The car is a 2000 camaro, stock 243 heads, t-rex cam, comp double 650 springs, forged pistons, rods, stock crank with 15000 miles on rebuild with no engine problems till now. The car had been sitting for a year awaiting a trans and converter rebuild. I replaced trans, converter, fuel pump (racetronix no hot wire kit), rear end and injectors. Ever since i put the car back together it stumbles and backfires through the exhaust at part throttle under load around 3000 rpm. It's fine at wot but has slow throttle repsonse at idle and is not throwing any codes or ses lights. I'm stumped! i've replaced plugs, wires, 02's, had the tune and fuel pressure checked, also pulled the intake to check for vacuum leaks or burnt wires. All of the plugs look very similar in color and have ohm tested plugs and wires. We lowered the power enrichment tables from 64% to 23% and that seemed to mask a lot of the backfiring but the lack of power under load and hesitation at idle remains. It seems to get much worse after the car really heats up (20-30min highway cruise). It seems that the number 7 coil is sparking a lot more than all the rest no matter what coil i swap it with. I'm also having problems with the converter staying locked while i'm cruising down the highway. If i let out of the throttle and just coast down the road the converter will unlock and try to lock back up but with no luck unless i tap the brakes or just let the car keep on decelerating. If i just let it keep decelerating the rpms will drop from 2300 or so depending on speed to 1000 rpm and the car will studder and the converter will lock hard and continue on locked up untill i let off the gas for more than 5 seconds. Tuner was having an issue keeping the car from pulling through the brakes at idle so he had to lower the iac's really low so it has to relearn each time it comes to an idle, this is making diagnosing the car much harder because i can't tell if the car is running like crap because of that or if it's a misfire at idle also. The car seems to run fine at WOT with good afr's. Any input would be appreciated, i'm out of ideas.

 

Data logging is where I'd start...

Also, a COMPLETE review of all wiring connections, vac lines/leaks, is in order.
Start at the pcm, and work your way out..
Grounds on the cyl head? [Did you take them of when the trans was done?]

 

i'm in the process now of checking the ignition box. My cruise control hasnt worked in a long time and I didn't really bother checking into since i barely used it. I found that the fuse for the cruise control had popped so i replaced it with another 15amp fuse. After about a 5 minute idle in the driveway and a quick 5 minute drive to walgreens smoke started to pour out from behind steering wheel. i quickly turned the car off and removed the fuse which seemed to stop the smoking. So i'm now on the hunt to figure out what the heck is going on in there. yes all 3 grounds are attached to one bolt in the back of the d/s head. I also pulled back some of the wiring loom for the d/s wiring harness that goes to the main coil plug. It seems like the pink and green wires leading into the main connector for the d/s coil from the engine harness has some discoloration. They dont seem brittle or to have any visible damage to them.

 

Wiper switch harness was fried and 1 of the 2 wires coming from the lock cylinder was broken. also found that all of my exhaust valve seal were blown. Still hunting down backfiring issues and possible converter failure.