The back story. I recently swapped a 2001 5.3 LM7 into my 1998 K1500. I rebuilt the engine but reused a lot of the peripheral components. Since initial fire up I have been experiencing a menacing P0300 code. I have an Autocal and have been able to monitor individual misfires on all cylinders. Initially the primary cylinder that consistently misfired was number 4. Now all seem to misfire randomly and 4 is no longer the primarily culprit. 7 has its fair share but again, all seem to now be misfiring at one time or another.

The conundrum: At initial start up the engine runs fine. However, once I get about a half mile down the road the misfire begins. The misfire seems to happen primarily between 1300 and 2500 RPM. If I put my foot in it, the misfire goes away and the truck revs to redline really well. Then back down to cruising RPM and it begins to misfire again. The engine revs fine in neutral and park. If I shut the engine off and immediately restart the misfire goes away, but comes back shortly after.

Corrective actions I’ve taken with no positive results. I have changed the plugs twice. It currently has AC Delco Iridium plugs. I have changed the plug wires. I have swapped the coils on cylinder four and the fuel injector on cylinder four. I have checked for vacuum leaks and found none. I checked the fuel pressure regulator and it is not leaking into the vacuum tube. My tuner said fuel pressure looks good. The fuel filter is new (AC delco part). I tried running the truck with the MAF unplugged and it helped for a little bit but then the truck started running even worse than before. The catalytic convertors are relatively new Magnflow parts that functioned just fine with my 5.7 L31.

The truck does not have EGR and when I had the engine apart the intake was thoroughly cleaned as well as the injectors. Truck also has a brand new Camshaft position sensor. I am assuming that if the PCM was bad my tuner would tell me. I’ve exhausted everything I can think of to remedy this misfire and I’ve searched for hours on the internet trying to find a solution so I’m here asking for advice.

Things I haven’t checked: Crank positions sensor, O2 sensors. Again, I am assuming if either of these were causing a problem the tuner would let me know.

I cannot understand why the engine does not misfire at all RPM, not all the time and not when revved in neutral and park. Truck also seems to idle ok. I would think if it was a problem with the harness it would be more consistent and would not subside just by turning the truck off and then back on.

Yesterday I replaced the MAF with a brand new Delphi part. I also replaced the fuel pressure regulator with a new unit. I'm now getting, P0300, P0101 and P0175. Truck is also backfiring through the exhaust under light acceleration. Seems like I'm going in the wrong direction. Misfire is worse rather than better. Also, I've noticed when I hit 3000RPM the truck has a surge of power up to nearly 6000, the feeling is almost like that of turbo lag, then a surge when boost comes on. Of course this truck is not turbocharged so I'm unsure what is causing it.