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[Roushman]

Hi I have a 1998 XJ Auto & She has a Rough idle/running problem that lasts for a few seconds if driven through.
The problem is Just after starting off (about 300 yards) she pulls back on the revs and then is OK, It is worse if you stop just before this happens i.e. at a junction. after the rough running you can drive though it and she runs perfectly. If I stop and switch off i.e. for a paper then start up again she runs OK if I stop for more than 5 mins she will again go through that rough patch then again runs OK.
Sometimes just as the rough running is about to end there is a clicking noise that appears to come just below and in front of my feet.
If it starts to run rough and you have not gone far and stuck in traffic I put her in neutral and give a good blip on the accelerator & that cures it. 
She has covered 85k and has been regularly serviced. The only thing out of the ordinary was a new Genuine Jeep rear exhaust section was fitted by ATS. Who had it for over a week and tried 5 different exhausts before managing to fit this one. 
Any Ideas on what the problem is would be gratefully received, its driving me nuts as she is otherwise very reliable & great fun to own.
many Thanks, Dave.

[Brynjaminjones]

May not be the cause, but just for peace of mind I'd take your distributor cap off and make sure it's not damp in there, plus maybe give it a squirt of WD40.


Other than that I'm guessing sensors - could something have been knocked by the chaps at ATS?

[Roushman]

Many Thanks,
 I have just fitted a new distributor cap, and that is all OK. I am thinking along the sensor line after finding a previous post.
She was running perfectly prior to the new rear section being fitted so may be they have inadvertently knock something, if the rain ever stops Ill crawl under and give it a good look.

[Brynjaminjones]

Excellent, you'll find a sensor sticking out of the cat (just in front of the back box) and also one in the down pipe near where it connects to the manifold. There may be more, but someone more knowledgeable will be able to help with that!

[JamesH]

Rear O2 sensor is the only thing they should have been near. Other sensors that could give rough running are TPS, CPS, front O2, MAP if you saw my TPS thread you'll have seen I tracked sensors and rough running for ages and it ended up being the fuel pump. Symptoms were different to yours though.

[gordy]

Sounds like upstream O2 sensor.  Checkout Bullshitkorners vid on this

and my thread on similar issue:

http://www.birtydastards.com/frm/index.php?topic=23022.msg283684#msg283684

[Jonny Jeep]

Symptoms are typical of a failed heater element in the upstream O2 sensor. When it fails the sensor doesn't get up to operating temp by the time the computer starts using it to gauge fuel mix. Engine runs rough till the exhaust gases heat the sensor to within it's operating range and the engine runs fine after that till the next cold start. I believe the normal resistance for the heater element (which the two identical wires to the sensor) is 5-7 ohms.