p0300 Misfire code

[Gamble97]

Ok this isn't my car but my nephews car. 96 talon tsi awd auto.
Mods:
bc272
small 16g turbo
ssac fmic
safc
680cc injectors
ported o2 housing
ported 2g manifold
3" turbo back
injen intake
tial bov

He brought the car over a long time ago and we put on the fmic,manifold, o2 housing,16g and safc (stock injectors). After it was done we went for a ride and around 5k rpm the car started to sputter like backfire really bad. So he had to leave and go back to school and the car sat at his house. I figured it was a bad boost leak from the fmic piping we welded together.
So he bought a ssac fmic kit, and switched to a tial bov. We threw that whole kit one, tbolt clamps everything, installed just how mine is. New tb gasket, new Jpipe gasket the works.
Started her up and she idled really well (only 8in of vacuum at idle though)
I reved it all the way to 7k and it had no problems. Took it for a ride and again in the upper rpm range it started sputtering really bad.
CEL has been on for some time with the p0300 only.
The plug wires have maybe 500miles on them at most, the spark plugs were just changed this weekend with some ngk7s. We also cleaned out the maf.

I know the tb seals are shot so a new tb is coming in this weekend. But how do we narrow down our problem. Car has 130k miles no mbc and is boosting at 20psi.

Any idea what would cause only 8in of vacuum also at idle?

 

[SethA]

I don't know if you have read through all of the P0300 threads but it is a pain to diagnose.

http://www.dsmtuners.com/forums/search.php?searchid=5444078

I did a clutch and a timing belt and got side tracked way too often resulting in my car sitting for a long time. Now it throws this code. I have the common problem of throwing the code at about 3000 rpms while cruising. There are also quite a few instances of throwing the code after a clutch job. If you could narrow down the conditions when the car throws the code that might help someone diagnose it.

Did you do a boost leak test?

Seth