2000 Trooper Running Too Rich

My trooper has a blackened tailpipe. I was told that this is caused by the engine running too rich and uncombusted fuel being pushed through the exhaust system. Recently it's started making a popping/puffing sound upon starting the engine. The mechanic I talked too suggested that this may be a little extra feul being trapped on top of one cylinder that fires out of sync. He went on to say that this could be the result of a leaking injector or malfunctioning fuel rail pressure regulator.

Three questions:

1. What does the blackened tailpipe usually indicate? keep in mind that my vehicle does nto smoke at all!

2. If it is running rich, how can this be corrected?

3. Has anyone else experienced the popping/puffing sound? If so, share your story.

Thanks much!