cause for frequent backfires at cranking?

[VetteMed]

I know the LT5 is prone to backfiring through the intake on cranking, but in the last 2 days, I have had backfires about 25% of the time (3 backfires in about 8 or 10 startups). Of course, the MAP sensor hose blows off. Also, there is some popping from the exhaust upon decel while in gear. None of this was an issue prior to my plenum pull last week (had injectors cleaned by FIC), rechecked vacuum system.

So, what is a likely cause for frequent backfires and pops on decel?

I am running with secondaries eliminated, Haibeck tune, headers.

thanks
Andrew

[VetteMed]

also found the HVAC system malfunctioning this morning, blowing only through the defog vents. Pulled the plenum, found the large diameter hard vacuum line detached from the check valve which supplies vacuum to the HVAC system.

I cannot be sure if this connection was the cause of the backfires (due to vacuum leak), or if it blew apart during one of the backfires.