I have an ZJ that has a missfire at idle only. I wanted to get information on here to see what your thoughts are on what might be my next step. Here is the information that I hope helps. The Jeep was rolled over last winter and I picked it up cheap. I know it is 26 years old and has 203,000 miles on it, but I like tinkering and I spend time thinking about what the issue likely is and for the price of the parts I like working on it enough to keep messing around with it. I plan to just use it as a winter beater and sell it in the spring but everyone needs a winter past time right?

I have replaced:
Head Gasket, intake gasket, IAC, MAP, Distributer rotor and cap including cam position sensor, spark plugs, and spark plug wires. I just put some fuel injector cleaner in this morning for the first time hoping that is where the issue is, time will tell. Compression is 135lb+ on all 6 holes.

Known Issues:
The harmonic Balancer needs replaced, it has a wobble to it.
According to the live data the timing at idle jumps around quite a bit but above idle it is pretty rock steady. I personally wonder if that is the end cause of the miss but not sure what sensor would make it jump around. I wonder if slack in the timing chain would cause the timing to jump around at idle?

Symptoms:
1. The miss is not constant on every cycle of the firing order (does not have a rhythm to it). It is of course much more noticeable in gear than idle because the engine is under load and you feel more vibration.
2. Over around 2,000rpm it is MUCH better but still has a small miss once in a while but seems good enough to consider it normal.

I have attached some pictures of some live data.

 

Poo Poo'ed injector perhaps. My 96 has 263K hard miles on it and still runs like new so dont sweat the miles.

 

Unless you have a scanner that actually records misfire counts, the miss does not go away at speeds, you just cant sense it. I had a constant misfire last summer and I too thought it went away above 2000, but i borrowed my buddies expensive snap on scanner, and it was there all the time just couldn't feel it. The timing always varies between 4ish and 10ish at idle on mine. Evens out in mid 30's over 1000-1500. Mine is a xj stroker with a 98 engine, had too small injectors running lean but only 1 cyl showed the misfires. Bigger injectors all happy..idle advance still jumps same. Your O2 sensor looks fine. I think you map is too low, will check mine tomorrow

 

At warm idle (670-685) my ign adv is jumping between 4.5 and 11 . Always has done so. Map is a pretty consistent 9.7 to 10. Engine off its 29.5 im at sea level. I could never find a reason for the cycling...could not find a written explanation about what control's it except the " computer " . Just like i could not find how the computer determines misfire. I have squashed all of the past guesses but still don't know. I stated your map is low, but don't know where you are, if high alt it might be normal. Whats it read with engine off