P0301 and rough idle

Hey all, first off I have a 98 XJ 4.0/5 speed 4x4 with 254k miles and still love it. However, in my 4 years and 40k miles of owning it, it has always displayed an intermittent P0301 code and CEL (cyl 1 misfire), but always ran great with a slight miss at idle as well as a rough idle. About 2 months ago, after a worsening rough idle, the CPS went out, and I replaced it with a NAPA Echlin part. Ran great with no CEL and no rough idle or miss at idle for about a month, then one day the P0301 code came back and so did the rough idle and miss. The CEL is now constantly on with P0301 code, and the rough idle/miss is the same as before, if not slightly worse. I have checked compression, switched injectors, has new cap, rotor, wires, plugs, popped the valve cover off and verified valve operation, seafomed, listened and sprayed for vacuum leaks at intake manifold, etc. It has all checked out. Rocker arms are still shiny and all cylinders have good compression both cold and warm. Ran it with the serpentine belt removed for a minute, engine runs quiet as new. I can still roast the tires from takeoff, but it won't pass CA smog like this and is therefore a major problem. I'm at wits end. Any suggestions? Timing chain maybe?

You've certainly addressed the usual suspects. I think I'd take it into a shop and see if some more sophisticated diagnostic equipment can help you isolate the root cause.

Are you sure the cam sensor in the dizzy is sync'd? http://jeep.blackonyx.net/pdfs/jcss.pdf

Awesome thanks guys. gradon, that is the one thing I have yet to really troubleshoot. Through all of my research it said the toothpick trick only works on the 00-01 with the coil rail. I just took a look at the cam sensor in my dizzy, and its slightly different than the one in the write up, with no apparent holes to line up. Have you or anyone else done this procedure on a HO model without the coil rails? If you have, please fill me in! Thanks again!

There are holes to line up in your dizzy--cap off, rotor off, gently pry the cam sensor plastic disc up and you'll see a half-circle wall(pulse ring) with a hole at the beginning edge of the wall(there are 2-3 holes, iirc) and there is a hole in the base of the dizzy. Make sure you turn the engine with ratchet clockwise to 0*/TDC for #1 at the end of the compression stroke(feel for air pushing past your finger with the #1 sparkplug removed as you approach 0*; if no air pushed out, you have to spin another 360*). Remove dizzy and you'll see the oil-pump slot in the hole. Use a long screw driver to set that to 11/5 o'clock from passenger fender perspective. With the cam sensor's pulse ring locked into place with a toothpick/rod/etc, place it into the hole with the dizzy-base shaft pointed to 1/7 o'clock. As you lower the dizzy in place, it will spin and drop into the slot and the hold down ears should be lined up with the mounting boss so you can bolt it in. When you put the rotor on, it should be at the 5 o'clock position, which is just past the #1 post on the cap. It's a tedious task, but a misaligned/unsync'd CMP is the culprit for many misfire/bogging/backfire into intake/rough idle/cutting out problems after engine swaps/builds/dizzy removals/loose timing chains. P030_ and P1391 are the typical codes thrown.

Are you, by any chance, having to top up the coolant tank?

I had a 97 that threw the misfife codesfor the number two cylinder. Traced it down to a coolant leak due to a failed head gasket.

I found it by pulling the plug un the offending cylinder with the engine at full operating temperature. Ran it up to temp, shut it down and immediately puled the plug. Was able to see steam coming out of the plug hole.

It was not overheating, just throwing the misfire code and eating coolant.

If it is not consuming coolant, then I would take gradon's advice.

One of the Craigslist fixer-upper XJs I bought had a blown headgasket between two cylinders, but no coolant leak, and would be rough at idle-1500.

Awesome!! Thanks so much gradon! Ive been busy at work today but will try that tmrw. I saw the 3-4 or however many holes on the pulse ring under the plastic piece last night and didnt know which one to line up, thanks again for the awesome write up! o-gauge I was thinking that possibility as well. I changed the coolant about 3 months ago and it has yet to drop. A few days ago I pulled the #1 plug on the hot motor to check and it didn't steam. But imo it puffs a little more white smoke on cold startup than normal (4-5 mins yesterday and it was maybe 65 out), so if it not the cam sensor, small head gasket leak could definitely be the culprit

Update: just completed the cam sensor alignment using gradon's write up, disconnected and reconnected the battery, and the 254k old 4liter sounds and runs like new. Standing next to the engine you can barely tell its running. No more rough idle, and no CEL after idling for about 10 mins. Im amazed. Thank you so much!

I have a '98 xj classic and it was throwing a cylinder 1 misfire code daily and running rough. It literally sounded like it had a big cam in it. The jeep wouldn't start the other day so I cleaned the battery cables for the third time in a year. Now, it runs like a sewing machine. Im just assuming that there wasn't enough juice to fire all of the cylinders. Everything else ignition wise is all new. Just my 2 cents incase you've tried everything that is cheap. Is do this before I'd pull the valve cover.

OBD-II can sense a partial missfire and call it a miss fire, and when it counts enough of them as a % (200 out of X passes), based on a slow or late CPS signal it throws the CEL light and code and shuts off the fuel injector to that cylinder, which then makes a smooth running engine, a very rough idle!!!!!

Then resetting the PCM (battery disconnect), starts the miss fire count all over by resetting the PCM. I have read many complaints lately about aftermarket, non dealer CPS sensors not being good enough, or going bad quickly, so if the problem returns, try a dealer CPS, and clean the CPS connector on the harness side.