my buddy jus finished a ls2 intake swap on his 99 t/a and we got all the sensors pug up all the vaccum lines on we beleive and we modiyfied the stock fuel rails to fit the problem is the car wont idle and it is backfiring through the intake help will be apreciated

 

is there a whistling sound under the hood= vacume leak.
usually a uneven idle usually means vacume leak.

Going to a ls1 to a 90mm opening will play with the tune a bit and it will be hard for the computer to catch the idle.

If there are no vacume leaks sounds like he needs a tune.

 

we dont hear a whistling jus like the car act like its goin to go dead but will run but it crackling and popping under the hood so we took the lead off and ran it with the mass air plug up but moved to the side and you can feel like a back fire out of the throttle body