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


NAXJA Forum User
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?