Hi. I have a 1993 Camaro 3.4L V6 with distributorless ignition. The primary oxygen sensor has been replaced but the secondary has not. New map sensor, coil packs, ignition module, EGR valve.My problem is this: after startup the engine runs fine in open loop mode, but once the ECU switches to closed loop the engine sputters and falls on its face. Any ideas on what to check? Appreciate the help