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[cornwalljeep]

Hi,

My xj 4.0 auto orvis, has had a misfire for over a year now.
It starts fine, but misfires at the same place (500 yards down the road). It loses power and I have to push the accelerator right down and it powers through and is then fine for the rest of the journey.
It has also stalled a few times when driving in traffic or slowing down for a junction.

However, it's never broken down so I've not had it looked at.

Tonight I noticed that petrol (I think it's petrol) was spitting out of the exhaust when idling in the drive. Then the engine started to splutter and spit even more petrol out of the exhaust.

Any ideas what might be causing this?

[YT]

Going basic here, have you checked the plug leads. I had a misfire on mine, started just every now and again, then got to the stage it was every outting at certain revs, but would drive thru it if ya gave it some throttle. It was no 1 plug lead. Could be worth checkin if any of the leads feel loose. Basic, but a cheap fix and simple to check.

[cornwalljeep]

Maybe it's water coming out?I'll check tomorrow

[Panic mechanic!]

cats produce water and you get a fair bit come out when the engine is cold.

[robsrotti]

your misfire will be one of the lambda sensors, i had the same thing on mine.

[Jonny Jeep]

It's a bad heater circuit in the upstream O2 sensor I believe. The O2 sensor needs to be above 300°C for it to be accurate. When the heater is bad the computer starts looking at the O2 sensor for fuel ratio data before it is up to temp. The engine runs rough till the exhaust gases heat the sensor up to the accurate temp range.

An free diagnosis is to disconnect the sensor and see how it runs. If it's OK disconnected. replace the sensor. The two wires the same colour to the sensor are the heater wires. If you test across them with an ohm-meter it should be 5-7 ohms +- .5 ohms. Don't test the other wires as the current that passes through could damage the actual sensor.

[cornwalljeep]

Thanks, that sounds like the most plausible explanation I've heard for the past 12 months.
I'm not technical at all, so I'll have to get a mechanic to do the test.

[robsrotti]

A Lambda sensor it is then, I think mine was the one after the Cat

[ChrisS]

Not on a Jeep, but had similar issues on a car a while back - symptoms not severe but could make it mis-fire and trigger the trouble light at the same spot on a journey every time.

It was a blown fuse for the lambda heater, rather than a lambda fault, but end result was the same, until it got hot, the lambda wasn't reading right. Took me a very long time to find that one...the fuse was cracked, rather than blown, so when I tested it with a meter, I got continuity that came and went, which I put down to a poor probe connection and put the damn fuse back in.....

[Jonny Jeep]

It was a blown fuse for the lambda heater, rather than a lambda fault, but end result was the same, until it got hot, the lambda wasn't reading right.

After seeing that I checked the 00 XJ FSM and Fuse 21 (15A) in the Power Distribution Center supplies the O2 sensor heaters. Check that and if it's OK check the resistance of the O2 heaters.