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. The primary cylinder that consistently misfires is number 4.

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 tune 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 fine. 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.

 

I had a similar problem after I swapped in my 6.0, a crank relearn fixed it.

 

Have tried the fuel filter? And how did you ran your fuel line?

 

Was your problem intermittent and only at lower RPM? I've considered the crank relearn. I guess I'll try that next.

 

The fuel filter is a new AC Delco, so I haven't messed with it. Fuel lines are set up as described by "foreverfalcon40" here:

GM Performance :: View topic - Officially Swapping in a LS & 4l80e in my 99 GMT 400 4x4 Tah

 

Did you leave the factory one still on the truck.?

 

Yes only happened at cruze on the freeway ~2000rpm.

 

Crank relearn did not solve the problem. Not sure where else to go.

 

Check spark plugs& wires

 

If those look good go to part store buy some brakes parts cleaner. And test for leaks. Intake leak or vaccum leak.p0300 is pita code