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Question about 02 sensor

whitexj98

NAXJA Forum User
Location
Cali
A while back i was having major issues with my engine, running rough, hesitation, and horrible gas milage. So i did a full tune up + upstream and downstream o2 sensors. It didnt change anything, i drove like that for about 6 months and then found out what the problem was a blown upstrem 02 sensor fuse. That fixed all my problems but im still getting horribe gas milage.

So my question is, could all of that driving on the new 02 sensors damage them?

1998. 4.0 AW4, throttle body spacer, cone air intake
 
If you were running really rich, its possible, they could be covered with a thick layer of carbon. Did you happen to get them from a place with a warrenty?
 
I'm not sure where the O2 sensors are on a 98, but if you could pull them out and see if they're covered with LOTS of carbon (normal use, they'll still get covered) but if it seems excessive, then that could be your problem.

Dont try and clean the carbon off, it will mess the sensor up even worse :)
 
The only oxygen sensor that has an effect on engine performance and gas mileage (fuel/air ratio) is the upstream one. The downstream one is there only to measure the efficiency of the catalytic converter.

These types of problems can get tricky. Do you have any codes? A lot of times, oxygen sensor problems are not as straight forward as them may seem. Just because you have a code on the oxygen sensor circuit doesn't mean that the problem IS the oxygen sensor. It can be anything that creates a mixture/condition that the oxygen sensor can't compensate effectively for.

You can test an oxygen sensor but you need to have the right equipment and know what you are doing. Any good shop could do it for you if you want to go that route. I would consider doing that, especially with the history you provided.

Good luck and post back what you find! :wave1:
 
birchlakeXJ said:
The only oxygen sensor that has an effect on engine performance and gas mileage (fuel/air ratio) is the upstream one. The downstream one is there only to measure the efficiency of the catalytic converter.

These types of problems can get tricky. Do you have any codes? A lot of times, oxygen sensor problems are not as straight forward as them may seem. Just because you have a code on the oxygen sensor circuit doesn't mean that the problem IS the oxygen sensor. It can be anything that creates a mixture/condition that the oxygen sensor can't compensate effectively for.

You can test an oxygen sensor but you need to have the right equipment and know what you are doing. Any good shop could do it for you if you want to go that route. I would consider doing that, especially with the history you provided.

Good luck and post back what you find! :wave1:
Hey Birch-- partial hijack here!
Since you seem to have some experience here, since I seafoamed the Jeep I have the occasional CEL. I had AZ pull the codes and it said 02 sensor 2. It runs fine but every now and the n the CEL comes on. I'm thinking replace and reset but after reading your post??? Whatcha think?
 
McQue, very good clarification.......

If you have 2 oxygen sensors (which whitexj98's does) then my "original" understanding is the downstream one doesn't effect fuel/air ratio. But now that this thread has me thinking. I found conflicting information, one source stating that the 4.0 downstream sensor (again, on a 2 sensor engine) along with monitoring catalytic converter efficiency, does have a "trim effect" on fuel/air, not as critical as the upstream sensor but it is involved in the calculation. Can somebody please confirm whether this is true or false? On OBD1, I don't think the downstream affects fuel air in any manner but now I'm wondering did that change when OBD2 came around?? (96+)

asatxj, since you are seeing a code for that sensor, it may not be a bad idea to test it. Test em both while you are at it. Sure you could disconnect the battery for a few minutes or reset your computer with a scan tool to see, but the light will probably come back. How many miles do you have on your engine/oxygen sensors? Heated oxygen sensors have an "average" (and this is strictly an average) life expectancy of about 100k. Most guys can get considerably more than that out of them and they may not be throwing a code, but keep in mind that they do become sluggish over time and lose their optimizing capability.
 
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The 2000 FSM implies that the downstream sensor is used, but it doesn't make sense that it would. The amount of oxygen present at the downstream sensor would depend more on the efficiency of the cat than the actual a/f ratio at the engine. It may be more of a sanity check to see if the downstream says lean (O2 present) while the upstream says rich (little O2). Or maybe it really does have some minor trimming effect, so long as the front sensor stays within an acceptable range.

"Upstream Sensor (Non-California Emissions):
The upstream O2S sensor (1/1 sensor) is located in
the exhaust downpipe before the catalytic convertor.
It provides an input voltage to the PCM. The input
tells the PCM the oxygen content of the exhaust gas.
The PCM uses this information to fine tune fuel
delivery to maintain the correct oxygen content at
the downstream oxygen sensor. The PCM will change
the air/fuel ratio until the upstream sensor inputs a
voltage that the PCM has determined will make the
downstream sensor output (oxygen content) correct.
The upstream oxygen sensor also provides an input
to determine catalyst efficiency.
Downstream Sensor (Non-California Emissions):
The downstream heated oxygen sensor (1/2
sensor) is located near the outlet end of the catalytic
convertor. The downstream sensor is also used to
determine the correct air fuel ratio. As the oxygen
content changes at the downstream the PCM calculates
how much air fuel ratio change is required. The
PCM then looks at the upstream oxygen sensor voltage
and changes fuel delivery until the upstream
sensor voltage changes enough to correct the downstream
sensor voltage (oxygen content).
The downstream oxygen sensor also provides an
input to determine catalyst efficiency."

(The Calf version says essentially the same thing except there are 4 sensors total)
 
iv heard that the downstream dosn't have an effect on the triming also, but that sounds like it actually does. If thats true then that could be my slight idel problem because i bored the cat out when it got pluged up. It throws a code, but i didnt think that would effect the running of the engine, but maby it actually does. It only throws one code, i cant remember what one but its somthing like to high voltage on downstrem sensor.
 
I'm more inclined to think it does not affect anything. A number of people who have eliminated the cat have replaced the downstream sensor with a simulator, which just randomly cycles between lean/rich indications. I have not heard of problems doing this.
 
Birch--
I have 105k on my Jeep. My mileage is unaffected but the CEL comes on about every week and stays on for 4-5 start ups and then it goes out. Since the code specifically is #2 apparently it has some function. Incidentally it's a 99
 
asatxj....oh the downstream has function, we were just kind of kicking around if it affects fuel air ratio. Lawsoncl, I tend to agree with you that it doesn't have an effect except for the confusion of the 2000 FSM stating that it does. Could it be that it has no effect on all years EXCEPT the 2000 and 2001 models? Maybe Jeep made the change in year 2000 in the ever increasing push to reduce emissions. Not a big deal either way but just would be kind of nice to know.

I would say that either your #2 sensor is heading south or your catalytic converter efficiency is on the edge which is what the downstream sensor is programmed to monitor. But with 105k, my money would be on the sensor itself being the cause. Still the same medicine if you want to get rid of the light. Have sensor tested. You could just replace it if you want but I kind of like to test stuff before chucking it. If it tests good, suspect your cat......not too much else it can be that I can think of except of course if you have a wiring issue. Never hurts to check all exposed wiring for intermittent problems.....
 
birchlakeXJ said:
Have sensor tested. You could just replace it if you want but I kind of like to test stuff before chucking it.


I've never heard of testing an O2 sensor, other than maybe checking the heater circuit. How do you test that its working properly? I guess if you've got the dual sensors on the Calif model you could compare the signals from the two.
 
lawsoncl: Two methods of testing an oxygen sensor are using an oscilloscope or a multimeter. An oscilloscope is the best method for testing. This will give you the exact output of the sensor along with its response times. A multimeter can also be used but this will only give an indication if there is an output or no output. The sensor will be switching too quickly for any response times to be actually measured. I don't have my FSM handy but my Chilton's has a procedure in it for the meter technique......
 
Yeah an o-scope lets you verify that it's switcing back-n-forth and actually doing something. I've used my scope a few times on the vehicle checking abs, speed and cps sensors. I did some searching and came across this procedure http://www.mr2.com/TEXT/O2_Sensor.html for testing an O2 sensor. The basic assumption seems to be that if the sensor is putting out voltage and responding quickly then it's probably good. That's probably true for the later O2 sensors, but some of the earlier sensors, like the Renix are HEGOs and have different voltages.
 
lawsoncl said:
Yeah an o-scope lets you verify that it's switcing back-n-forth and actually doing something. I've used my scope a few times on the vehicle checking abs, speed and cps sensors. I did some searching and came across this procedure http://www.mr2.com/TEXT/O2_Sensor.html for testing an O2 sensor. The basic assumption seems to be that if the sensor is putting out voltage and responding quickly then it's probably good. That's probably true for the later O2 sensors, but some of the earlier sensors, like the Renix are HEGOs and have different voltages.
I have posted this at least 1/2 dozen times the last 6 months here. Renix is 0-5 volts, 87-90,

Everything after 1990 (jeep) is 0-1 volt. First test is for 12 or more volts at the third wire to power the heater in the O2 sensor. If there is no 12-14 volts on that third wire, then the O2 sensor can not work properly. Next is to use a high impedance analog volt meter. Seen them for $10 bucks at some places lately. Look for a swing in voltage from 0-1 volts about once per second at idle on 91 and newer models, look for apx a 1-4 volt swing in one second on the 87-90 sensor. As the idle is increased the swing should tighten up and hold closer to 2.45 volts on 87-90 models, and 0.45 volts on the 91 and newer models. It should also swing back and forth much faster as the RPMs increase. If it swings back and forth at a regular cycle of less than 1 second than everything is in closed loop and the O2 sensor is working. If not, it could be a bad O2 sensor, or could be bad wiring, or other bad sensors keeping it in open loop. Any defective computer sensor keeping it in open loop would throw the system into a rich mode and the O2 sensor if it is working would show a high voltage bias of 1.0 volts (91-200x) or 5 volts (87-90) while running rich, IIRC. A constant 0 volts on a 91 and newer O2 sensor is probably a defective O2 sensor since it is a voltage producing device, although it could be a damaged output wire, grounded to the body. The 87-90 is a variable resistor and could be tested for open circuit (infinite resistance) with all power off and engine off, since it would read 5 volts under power even if the O2 sensor was burned out, open circuit, as the ECU supplies the 5 volt reference signal for the 87-90 O2 sensors.
 
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Update for me. Replaced the downstream o2 on my 99. Now a few weeks later it stumbles on take off and gas mpg is down about 1.5. I'm thinking I have a plugged Cat. Any suggestions on a replacement? Don't want to spend $100s on it if I don't have to as I need a muffler and tailpipe too.
 
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