my buddy has a 2003 ranger edge xlt with the 3.0 v6. the engine had around 130xxx miles and lost number 1 and 4 cylinders. Rods broke. He went to a junk yard and picked up a 04 3.0 v6. he used everything off of his engine that blew sensor and electrical wise. the yard engine was missing sensors. last night it got fired up and is running but it has a backfire that comes out of cylinder number 2. once you start revving the engine it backfires or pops once out the intake. we have played with firing order but put it back the way it was stock. and the other thing is that the check engine light does not come on when a code or codes are thrown. i am stumped on the check engine light issues and the back fire issue.My questions are as follows and thank you. Is there a difference in the 2003 and 2004 3.0 v6 engines? Why would the Cel work before swap and not after? Could a test drive of the truck make anything "Correct" itself?