Help needed! Stranded XJ with horrible idle/miss
 

Hey guys,
I have a 2001 XJ with the 4.0 petrol and as I was driving felt a big miss. The car now struggles to get up to speed but will every now and then clean up if I give it a boot. It struggles to hold idle if in gear but as soon as the load is off it in park or neutral it will idle OK again.
Driving at a constant speed it hard too with it surging or missing really badly.

Any help is greatly appreciated as I am on holidays with the family and am a fair distance from home with the jeep as our only means of transportation!

Blocked fuel filter, man.

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It could also be a vacume leak. Have a quick check under the bonnet for a hose that has come off. Check the PCV valve and hose that runs from the top rear of the rocker cover to the intake manifold.
Symptoms of a vac leak can be an OK idle with no load because the IAC valve may compensate. Missing under light load and ok running when booted.

Thanks guys, I really appreciate your replys and will check both in the morning.

Check to see if it has thrown any error codes.

Thanks for the replies guys, have checked codes and is not throwing any at this stage.
I feel like I'm down to the usual culprits being the CPS, CAS, upstream O2 sensor or fuel filter. Being a 2001 it's going to be hard to get to the fuel filter where I am on the goldy.
I have tried to start it again this morning and it will only just start occasionally and idles horribly now if at all. Anyone know where I can get a CPS and CAS over the counter on the gold Coast??

Try resetting the computer. If it runs better, it is not your O2 sensor.
A coil pack could be cactus. I would have a workshop check your fuel pressure and do a live data scan before throwing parts at it.

Thanks rustynuts, I have the battery disconnected now and will put it back on soon to reset.

When I went to service the injectors, I discovered the fuel rail had a LOT of gunk in it.http://www.ausjeepoffroad.com/forum/...6e0c2ccea3.jpg

Damn that's a little concerning Werd!

I have hooked up the battery and it ran just as bad. I unplugged the O2 sensor with the same result however once I plugged it back in it seemed to run a lot better with just a flat spot when reved. I have driven it yet but I am thinking a new O2 sensor might do the trick.

Maybe my approach is antiquated but breaking down under load sounds a lot like a possible ignition/spark issue, when where the plugs/leads last changed?

When I was younger we used to use the problem to help localize the issue, specifically a lead with high resistance will arc out elsewhere and this is readily seen under low light conditions.

Alternatively poor fuel delivery will cause identical problems (as previously mentioned), how old is the pump in the fuel tank? have you run the tank dry recently (rubbish can get into the pump)?

does it backfire ? is the O2 sensor over 100000k old? you might be right
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If the O2 sensor is really bad, it would usually throw a code. When the computer is re-set the engine will run on its original settings and all should be good. If it starts to run bad again after a while, it may be the O2 sensor.

Only my personal experience, my 96 had similar problems. I disconnected the battery over night and unplugged the upstream OXY sensor and left it unplugged when I reconnected the battery the following day to ensure the pcm was not using sensor at all. Not sure if you have done this test in this particular order or if it makes a difference?

The xj ran rich, however the symptoms I was experiencing had gone. During this process I found my headers were also cracked which may have been contributing to the problem.

A new set of pacemaker extractors along with a new upstream OXY sensor later saw the xj running better than ever.

How's your battery? I've found these engines hate batteries on their last legs. Very long shot but worth knowing the battery is in top condition and is easy/cheap to rule out.

Good luck, let us know how you get on 😀

So I took it for a blast with the O2 sensor unplugged and it ran better that when it first started playing up but still missing and stuttering a bit.
I decided to do a bit more elimination of other faults and when I unplugged the coolant temp sensor it ran great. Started first go and had no miss under load. Still had a slight hesitation every now and then but nothing compared to what it was like before. I assume the base map when the coolant temp sensor is not detected is pretty rich so I think that could be the source of the hesitation.
Now just to replace that and get it home on Sunday!

Fuel filter.