O2 sensor question

I have a newbie question, I guess. My 93 (4.0, AX15, 231, 6.5" and 4.10s and 33's) has 140k miles on the original O2 sensor, and I get super sucky gas milage lately, like worse than I should be. I know that I should change it just because of the age of the unit, but my question is what will this do? I pass the local emission tests every year with flying colors. Can I realistically expect to notice a difference if I change the sensor? My plugs are fine, wires are fine, CPS is new, other sensors are original, but the old girl just feels tired and I get about 10-12 mpg around town. Any advice?

Thanks,
KY Chris

Forgot to add that I think my cat is toast. It rattles like a coffee can with rocks in it at idle. Should I replace this as well and get one with a downstream O2 sensor, as well? My current 93 just has one O2 sensor, as far as I know. I run Rusty's catback system.

Thanks

IMHO, The Cat would be the first thing I'd replace - the substrate in the converter passes exhaust in only one axis, so if it's rattling, the flow will likely be restricted...
... Additionally, while you're working the cat replacement - you should make efforts to inspect the muffler - new or not it's a favorite collection point for little bits of fractured cat-guts (also why many off-road event require mufflers - to catch that hot debris and to act as spark/flame arrestors)

You may wanna retrieve the codes fom your computer -- replacing the o2 sensor is not an unsensible thing, but it may not yeild much of an increase in MPG - it's not uncommon to see 1 or 2 MPG from an o2 sensor update when the computer hasn't thrown errors on a bad sensor though, and I have heard of folks gettin more than that (hearsay, I've not experienced more than 3 without a code in an early HO).

Your Computer is looking for (expects) only one o2 sensor before the catalytic converter... adding a second (or changing locations) would be of no benefit - the 2nd one would have no connection point for the computer and is used to monitor the efficiency/performance of the converter, which has entirely different data values than what your ECU would expect to see from your before the cat sensor.

There are also some stupid simple things that you can do or check. like hows your air cleaner looking? What about the quality of fuel that you are running, Tire pressure? Dumb things like those can make a difference and help with squeezing some extra MPG's out of your Jeep.

The O2 sensor is supposed to be changed every 75,000 to 80,000 miles. The fact that yours has not been changed may explain why your catalytic converter has gone south -- my original cat lasted 200,000 miles and was still okay, but I had to replace everything else so I decided to change out the cat while I was under there.

Poor gas mileage may also very well be due to the O2 sensor. That's why it kills cats -- when they fail, they allow the engine to run rich. This hurts gas mileage, and it also floods the cat with more unburned hydrocarbons than it's designed to handle, and it burns up.

ALWAYS change the O2 sensor if replacing a catalytic converter. Otherwise, you're just asking for the new cat to fail prematurely.

What brand of O2 sensor to get? What is recommended? what have you guys had experience with?
TIA

In a '93, I'd either run a Bosch or the factory replacement (dealer part).

The Bosch units provide good quality signal through some situations that may foul other vendors....

sorry chris, my business is rope.

Take your CAT off and poke all the stuff out of it put it back on.
Had mine hollowed out for 5 yrs still passess. If you put it back on they'll never no its hollow. same gas milage but sounds better.

I bought the Bosch O2 sensor, but I guess I'll hold onto it till I have time to throw a new cat under there. Prob is that I wanna reuse my cat back set up, so I need a cat the same size as the stock one, or a little longer. Are any of the high flow cats that size? I can't just gut mine. I'd get thrown out of the Society of Toxicologists if I did that. Maybe I could just fill it with some rope, Beez?
KY Chris

BIGXJ said:

Take your CAT off and poke all the stuff out of it put it back on.
Had mine hollowed out for 5 yrs still passess. If you put it back on they'll never no its hollow. same gas milage but sounds better.

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The above advice is BIGXJ's personal opinion only. Doing what he suggests is illegal everywhere in the United States and is not condoned by . If you choose to do this, you are doing it at your own risk.

KY Chris said:

I bought the Bosch O2 sensor, but I guess I'll hold onto it till I have time to throw a new cat under there. Prob is that I wanna reuse my cat back set up, so I need a cat the same size as the stock one, or a little longer. Are any of the high flow cats that size? I can't just gut mine. I'd get thrown out of the Society of Toxicologists if I did that. Maybe I could just fill it with some rope, Beez?
KY Chris

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I bought a "direct fit" Catco from Summit Racing that was ... well, a direct fit. The cat itself is smaller than than the '88 OEM unit, but it has longer pipes so it matched up perfectly with a stock exhaust.

yeah - even a universal cat of the same diameter can be fitted with some extension pipes for the same effect and stil fit-up to the after-cat hardware very nicely...

dont get me wrong I dont want anybody to break the law for what i posted. But here in oregon if you live out of the city limits
you do not have to go threw those checks. therefor you need no cat. Besides the cherokee is WAY over resticted with that crap...

Just because nobody checks on you dosen't mean it's leagal.

I bought a universal fit Catco converter and the O2 sensor is AFTER the cat where as the stock O2 sensor is BEFORE. Bosch offers a different sensor for either situation for the 4.0. If I put on the new cat could I just replace the sensor with a Bosch "after-cat" unit? And, does anybody know of the best place (readrice) to get a Bosch O2 sensor?

Thanks,
Ashley

98xjlimited said:

I bought a universal fit Catco converter and the O2 sensor is AFTER the cat where as the stock O2 sensor is BEFORE. Bosch offers a different sensor for either situation for the 4.0. If I put on the new cat could I just replace the sensor with a Bosch "after-cat" unit? And, does anybody know of the best place (readrice) to get a Bosch O2 sensor?

Thanks,
Ashley

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Your '98 needs both sensors. The sensor before the cat is the one that controls the fuel delivery to the engine -- the one after the cat basically is just to check that the cat is functional. You'll have to get a bung welded into the pipe ahead of the cat if you want to use that universal. They built it assuming the first sensor goes in the down pipe rather than the cat itself.

Eagle,

What is a bung and how/where does it get welded? What size do you use and does it effect the performance of the 2nd O2 sensor?

A "bung" is just a nut that gets welded or brazed onto a hole in the pipe, as a place to screw in the O2 sensor.

The point isn't whether or not the front O2 sensor affects the rear one. The point it that the PCU (Powertrain Control Module) uses the data from the front O2 sensor to monitor the fuel mixture and control the fuel injection for best running. The second O2 sensor serves only to compare between the front one and the rear one to verify that the catalytic converter is functioning. If you leave off the front one, I don't know if your Jeep won't run at all or it'll run but run lousy. You will definitely have a Check Engine light situation.

I will have to throw another vote for replacing the CAT THEN the O2 sensor. The CAT plugged out will mean the motor runs like dirt and there is no way around it. But if you replace it than maybe you will find out that the O2 sensor isn't as bad as you thought. Plus, if the O2 sensor is bad will just burn more fuel than normal but your motor will still breathe.