No check engine light but assume o2 sensor is bad?? help

98 cherokee 4.0 aw4 180,xxx miles well taken care of

jeep is surging at stop lights, get's a slight miss more often when i'm going a constant speed like 70mph on the interstate. one of the o2 sensors is new within 2 years the other is probably factory. the cat is gutted and has been for 4-5 years. i'm really thinking it's the other o2 sensor having intermitten problems. been doing this for several month's. if i run a bottle of fuel treatment (cheap stuff) through it the jeep ran like new. now it doesn't really help. Gas mileage seems to be the same as it always has been about 16 mpg on 31" tires.
i hate to buy a new sensor and that not be the problem. My check engine light has not come on and hasn't been on in the past 2 years.

Any one have any suggestions on what i should do? anyone every had a o2 sensor go bad and it not turn the chech engine light on??

Thanks

We frown on gutting CATs here but beyond that, I would say run a can of BG44K throught he gas. The cheap cleaners help for the short term but can actually cause their own problems.

O2 sensors tend not to set a code if they die a natural death, as the failure is slow. The sensor that controls the mixture is the first one in the series, and the second one, just past (or in) the cat merely tells the ECU that the cat is working. Your gutted cat should have set off the CEL. When's the last time the throttle body was cleaned? As in taken apart, and the idle air chamber and plunger cleaned and de-gunked. X2 on the BG44K. I ran a can of that through about two months ago, and it runs smoother, and got rid of a barely audible mis-fire.

Good advice here--when was the last tune up? Cap (not the cheap aluminum contacts), rotor, plugs? Brand of plugs?

As said, o2 sensors die a slow death and you could (most likely) NEVER get a CEL or code for it. The o2 sensor is a maintenance item, off the top of my head usually 80 thousand miles between replacement--about 0.000625 cents per mile. While you can keep driving with one past its replacement schedule, it will cost you more in wasted fuel if you do.

yeah thanks for the advice
plugs, wires, rotor button, cap, distributor are all good and recent. it's not really that kind of problem i'd think if it were one of those i'd have the problem more often. the idle surge and miss is sporatic and is really very hard for me to ever know when it's gonna happen. it might happen all week long and then run excellent for a week or two. i'm gonna craw underneath it and see which o2 sensor i changed (about 2 years ago) if it's the one after the cat then i'll for sure buy the before the cat sensor and change it tonight. I'm also gonna buy some higher dollar fuel treatment and give that a try too. if that don't work i'll move onto checking/cleaning the throttle body.
I had this same problem last fall and then it went away all winter until about 2 month's ago. who knows? I'd still drive it across country and back before i'd drive the wife's new camry!
Thanks for the input.

If the parts stores around your area don't carry BG44K usually the dealerships will have it available in their parts departments.

FYI--I have seen those aluminum contact caps burn out their contacts very quick, particularly in vehicles with a little wear in the distributor shaft. You should check the cap's physical condition and don't a**ume it is good based on when it was replaced.

Some good advice has been given here.....

Oxygen sensors get "lazy" over time and yes, you definitely can have a slow oxygen sensor and not have it be bad enough to set a check engine light. If the pre-cat sensor is old, swapping it out is just a good idea as a matter of course.

Gotta tell you that a common cause of "idle surge" is definitely the idle air control. They get carboned up over time and miles. I would invest an hour of your time and the $5.00 for a can of throttle body cleaner and try cleaning it and also the throttle body.

The Idle Air Control (IAC) is mounted on the back of the throttle body. The valve controls the idle speed of the engine by controlling the amount of air flowing through the air control passage. It consists of a stepper motor that moves a pintle shaped plunger in and out of the air control passage. When the valve plunger is moved in, the air control passage flows more air which raises the idle speed. When the valve plunger is moved out, the air control passage flows less air which lowers the idle speed. Over time and miles, the IAC can get carboned up which can have an adverse affect on idle quality. Cleaning the IAC may restore proper function and is an easy procedure to perform and good preventive maintenance so it is never a bad idea.

CLEANING THE JEEP 4.0 IDLE AIR CONTROL

Remove the air filter cover, associated hoses and the rubber boot that goes from the air filter cover to the throttle body. Remove the IAC with a torx driver (2 bolts; one can be kind of hard to get to)

"Gently" wiggle out the IAC from the throttle body. Gasket on the IAC can be re-used if it is not damaged

Clean the IAC with a spray can of throttle body cleaner; inexpensive and available at any place that sells auto parts. Throttle body cleaner is recommended rather than carburetor cleaner as it is less harsh, safe for throttle body coatings and is best for this task. Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle that protrudes from the IAC as it is fragile and you could damage it.

Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner

It is also a good idea to clean the entire throttle body itself, the butterfly valve inside of the throttle body and all associated linkage as long as you have things disassembled

Reinstall IAC and check idle quality
　

Great write-up! The only thing to add, if I may, is that I had a situation similar to, but nowhere near as bad, as the OP. The Throttle Position Sensor-TPS- started to fail, giving a similar symptom. But, mine threw a code. The vehicle continued to run good till I got home, and read the code. I had the time to take everything apart, clean it, replace the offending part, and all was well. The original TPS had a definite dead spot at right about where it would be at highway speed. And, it didn't hurt that the rest of the TB got a good cleaning.

Just recently, about 2 weeks ago, my 98 did something very similar. It would almost die sometimes when I would stop and sometimes on take off would sputter but very sporatic. Would sometimes go 2-3 weeks with no "episodes". Just cleaned the throttle body out real good and not a single problem yet. The body was pretty gummed.

If you gutted the cat, and it never threw a code for that, perhaps your CEL bulb is gone/burnt out? Does it light up when you turn the key on and all the lights light up? Never know could be throwing a CEL now and not know it if the bulb isn't lighting.

Well i replaced the up stream o2 sensor. didn't fix the problem but the old one had 50 to 65k miles on it so at least now it has a new one. i've got a 100 mile drive today after work and the tank is full with some higher dollar gas treatment in it. i'll see how it does, if it's still acting up by tomorrow i'll do a good cleaning of the tps/throttle body. I've cleaned the throttle body before but it's been probably 30k miles ago and it looked pretty darn clean then. We'll see this time.
Another thing to add:
When it starts acting up i can put the tranny in neutral and rev the motor a couple of times up pretty high and it'll run good for a bit (maybe 2 miles or even 20 miles).
Has me thinking its in the throttle body more and more.

My Check Engine Light works when i turn the key on so it's not burnt out.

Thanks

98XJSport said:

If you gutted the cat, and it never threw a code for that, perhaps your CEL bulb is gone/burnt out? Does it light up when you turn the key on and all the lights light up? Never know could be throwing a CEL now and not know it if the bulb isn't lighting.

Click to expand...

My 97's the same way. I know the cat isn't catalyzing for a fact. I can look at the live data stream and see the rear o2 jumping up and down along with the front, but the pcm plays dumb and keeps the check engine light turned off.