P0174 means the computer has reach a preset limit on adding fuel to cylinders 4, 5, 6(bank2), and O2 sensor is still telling it there is too much oxygen in exhaust.
Too much oxygen can be from a vacuum leak OR a miss fire with no or low fuel content, so cylinder is basically just pumping air into exhaust pipe, 2 cylinders firing and 1 cylinder pumping air = too much Oxygen in exhaust.
Since you do have a miss fire, P0306, you can feel it missing right?
Then the P0174 code means the computer and O2 sensor are working fine.
I doubt MAF sensor is involved or you would see bank 1 and 2 both with a code, the MAF sensor gives the computer it's Floating Zero for 14:1 air:fuel mix based on air flowing into engine, computer then reduces fuel a "-" or adds fuel a "+" to that 0 based on O2 feedback, when computer gets to about +25(or -25) it will set the code for a problem.
On most scanners you can watch live data, look at STFT(short term fuel trim) and LTST(long term fuel trim), these are specific to banks, in your case bank 2 would be the one to check, but look at bank 1 for what it should look like.
Generally the STFT should be switching very fast and stay between -10 and +10, more on the "+" side at idle, it is normal to run richer idle.
Slow switching numbers usually mean O2 sensor is wearing out.
Fuel pressure is mentioned because +25 is actually the amount of time the injector is opened, on your '99 3.0l the computer assumes 65psi of fuel pressure, it has no fuel pressure sensor, if it was only 30psi then +25 is because less fuel is flowing out of each injector when it opens, so computer has to open each injector longer.
In you case this wouldn't apply since you get a specific P030"6" miss fire, low fuel pressure would cause random miss fires.
Have you done the wet spark plug test?
It is looking more like an injector issue
You also haven't mentioned the Cat converter, if #6 injector is working and miss fire is because of low compression then Cat should get very HOT, as it has to burn off the #6 fuel that was dumped into exhaust stream, Cat would just be normal hot if injector was not working well.
Last edited by RonD; 10-26-2014 at 11:32 AM.