I have a '90 IROC (5.7) with a Vortech SQ blower installed. Basically a stock engine with the stock 6 psi Vortech kit. It has run well for a couple of years, but suddenly in the last few outings (it's my weekend car) the power has started dropping off and it has begun backfiring (technically 'afterfiring') out the exhaust during deceleration. These symptoms are not dramatic yet, but each time I drive it it gets a little worse. I know the inline fuel pump is running, but I have not yet had the opportunity to check fuel pressures or hook up a scan tool. The backfiring suggests rich mixture to me. Anybody recognize these symptoms? Could the FMU be out of calibration, or might a leak in the boost reference hose to the FMU cause this? Thanks for any suggestions.

 

I forgot to add that it also surges while accelerating from low rpm, and tapers off until it's pretty much unnoticeable above about 2000. Thanks.