104k miles on it and has been running great. Until... Yesterday during a lunch run the Check engine light started flashing. Ran a code on P0303, misfire cylinder 3. Have some extra parts available so the first thing was to pull the plug and look(the middle one in the back correct?) Plug looked fine. pulled one in the front, it looked fine, gave them a bit of a clean on the wire brush checked gaps, swapped them and tried again. Again P0303. Yes we were careful to clear the codes each time.
Grabbed one of several coils i have and swapped out the coil(center one correct?). Again P0303.
Pulled the plug wire and tried another one. Again P0303. I was interrupted druing my earlier plug swap, so second guessing that, swapped it again for another plug and while at it tried another wire and swapped for yet another coil. P0303 again. So far 3 plugs, 3 wires and 3 coil all giving me p0303. Since it was all there, i pulled all 3 coils and replaced the coil base that has the module in it. Again P0303. The code shows up the moment there is a load on the engine, No load its fine, drop it into gear and you almost immediately get the misfire code. Any help appreciated...

 

You should do a compression check, could be a burnt valve? what has changed if anything regarding gas?

 

Nothing has changed at all in the way of gas or the way it is driven. My wife has been using it regularly. Drove 20 minutes to work and parked it. Went to get lunch 5 minutes away it was fine. On the way back from picking up her lunch she saw the CEL flashing. Couldnd do anything at the time so she drove it home after work. Not sure what the compression ought to be either, any other thoughts anyone? Brother in law suggested a bad injector.

 

Welcome to the forum. I had the same problem on my 2002 Chev Tahoe with the 5.3L V8. I went through the same preliminary checks as you did. Swap the plug on the bad cylinder with one that had not problem, cleared the code, it came back. I then swapped the plug wire and coil, clear the code, ran it again, same problem. I then tried one more thing and got a new fuel injector. With fuel injection, the injector is fired for that cylinder when that cylinder is ready to accept fuel. So if that injector didn't open when it was ready to accept fuel, then you would get a misfire, especially when you load it up. After I changed the injector and cleared the code, it never came back.

 

so far 3 votes for the fuel injector, 1 here, 1 form a mechanic i talked to and 1 from my brother in law who thinks it is some sort of an electric related mater like the fuel injector as it fails to fire because the code comes back so quick. His beleif is that if it were valve related it would take longer for the CEL to light.

 

Put in a fuel injector and after a short bit to pressurize/work air out and burn off the windex(used to lube the injectors and holes to slide them back into place) the power was back and it runs much better. 20 plus miles and no CEL. Just noticing a bit of a pulsing sound and feel at idle in drive or reverse after driving it. Once in park it smoothes out again, but it never did that before. The old injector has some bluck crud on its face obscuring the holes. It wiped off, but that made it apparent the injector wasnt firing.