Got an LT1 with a 292 XFI Comp Cams cam and everything from fuel pump to the water pump modified on this car. I had it tuned and now it backfires or coughs from the intake when the engine is at operating temp and you are accelerating or revving from idle. It sets the engine light for about 8-10 secs when car returns to idle. I have a MAP sensor code for low vacuum and wondering if it is the MAP sensor or do I need a vacuum pump to fix this. Or did the tuner perhaps improperly set the timing?

 

Adjust the valves recently? This all of the sudden just started happening after the tune?

 

It started after the tune. I'm wondering if the poor state of tune was masking another problem or this resulted from the tuner retarding the timing too much for the Nitrous setup. It's got brand new TrickFlow 2.02 1.60 heads with about 250 miles on the motor. The valves were adjusted after assembly. ProMagnum 1.6 rockers, LS7 lifters, 58 MM Holley, 30lb/hr SVOs, Moroso Air Tube, long tube Pacesetters, Digital 6 ign, LT4 knock sensor, EGR delete, electric WP, etc. if that helps at all. Cam is in .584 ex .579 , DUR is in 242 ex 248, lift in 3660 ex 3630, 113 lobe sep. Thanks.....