My son has a 91 Explorer with the V6. The other day he went to start it and it back fired (violently) and blew the muffler apart. When he shut if off the motor kept running (dieseling). He changed out the muffler and it still does this. Any ideas on what it could be? Thanks for the help in advance.

 

is usually caused by retarded timing. so first thing i would do is get a timing light on it and see if it has jumped time. check you manual for specs. should be some where arround 12 degrees