diesel backfires and smokes
 

i have a 1992 f350 7.3 diesel with 107,000miles. its starts up fine but makes a popping noise out exhaust i thought was cracked manifold so replaced it but noise now comes out tail pipe. i pulled all injectors and 3 on pass side were soaked in oil, may have been diesel but was black. what would cause the backfiring? ive asked a few people and they never knew a diesel to backfire at all... please help me!!!!