Flashing means multi misfires which trips a P0300 DTC, not a single misfire, multi is seen as worse case for it would cause damage to the CATs so reason SES flashes.

Several things can cause real and false misfires, anything that causes an uneven rotation of the crank from being too rich, ign issues, custom CAM, slipping belt to underdrive pully that has caused the crank to be out of balance.
Even a warped brake rotor can triiger false misfires.

Lastly if the PCM lost the crank position sensor sync the PCM has to be commanded to do a CASE relearn.

 

sorry gone for the weekend...

little more info

car originally started misfiring BEFORE headers or any changes to the emmissions.

the car only starts misfiring when the car is reved over 4000rpms (give or take)...if you keep it low...it runs perfect for weeks on end.

i can scan the car, it is misfiring on cylinders 5 and 7 only

when it does misfire, it will misfire for a few seconds, then stop for a few seconds...then start again. it will do this for 20 min or so (varies greatly) and then runs perfect untill you rev it past 4k again.

since it first started, i've changed plugs, wires, coils, swapped injectors installed LT headers and ORP, removed AIR and EGR (but have not reprogamed).

the flashing SES has only happened 2 times since this started...probably due to the dropped cylinders and un-burnt gas...

the big hassle with it is why does it run fine till you rev it up?

 

Have a good shop check the compression in those cylinders.

 

This is a stretch but could it possibly be something in the valve train. Perhaps weak valve springs or incorrectly adjusted rocker arms. Maybe the higher rpms are causing the valves to be sucked open if the springs on those 2 cylinders are a little weak. That is probably way out there but it is something to think about especially if you have missed a shift and over rev'ed.

The only other thing I can think is maybe there is a crack in the head or head gasket that is causing a loss of compression between cylinders 5 and 7. Since they are beside each other maybe there is a problem in the head or gasket somewhere. Have you done a compression and leakdown test on each cylinder to rule that out.

I would think that last one may be a little more realistic in theory than the first since the misfire is on 2 cylinders next to each other. Something to start looking into though maybe.

 

Well if you don't have cats then you can't hurt them, can you?

With a bigger cam you will always get some misfires,especially at idle due to the increased overlap, but you don't have to set a code with them.
Most tuners will increase the number of misfires necessary to set a code.

 

"This is a stretch but could it possibly be something in the valve train. Perhaps weak valve springs or incorrectly adjusted rocker arms. Maybe the higher rpms are causing the valves to be sucked open if the springs on those 2 cylinders are a little weak. That is probably way out there but it is something to think about especially if you have missed a shift and over rev'ed."

finally...someone suggesting the same theory as me. this is what i think may be the problem. either a weak spring or a bad lifter...since the engine runs fine UNTILL you rev it up...not neccesarily as rpms go up...but after you rev it, doing it at idle and cruising rpms following a hi rpm run

figures it has to be the hard cylinders to get to

gonna work on the compression test this week (assuming this next hurricane doesn't mess us up)

how do i check compression on a FI engine? and how would i check the adjustment on the rockers? is it with a feeler guage like the older style?

 

To check compression you get a compression tester, like this, remove the spark plug of the engine you want to test the compression of, screw the hose from the tester into the spark plug hole, then remove the fuse for the fuel pump and turn the engine over with the key while someone watches the gauge.

As for the rockers I got this straight from ls1howto.com

 

it is easier if all 8 plugs are removed so engine cranks better
Remove fuse for fuel pump, disconnect coil pack wiring connect.
Hold gas pedal to floor so engine gets air and allow engine to go 4 full 360 cranks ( 4 compression strokes) and pressure gauge will
hold what the pressure was until you press the release valve on gauge.

If stock all 8 cylinders should be around 160 + PSI, any one about 15% less I would look for ring or valve damage.
For a stroker engine like mine the PSI is 200.