I have a 04 tahoe with a magnason charger in it just recently it starting misfiring on build of boost when I drive like a grandma it's fine but anytime I on it it starts heavily misfiring ! I get a p0300 n have monitored misfires n they are heavy across all 8 cylinders! also w the p0300 shortly is a p0420 cat efficiency code n I know it's due to excessive fuel bc it will go away after normal driving for a little while! I changed the fuel pump because the fuel pressure is at 48 psi on wot! I have monitored the 02s n saw nothing standing out at me it has lng tube headers! I have changed the spark plugs and wire to ac delcos n nothing new also changed the maf and still nothing so I am in need of assistance as I bought this truck w the charger n am not to familiar w it ! Thanks for any input

 

What gap are the plugs at?

Being that the motor has forced induction, a factory gap is too large and is more likely to get blown out.

 

How many miles are on the cats? You said you monitored 02's, what were the readings at wide open?

 

I don't rem the readings but there is 136k on it n it started before I rwplaced the plugs n it won't happen in open loop only In closed! When this prob started it would only happen in higher Rpms like 5k then progressively worse now it will happen at 2k and 50 percent throttle or more

 

i would bet clogged cats

 

I'm gonna go that direction thanks

 

you can have your exhaust pressure tested to confirm. when i installed my stainless works y pipe i toasted both cats in about a week. one had a melted glob of metal that looked like a small beer can.. i had misfires once i got 2500 rpms or over

 

Sounds exactly like what I'm dealing w ! Thanks for the info!