Super light misfire at idle and blinking CEL on highway - tried everything

[pddenhar]

I've got a 2000 Cherokee Sport 4.0L with 275,000 miles on it. It runs strong and pulls well on the highway, but like I said in the title it blinks the CEL on the highway for a misfire and feels like it has a very light single cylinder miss at idle.


The codes are P0300 and P0301-P0306 (misfires registered for every cylinder).



I've read about 100 threads by searching through the forums and have found no solutions that work yet.


I've tried:

• New plugs and ignition coil assembly
• New fuel injectors (mine were cracked and dirty, but this didn't clear up the miss)
• Compression test:
  • 1 - 125
  • 2 - 130
  • 3 - 140
  • 4 - 125
  • 5 - 135
  • 6 - 135
• Valve rotation from TSB 09-003-3 (one exhaust valve in cylinder #6 was not rotating so it was worth it, but this did not clear up the miss)
• Ran engine with the valve cover off to check for a dead lifter. All rockers seemed to move normally.
  
• Checked the crank signal with a oscilloscope (4 pulses per trigger, the signal looked very clean)
• Checked the cam position sensor with a scope (nice square wave, looked clean)
• Checked each injector's trigger pulse with a scope (same story)
• Aligned the cam position sensor to 0 degrees TDC (mine was a bit off, but aligning it did not clear up the miss)
• Checked for a vacuum leak around the intake with carb cleaner. I couldn't find any spots where the carb cleaner sped up the engine.

I'm at my wit's end here. I think I've tried everything anyone has ever posted on the forums and then some. The only thing I can think of doing now is removing the head to replace the head + intake gaskets and lapping the valves.



Anyone else have some words of wisdom for me before I really get into this thing?