I recently replaced the 22re in my 87 pickup and the new (300,000k) motor backfires. It runs great and is completely stock, except for a custom air intake and a header with 2 1/4" exhaust and a Dynomax muffler. I had the same intake and exhaust on the old motor with no problems. The backfire is never a loud bang, just a series of small pops when decelerating in gear. Thanks for the help.

 

Try adjusting the TPS. I had the same thing happen while my TPS was misaligned. Also could be the dashpot.