My Z-28 was sputtering and backfiring on cold start...wouldn't do it if it warmed up for 5-10 minutes. Dealer diagnosed water pump leaking on opti-spark...said computer confirmed. My mechanic replaced water pump, opti, plugs, plug wires. But, it still has the same exact problem. Any suggestions on what we might be missing?

 

Well since you have already replace the opti,waterpump, plugs n wires, that leaves the coil and ICM which are both about 20-30 a piece.. Might as well try that stuff too... Weird how it runs like crap in open loop but runs fine in closed???