I have an LS1 5.3 with a Z06 cam, 799 heads and LS6 intake, injectors and throttle body. The engine is transplanted into a 1985 Trans Am. When the engine first cranks up it runs fine. After the first time I hit WOT it throws all four knock sensor codes and starts backfiring. Clearing the codes at that point does no good. Fuel pressure checks at 55-58 at idle and holds at 58 as I go through the gears. The car has been tuned at a specialty shop. When I let it cool down the cycle begins again. I have checked all the wiring and can't come up with anything wrong there. Any ideas as to what may be the problem.

 

Find out what octane level the shop tuned for, and see if you are using the wrong level of gas is about the only thing that comes to mind.

 

My tuner is going to run it with his wideband O2 sensors to see if he can figure out what's going on.
It runs much better when the MAF is unplugged. Put a brand new MAF on and still runs like crap with the MAF plugged in.