I have a '92, 3.9L that backfires during acceleration until the engine light(right bank 02 sensor) comes on and the backfire quits. The engine is rebuilt (machined,new pistons, rings, bearings, cam, and lifters) with less than 100 miles. The only engine, exhaust or electronic parts that have not been replaced are the mass air sensor and the ICU. The mass air sensor was tested for voltage change and seems to be working. Compression tests indicate 120 psi in cylinders 3 and 4 the remaining are 145 psi. The only remaining items I think the issue can be is a leaking valve or the mass air sensor. Can someone tell me would the check engine light (O2 sensor) correct the backfire caused by a leaking valve?, seems unlikely. Last mechanic thought the vacuum advance on the distributor may be faulty, however I replaced the distributor with no change. Any ideas would be appreciated.