The Problem (1990 5.7 TPI Iroc-z)
It starts immediately, it idles, it revs. In closed loop, it doesnt idle smoothly, with a miss every couple seconds. When I rev it say above 3.5k and let off, it usually will backfire very loudly. When driving, it seems to hesitate when slowly leaving a stop, and has a lack of power under acceleration.

I Recored a Data Log Here

Every time it misses, I notice the Spark Advanced had jumped to 24*
The green lines coencide with the missfire.

I get no SES, Car goes into closed loop, TPS reads 0%-100% variably with no snags, etc

What Ive done:

• Plugs (gapped to .035)
• Wires (Yes they are in order. I have checked 5-6 times. It idles too well to be off)
• O2 Sensor
• Cap/rotor
• ICM 2 years ago with distributor
• Injectors were replaced in 2008, and all Ohm 14.7-14.9ohms
• All Battery cables seem connected well
• No apparent vacuum leaks. Intake gasket and hoses are <2years old. Sprayed with Carb Cleaner and no hesitations. Pulled main vaccum from intake and plugged with finger, made no difference (after initial surge)
• Fuel pump+Filter changed this past summer.
• Timing with EST disconnected is 8* advanced
• Distributor doesnt seem to have any play.

-Both Cats are gutted, single Air Tube is run from one to the smog pump. The other tube is welded shut.
-Exhaust leaks a bit on both sides at Y-pipe because the Magnaflow "direct fit" is a larger diameter and doesnt fully seal. BUT It has been that way for 5 years and driven fine until this past month.
-Besides a muffler, car is otherwise stock.

Im running out of ideas. I am also out of parts to throw at it. I asume there is an ignition issue, because of the miss and the backfire from the exhaust.


*Sorry if its bad, but I removed my thread in the other section because this one is more appropriate*