1995 XJ 4.0 - Misfire at idle

[Sobek]

Hey guys,

Wondering if you've got any advice as to how I should go about diagnosing this one. The old Jeep's been driving beautifully for a long time now, no worries. But about a week or so ago it suddenly developed a slight idle misfire. On the morning of the day this started, I was cruising down the motorway off-ramp and smelled this fairly strong burning / electrical smell which lasted for about 20 seconds... I got worried, but everything stayed in check, no warning lights, no temperature problems, no power loss, nothing. I figured it must have been a mix of smells from all the cars in front and thought no more of it.

So now it's got this misfire while it's idling, but if you give it even the tiniest hint of throttle it's absolutely smooth - will hold whatever RPM you go for perfectly fine. No check engine lights, but I haven't yet tried to read the codes to see if anything is stored anyway (will have to read up again on how to check those error codes!).

I pulled all the ignition leads one at a time, and 5 of them were perfectly fine, however the 6th had a thick layer of white chalky powder built up on the nipple of the rotor cap (rotor cap was replaced not too far back now, it's in good shape). I checked the lead on the spark plug end and it was clean, gave the rotor nipple a clean with a wire brush along with the ignition lead and it wasn't too bad, a hint of surface rust in the prongs inside the lead. Put it all back together but it's still missing.

Anyway, I'm not too familiar with diagnostic and maintenance procedure on these XJ's as I mostly work on BMW's and the Jeep's always been off to the mechanic for any and all troubles. Would like to sort this one out myself though... Can anyone give me any pointers for what to inspect or diagnose next? Really appreciate the help, cheers

[anthonygubbin]

My guess would be the cap. I have had them stuff up within a few months of replacement. The other thing you should try is cleaning the throttle body. Take it off and get some throttle body cleaner and a tooth brush and scrub the inside.

Regards A

[Sobek]

Thanks Anthony.

Had the TB off about 2 months ago, gave it a good clean and also removed the air box venturi at the same time to get it breathing a little better. Would a failed / failing distributor cap be capable of producing a burning electrical smell? Just curious what could cause that, if it was indeed coming from my car.

Sorry if these are 'duh' questions, I've never worked on a vehicle that's used a cap & lead system before!

[xjdavid]

Have you removed the idle circuit device on the side of the throttle body and cleaned it out. You will need some security torx bits.

[Escapable]

I go for the throttle body clean, remembering that if these leads are not wire (refer to FSM as I think they should be resistive anyway) then the best way to test them is with an Ohmmeter. Muck on the ends will indicate these are not firing correctly and if in doubt replace them (part of a good regular service protocol anyway). The cap has been replaced but while your in there add the rotor to the list as these are all relatively cheap parts to replace and chuck the old bits under the back seat or somewhere you keep those strange tools you may never need like the spare crank sensor!

[Sobek]

Hey guys, just a little update. I took the throttle body off earlier today and gave it a thorough cleaning. It was... dirty. Lots of gunk built up and baked on, and the idle control valve was almost certainly not functioning as when I finally managed to pull that out, it was basically just a big hunk of blocked black gunk... Took me an hour but I got everything cleaned up and moving smoothly. Put it all back together but it made no difference in the end. Still, good little maintenance item done.

I also pulled all the spark plugs to check the end condition and wear, but they all look fine. The electrodes were mostly white, but they seemed otherwise good. Unfortunately, the ignition lead on the cylinder closest to the radiator basically tore to bits as I tried to remove it. I may have overdone it as it's a ridiculously tight spot, but regardless I now need to replace it so I guess I'll just get a whole new set of leads tomorrow anyway.

Distributor rotor looks fairly tidy, and the points in the cap have only the slightest hint of wear, nothing to write home about. I'll start with replacing the leads then see how that goes and move on from there. Also, I noticed that no matter how much it stumbles or shudders with this misfire, it's always holding perfect RPMS - they never dip up and down, not sure if that means anything. I also checked the codes and the car is reporting nothing but 55 (no codes). Guess whatever's wrong isn't bad enough to trigger any codes yet.

Will keep you updated

[rainman]

When it is idling and you turn on the ac does the idel speed change?