuel filter and air filter have been replaced. It has been doing this for a good while. I have tried diesel additives with no luck. Is it time to replace the injectors? How much does a dealer charge to test them? How much are new injectors and where is the best place to get em? If that is the problem i will try to do it myself. Thanks for any advice.