4.7 backfire through intake and exhaust

Was the engine from the same year as the truck, and which flex plate did you use?

There are differences in the number and spacing of the notches in the flex plate, depending on the model year. Normally when the wrong flex plate is used, the Engine Computer can't figure out which cylinder to fire, so you have a crank/no-start, but I'm not sure how that works out for various years.

The 8th digit is a N so Mopar said that they would interchange and yes it will run it just backfires and every time I give it acceleration. it will shoot a 4-foot flame out of the exhaust pipe.

What would interchange? What size engine do you have?

4.7

Please don't be stingy with the information. I'm trying to figure out if the flex plate could be the issue or if we have to look somewhere else. For 2002, the 4.7L and 5.7L use different flex plates, but they are the same for 2003.

Was this a used engine, rebuilt one, or a crate engine from Chrysler? Did you install the same size as what came out?

It is the same size and it is a used motor.

Different flex plates were used in different years. If the engine is from a different year, to be safe you have to transplant the original flex plate onto the replacement engine.

The next best suspect is the crankshaft position sensor. Some earlier versions needed a thick paper spacer stuck on the end to set the critical air gap. Yours uses a different type of mounting bracket that sets the air gap automatically, but the sensor can be damaged or bent if it isn't removed during the removal and installation of the engine. The back-firing you described can be caused by an incorrect air gap, and it can be caused by a cracked magnet inside the sensor. I don't know what causes them to crack, but I've read about that solution more than once.

Thanks.

Please keep me updated on your progress.