I have done the smoke test for a vacum leak , tested or changes all spark plugs, wires, coil packs and injectors, cleaned the o2 sensors with gas and a vibrating massager, run seafoam through the intake and has tank, still a code.

the engine ran fine until i got exhaust leaks on both manifolds fixed. Then the p0300 code pipes up. Since I have cleaned the o2 sensors, now the lean codes show up. The engine seams to run fine at higher speeds, but smells extewmely rich, even in the cab.

the only weird thing I have noticed is the engine runs better with the pcv line disconncted. Please help.

this is an h2 6.0 liter 2004. I also pulled the valve covers and everything was okay, as ell as cleaned the mad sensor and throttle body. Please help

 

Check the MAF. Make sure it is lined up perfectly and that it is clean.

Tim

 

That and instead of trying to clean the O2 sensors, I'd replace them. They are based on resistance and over their life which is around 100,000 miles they slowly lose their resistance to the point that eventually the computer just ignores them, runs your engine super rich and kills your economy. Its proven that trying to clean O2 sensors when they are pretty old is not very useful. On newer ones, pulling them and cleaning them every 15 to 20 thousand miles can provide marginal benefits.

The downstream ones are more critical and the first two to replace. The computer is evaluating the difference between the resistance before and after the catalytic converter. If it thinks incorrectly that you are running too lean it will force it to run rich. This also causes a lot more unburned fuel to get through which will also rapidly shorten the life of the catalytic converters.

Another to make sure of is that when you put the air pipe back on the throttle body, that the bottom edge did not get hung up which is somewhat common. Obviously it needs to seal all the way around.

One last thing - since you made a bunch of changes I'd disconnected the battery for about a half an hour to force the computer to reset and relearn the long term fuel trims. Just clearing the code with a reader will not force it to reset those. It will probably run a tiny bit rough for just a short bit while the computer establishes new fuel trims and adapts to the new resistance readings in the sensors.

 

I just fixed a p0300 and this time it was found to be a fuel pressure regulator. The regulator was spitting fuel directly back in to the intake manifold through the vacuum tube making the vehicle hard to start, rough idle (flooding), and burning rich. You can easily check this by following a diagnosis on a similar vortec engine here: