96 Cherokee Sport 4.0 misfire
 

My 1996 4.0 XJ broke down on the road and didnt wanna start. I changed coil, pick up coil and finally it proved to be crankshaft position sensor. It started up, but started misfiring heavily, something that has never happened before. I did a tune up - new spark plugs, cables, distributor cap and rotor. Still misfiring badly. Then check engine code mentioned something about map sensor, so i changed all three sensors on throtle body - map, control idle valve and throttle position sensor. Car ran perfect without a single miss for 500 miles. I noticed however it did miss when i pressed windshield power washer pump, which really puzzled me. Then after 500 miles i plugged in to charge my phone and it missed half an hour later. Bit by bit it started missing, too much to drive normally, but not as much as the first time around. It can miss on straight highway at 65mph, or getting going at traffic light, totally random, no patter what so ever. Any ideas? Thanks.

i have an update... it turns out mchanic didnt change rotor when doing tune up... whe taking it out this time, the metal piece on old rotor was completely gone! and its not down in the distributor. i changed it now with autozone duralast rotor hoping its the solution to my months of problems but instead the car became undriveable! its misfiring terribly as soon as you touch accelerator pedal! it runs rough idling too. before i could at least drive it as it missed sporadically... i dont know what to do... a mechanic told me to have him clean my valves of carbon deposits and replace springs according to chrysler tsb, but somehow i feel thats not the solution... btw, no engine code came up since new rotor... previously it was reporting random misfires and individual cylinders occasionally...

update
 

here is an update... i checked the fuel pressure, its at 49psi... i pulled out a spark plug, they are black although the plugs are brand new... i am guessing it runs rich... i dont know if it might be the fuel pressure regulator thats the culprit

Looking at my Grand Cherokee manual, it lists normal fuel pressure to be 31-33 with vacuum hose attached and 39-41 with hose disconnected. With your high reading, I would check your vacuum system to see if you have a leak. Some of the sensors you replaced are connected to vacuum.

Also, open distributor and check for erosion, burnt marks or carbon tracks to see if you have a problem there. If you have a cam position sensor in the distributor, can the rotor pieces get to it and foul things up? Check each plug wire to see if each is sparking.

Remove all your plugs and see if they are all black. Black means rich. So you have to look at what causes a rich mixture. Oxygen Sensor is one, other sensors, plugged catalytic converter, maybe a malfunctioning evaporator system if one is installed.

Just throwing ideas out where to look.

Good luck, hope you find the problem soon.

Update
 

Hi everyone and thank you. Here is an update... It seems that one of my sensors on throthle body was upside down, and that was causing all that terrible rough idling making the car undriveable. However, the jeep continued misfiring, but not too badly... car wouldnt shake violently like before. I changed the distributor, it was kinda loose and thought that might be causing the short with the picku up coil / stator / camshaft position sensor. For a while, it seemed like that fixed the problem. But alas, the mysterious misfire occured again. No rules, just happens everynow and then. Again, its not too violent, car doesnt shake much. It reports check engine code - random misifre detected, and occasional individual cylinder. My mechanic says that there is Chrysler tsb (technical service bulletin) issued which recommends procedure for cleaning the valves from carbon deposits and replacing valve springs. Since thats like $400 or so job, i put it off for now. Hope my experiences help someone.