I just recently had a new crate motor 6.0 vortec installed in my work truck. the mechanic is struggling on finding out the reasoning behind the random missfire code p0300
we have done the following and still have a random missfire on cylinder 4 and 7 but only at 70-75+ mph on the interstate. otherwise no missfires

new crate motor
new plugs and wires
No.7 has a new coil
Tested each injector for fuel pressures which was all running same pressure

any suggestions on what to check now?

 

did he do a crankshaft variation relearn. it must be done or false misfires will set at high rpm.

 

I will find out about that. Haven't heard that one

 

I believe that solved the problem. Still driving and testing it but so far so good. He didnt run that test initially