Take it to Autozone or one of the others places that do a free code check. Also make sure you didn't mix up the plug wires.

 

I Switched my #5 injector with #6 and drained the fuel rails. No misfire since then. Only a lean code on banks 1 and 2. That was two weeks ago

 

Did you ever figure out the problem? I have a similar problem with 2006 Silverado V8 5.3L. Getting the P0300 code, checked everything we can think of. Put a new MAF Sensor in, cleaned evap system, replaced spark plugs and wires, checked injectors, O2 sensors, Cat, everything is working. When hooked up to scanner it reads mis-fire in cylinders 5 & 7.

 

I'm having the same problem with my 2000 Chevy with 5.3. I've changed fuel pump,map,idle,idle control valve,plugs,wires,Knox,and all 4 o2 sensors.You can hardly see the misfire at an idle.But when you hit 2000 rpms its all over the place. I've checked the injectors,swap out coils and still the same.Oh yeah I even had A muffle shop open up my cats to see if clogged.Ready to run it off a hill. Their is a lot of post on this po300. But no one ever tells their out come if it was ever fixed. PLEASE HELP. Any one.

 

I fixed my misfire. It all boiled down to fuel. Start running 100% gasoline through it and your ck engine light should go off if it fixes it.
These trucks seem to be very sensitive to ethanol and they do not like it.

 

I had a similar problem with my 2001 silverado 4.8. The engine all of a suddun lost power and started running rough. I got the usual multiple random cylinder misfire codes and went to work on fixing it. We changed the plugs and wires. No help. Then changed the injectors. No help. Then checked the fuel presure which was fine. Then changed the coil packs. No help. Then pulled the O2 sensor in front of the catalytic converter on the drivers side cause it was the side with the apparent problem. Problem solved. It was the converter clogged due to the bad injectors to start with. We will either replace them or just punch a hole through them. Not sure yet which. But just the small amount of air flow bypassing the converter should let you know if it's the converter or not.

 

 

 

i hope 7 years later if it was a blown head gasket he would of figured it out by now

 

Throughout that process, I changed my intake manifold gasket, plugs, wires, and cleaned the Mass Airflow Sensor wires. no change. For whatever the reason, I decided to start running non ethanol gas and this seemed to clear up my issue. maybe a coincidence, not sure, but my issue was fixed.