I thought I had this fixed on Friday...

To recap - last week the idle on my 98 XJ got really rough and the check engine light came on. I plugged in my trusty OBD-II scanner and it tells me that cylider 3 is misfiring (I have the I6 engine, 93k miles).

I replaced the distributor rotor & cap, wires, and spark plugs. After I got the wires connected right (on the 2nd try) the engine started up and seemed ok (no more check engine light, smooth idle).

However, when I showed a friend of mine my nifty OBD-II scanner this week, I realized the engine computer is still reporting Cyl 3 misfires (code P0303). When I listend to the idle this morning on the way to work I could hear what sounded like an occasional cylinder miss, but there isn't a jaring vibration like last week. I can't tell for sure if the cylinder is missing at driving speeds, but I'm trying to pay attention to that now...

I exhausted the "simple" things I could think of doing last week. The only other recent work I've done on the engine was an oil & filter change (I used Mobile-1 5W/30)...

On the upside, this doesn't appear to impact the drivability/performance of the vehicle, but I don't want to dismiss this and get bitten later...

Any ideas what I can try next? Or is this one of those things Jeeps do (I haven't had it for very long - only 3 weeks)?

Thanks,

JW

What does the #3 plug look like? Is your #3 plug wire laying across the manifold or something that may be allowing the spark to jump?

I just replaced all the spark plugs, plug wires, distributor cap & rotor on Friday. When I installed the new plugs I checked the gaps before putting them in and made sure they were in spec per the Haynes service manual.

I arranged the plug wires as neatly as I could. There isn't anything obvious that could be shorting out the plug/wire that I can see. I was pretty careful to be sure the wires were plugged in at both ends and making contact when I installed them.

I suppose I can pull the #3 plug again and double check that it is getting a spark fairly easily...

JW

what brand gas are you suing and is your fuel pressure proper?
bad gas or low pressure will cause a lean miss on ocasion. try switching brands of gas or putting some high octane in. if it still is missing you might have to rent a pressure gauge and check your ful pressure. it could also be a clogged injector. can you see long term fuel trim with your OBD-2 scanner?? if so is it driving the car rich? sounds like your ignition system is in fairly good hape. im now thinking it might be fuel related as well.

I usually get Shell or Mobil gas... Last time I filled up it was Shell and I put in 89. I've been using the same service station for a while and haven't had any problems - they do a lot of business so I know they go through a lot of gas.

With the scan tool I can see the long/short term Fuel Trim Status (among other things)...

The data I have handy is from Friday, after I replaced the parts. Both long & short term fuel trim for Bank 1 report 0.0%. (I wasn't sure what that meant). The engine was cold (55.4deg), ~1400 RPM idle. Calculated load was 15.7%, intake manifold pressure was 8.7 psi. Fuel System 2 status is Open Loop (says it hasn't met conditions to go closed loop).

I had thought it might be a clogged injector... I was thinking of dumping a can of injector cleaner in since that is cheap & easy.

I'll read the Haynes manual and see what it says about checking the fuel pressure again... If I remember correctly, one of the fuel systems is under 50-60lbs of pressure at all times, but I can't remember which.

Do you know if there is a way to isolate the injector? Sort of see how it performs vs the injectors on other cylinders?

JW

At the tail pipe do you smell raw gas?

I had a similar problem with my chevy turned out it was the rocker arm for the exhaust vavle.

If everthing else fails pull the vavle cover off and check them.

let it warm up proper (hit closed loop) and check it agian. if you have a dead or clogged injector your car will run lean. so in order for the O2 sensor to read the right thing you have to drive the engine a bit rich. so your long term and short term should read higher, but since you weren't in closed loop those readings could be inacurate. same situation for a lean-mis.

if your OBD-2 tool has bi-directional controls you should be able to cancel the operation of one injector out of the lot. if not, you can manually remove the plug from each injector( it wont hurt it, but don't short it.. that will)

i know my 91 runs 38PSI regulated. and 42PSI topped.

The exhaust doesn't of smell gas as far as I can tell...

I forgot the OBD-II interface at work, so I'll have to wait until tomorrow to collect the data when the engine is warm. I know I don't have the ability to disable the injectors with this particular tool.

Will keep you posted.

JW

I see what you're getting at...Unfortunatly, I can't get a closed loop reading... I'm consistently getting the Cyl 3 misfire though (w/ open loop reported).

Unfortunatly, what I have isn't a tool like a mechanic would use. It plugs into the car's computer port and it collects data. Then I can unplug it and connect it to my PC to review the data.

I've tried waiting for the engine to be warm before connecting it, but it doesn't seem to work they way I would want (e.g. get enought data).

If the O2 sensor went out, would the engine run?

JW

yes the engine would run. but it would not run closed loop. maybe thats the probelem. how hot is hot? it should go into closed loop within 5 or 6 min. max. MY OBD1(SBEC-2) goes into closed loop from a compltely cold state in about 4 min. if i had ran it that day or it was warm out (70+) it will go into closed loop in 1 min or less. i know that a 2000 pontiac grand am will go into closed loop from DEAD cold in about 6 seconds. so if you run your OBD-2 car for more than 5 min. and it still isn't in closed loop there is probably a problem there. basically the engine needs only a couple of sensors to run. Crank shaft position snesor, Cam shaft psoition sensor, and the MAP or MAF. with that much data the engine will run open loop and be barly drivable.