[Dhorn33]

I posted on JU about this but basically my TJ will start to cut out, sputter and sometimes stall while driving it around. I thought it was only while driving down the highway - but today over lunch it did it just taking off on a side street. There is no CEL and generally if I shut it down and restart it runs fine. It idles fine - unless I try to idle it while the condition is occurring. The Jeep does not do this all the time - it is erratic and irregular occurences. One of the more common suggestions is o2 sensor so I just unplugged that to see what happens. With the o2 sensor unplugged if I don't get the problem to occur can I safely conclude it is a bad upstream o2 sensor?

Another suggestion was TPS - but wouldn't the Jeep idle erratically (rev to higher RPM or nearly stall to lower RPM)? That has been replaced once before a few years ago.

Also - my cat is gutted and has been for 6 or 7 years so I know it isn't clogged.

I have not tested fuel pressure but I guess if this o2 test fails then that will be next on my list?

Any thought or suggestions?

[Jerry Bransford]

A bad 02 sensor will cause it to run with less power and worse fuel economy as that normally triggers a full-rich condition. Sputtering too doesn't make me think of a bad 02 sensor. When you say 'sputtering', is the engine missing (not firing on all cylinders) or is it just idling roughly with eratic acceleration? What year is your TJ? Your Jeep Forum Profile is still blank so please complete it so we'll not have to ask you how old your Jeep is or how it's configured.

[Dhorn33]

The Jeep is a 97 Sahara with 4.0L, 5 speed and 91xxx miles. It has 4.10 gears (8.8 rear, Dana 30 front) and currently rides on 31's that I just put on to get better fuel economy than I was doing with the 35" BFG MT's I had.

What happens is I will be driving down the ride with everything peachy - and all of the sudden the Jeep will lose power and the more I press the gas pedal the more it bogs out - like it isn't doing anything at all. There is no CEL and all the gauges still function perfectly. If I press the clutch and rev it it seems to rev okay - then I try to go again and sometime it will be fine and sometimes it will bog out again. A few times I have had to pull over, shut down and restart and it normally runs fine at that point. There are no codes stored and no CEL. Until today this has always happened driving down the freeway - but today it happened on a sideroad. It doesn't happen everyday and it doesn't happen at a certain temp or timeframe from what I have noticed. (one day it happened within 5 miles of home - then one day it was after driving 20+ miles?)

[Jonny Jeep]

When my O2 sensor went bad it sputtered and backfired, but fuel economy didn't seem to alter. I think it's because the actual sensor circuit hadn't failed but rather the heater circuit.

If it's your heater circuit that has failed it will cause the sputtering etc just as the engine is getting up to temperature. The heater circuit should warm up the O2 sensor to between 300-350ºC so that the engine can switch from using pre-programmed data.

When the computer switches over to using the sensor data the sensor isn't actually warm enough to provide accurate information due to the failed heater circuit. The engine runs rough till the exhaust gases have heated the sensor above 300ºC - hot enough to provide accurate information. With the computer sensing the O2 sensor is unplugged it just runs on pre-progammed data which is why it runs fine.

Running with the sensor unplugged should cause a code to be stored. In fact in the FSM there is reference to an oxygen sensor heater diagnostic trouble code. Didn't get any code at all on mine, but from reading about the symptoms and cures on this forum I swapped out the sensor and all was good again.

Edit: having read your second post regarding when it occurs I'm puzzled slightly. For me it always happened at the same point down the road from home. As you don't get symptoms with the sensor unplugged I still think it's the best place to start. 91k miles is close to the end of life for O2 sensors anyway - 100k miles is manufacturers recommended replacement. Mine went at 61k miles.

[Jerry Bransford]

The lack of a CEL or diagnostic codes is surprising. As suggested, maybe it is the heater circuit for the 02 sensor. Any blown fuses anywhere? How's the wiring going to the 02 sensors?

[Dhorn33]

No - I haven't checked fuel pressure partly becuase it would be extremely difficult to do that because the condition happens spuratically and if I shut off the Jeep it usually fixes the problem.

With the o2 sensor unhooked the CEL is now on.

[Dhorn33]

I updated my profile too now...

[Dhorn33]

One more thing - the Jeep has been driven very little in the past 5 or 6 years. In fact - I have put on less than 8,000 miles in that period of time. The o2 sensor was replaced at some point - maybe 10 or 15k miles ago? I remember replacing the TPS at some point too but I can't remember when that was?

[Jonny Jeep]

I'd suggest driving for a few days with the sensor unplugged to see if the problem manifests. If it doesn't then it's almost certainly the sensor or wiring going to it.

[Dhorn33]

I'd suggest driving for a few days with the sensor unplugged to see if the problem manifests. If it doesn't then it's almost certainly the sensor or wiring going to it.

That's what I was thinking too. Good times.

[Dhorn33]

Well so far with the 25 mile trip home and then another 25 miles back to work today it has not acted up. Another day or 2 and I may be buying an o2 sensor I guess.

[Jonny Jeep]

Make sure you post back and let us know what you do, and if it fixes the problem.

[Dhorn33]

I just drove another 50 miles over lunch (I was picking up some Jeep parts!) and it continues to run perfectly since uplugging the o2 sensor.

[thejafe]

Is your throttle body nice and clean?

[ninemmbill]

Any more update? I have been having similar problems and have also just disconnected my O2 sensor to see if that isolates the problem.