[old_master]

Typically a backfire on deceleration is associated with a system that incorporates an air pump, (A.I.R. or air injection reactor). However, 1998 Blazer is not equipped with that system. The AIR system is designed to pump fresh air in to the exhaust manifold on acceleration and cruise only. If the diverter valve fails, it will allow fresh air into the exhaust on deceleration also, causing a series of small backfires. When the relatively cool fresh air is introduced to the hot, fuel-enriched exhaust, a “thunder storm” of sorts happens. If you have been around a vehicle with open headers, they backfire on deceleration also. Same principal, very short exhaust pipes and the exhaust does not cool down gradually, as it does in a full exhaust system. Point being, I would look for an exhaust leak, possibly a cracked manifold, or a leaky gasket; it doesn't need to be large to cause backfiring.