chev 1998 pu, emissions control, air leak

i have purchased a 1998 chev pu it is a 5.0 liter 305 v8 and when i am going down the road it will sometimes sort of like back fire it only does this once in a while but it is driving me crazy the fuel pump is new the injecters where cleaned could it be the maps or ? some times it really cranks over a lot to start to oh new distributor was installed and new plugs whats left code reader gave a 1345 code but they said if no check engine lights were on it was ok Help !

Answer
    There is probably an air leak in the exhaust system.  This is the most common source of backfires.  It could also be the idle air control.  If it is plugged up the throttle blade can completely cut off air to the engine, resulting in very high vacuum signal.  This, in concert with the vapor/emissions control system could also cause a backfire.  Another possibility is that the timing chain is stretched.  You don't see this too much anymore, but it can happen.
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