Large misfire count on cylinders 4 and 7

[bubba68ss]

I have recently done the 24x conversion on my 5.7 and now run a 2002 5.3 truck tune with the 12122156 OS. I am getting p0300 with flashing MIL every time I drive it and go above 1600 rpm.

According to scan tool I am only misfiring on cylinder 4 and 7 (roughly 4000 misfires in a 3-5 minute log on each cylinder). It idles great and I can only barely feel a slight vibration when MIL begins flashing and I am misfiring. Other cylinders mostly have 0 or less than 5.

I have swapped coils with cyl 4 and 8 to see if the misfire followed the coil, installed new wires, tried my old ckp with new crank relearn, watched wideband O2 read 14.6-14.9, seen o2's cycle between 0 and 1000mv, etc...

Is there any relationship between cylinders 4 and 7 (besides having to change the firing order on 4/7 and 3/2)? How about any known problems?

Im just looking for anything to test next.

My log file. Anything unusual going on?

17660

17659

[bubba68ss]

Is it possible I am getting false misfires? I am using the misfire tables from a 5.3 and not my 5.7 tune. Should those tables stay with the 5.7?

Also interesting that 7 and 4 are on the same spot on the crank reluctor, correct? Firing order of my 5.7 is 18436572 and ls is 18726543.

[joecar]

Misfire tables should stay with engine.

I take it you were able to do a CASE relearn.

Yes, #4 and #7 are "companions" (they're both at TDC at the same time), so they share the same spot on the CKP reluctor.

To find companion cylinders, split the firing order into two groups, and then pair them up:
SBC: 1843 6572 --> 16, 85, 47, 32
LS1: 1872 6543 --> 16, 85, 74, 23

[bubba68ss]

Misfire tables should stay with engine.

I take it you were able to do a CASE relearn.

Yes, #4 and #7 are "companions" (they're both at TDC at the same time), so they share the same spot on the CKP reluctor.

To find companion cylinders, split the firing order into two groups, and then pair them up:
SBC: 1843 6572 --> 16, 85, 47, 32
LS1: 1872 6543 --> 16, 85, 74, 23

Yes I was able to do the CASE relearn with help from Lextech and his suggestion.

If 7 and 4 are companions on the reluctor could that be a 'bad spot' that is making false misfires?

[bubba68ss]

Put my 5.7 misfire tables in and I no longer have flashing MIL. I do still have about 10% of the misfires that I had before so it must have done something. I'll have to tweak it a little bit more to tune them out.

[joecar]

...

If 7 and 4 are companions on the reluctor could that be a 'bad spot' that is making false misfires?
This is a possibility... if you have an oscilloscope you can view the waveform produced by the reluctor... also eyeball the reluctor.

[joecar]

Put my 5.7 misfire tables in and I no longer have flashing MIL. I do still have about 10% of the misfires that I had before so it must have done something. I'll have to tweak it a little bit more to tune them out.Interesting... redo the CASE relearn.