2007 H3 79K, Just replaced "EVERYTHING"! Was running great for a few days, then started stumbling and the check engine light came on. Throwing the dreaded P0300 with an occasional P0304. Seems like a common problem from what I've been reading here. Just put in new plugs and coils so I checked them all and swapped them around with no change, stayed on #4. I'm hoping that all the new stuff and a lot more driving has just dislodged some carbon on the intake valves or sucked in the intake gasket somewhere, which I'm pretty sure is original. gonna swap out the intake gasket, check for cracks, torque it all up and do the SeaFoam intake hot soak along with some BG 44K and check all the connectors for shorts/grounds. With such low miles, I'm hoping its something like carbon or a leak or short, possibly a gummy injector on #4?! I don't think its the head yet under 80K but we'll see.

 

check for a plugged catalytic converter

 

I will, thank you.

 

Make sure your using acdelco sparks. 41-103
I'd strongly advice not buying online and getting them at a store. Lots of bad parts floating out there.
C.O.P should be genuine acdelco. Do not use cheap stuff.
Have you replaced Lil hose at back under resonator box?
Follow HummerZ's process with cleaning intake system.
clean maf and throttle body too.
Replace injector with only quality replacement. Not cheap web stuff.

 

I used NGK’s and they look good but I’ll put some Delco’s back in and see what happens. I try and use all Delco or high quality stuff, no Amazon knock off’s. The tube on the resonator looks fine, I’ll have time to tear back into it this evening and give an update.

 

Take a look at this...... Acts like it wants to stall/rough idle -No CEL -What To Do:

 

Good info, thank you!

 

Long story short, use Delco. I replaced so many things in the last two weeks and my one mistake was falling for NGK platinum laser cut extra power BS! I moved the new coils and NGK plugs around 3 times trying to isolate the P0300, in the end, replaced the bogus (brand new) plugs with new AC Delco’s and it’s smooth sailing now, lots of stress for nothing!
Everything else is Delco so I should be good! Still gonna HummerZ the top end for piece of mind. Thank you for all the input, this site and community is great!