Frustrated...04-LS6.0

[FD-Man]

Have a 2004 Chevy Silverado classic 2500 with a 6.0 motor… Recently rebuilt, getting a flashing engine check light throwing code P0300 which is a misfire only does it around 80 miles an hour when you back off the throttle. Like I said motor was just rebuilt new plugs new wires and new coils, throttlebody is also new, ran a full tank of seafoam through the motor to clean the injectors, gas cap is secure in tight, Anybody else have any ideas on how to figure out why it’s miss firing and how to figure out what cylinder or cylinders it may be… We don’t notice any loss of power at all or shattering in the motor…

[davester]

you might check fuel pressure, both at idle and when you rev up the motor, should dip briefly while accelerating motor, but quickly return to full pressure once the rpm has stabilized at a higher rpm.

 

maybe also check if there are any air leaks in the intake and/or exhaust (missing exhaust manifold bolts).

 

after that, I would be checking how the knock sensors are working and fuel trims.

 

not suggesting throwing parts at it, just things to check .

[Formulabruce]

I have seen new aftermarket coils do this.

[CamGTP]

A live data scan tool will let you watch each cylinder for misfires.