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What are the symptoms of a bad 02 Sensor?

xylonjay

NAXJA Forum User
Location
Minneapolis,MN
I cant find anything in the forums that explains what the symptoms of a bad 02 sensor are. My 94 XJ with 207,000 miles is hesitating randomly, most times it hesitates at freeway cruising speed, and sometimes if you are taking off from a stop light. Im stumped. Any help would really be awesome.
 
xylonjay said:
I cant find anything in the forums that explains what the symptoms of a bad 02 sensor are. My 94 XJ with 207,000 miles is hesitating randomly, most times it hesitates at freeway cruising speed, and sometimes if you are taking off from a stop light. Im stumped. Any help would really be awesome.
Just unplug the O2 sensor and drive it. If the hesitation stops it is, was the O2 sensor. Bad O2 sensors kill the gas mileage big time!!!! That is the biggest clue that they are toast.

Hesitation is frequently caused by a bad TPS sensor. Search "Throttle Position Sensor testing" here for more details. Also check into the EGR valve (I think 94's still had EGR valves). Search Exhaust Gas Recirculation or EGR* (Three letter search words like EGR do not work here).
 
A bad o2 sensor will not necessarilly cause a hesitation, but it will cause you to run rich, or lean depending on how it's reading. Earlier xj's only use one oxygen sensor to monitor emissions and adjust fuel trim. With access to a scanner with 'live data' capabilities, you can monitor the o2 sensor volts and make sure it's switching properly, but a lazy o2 sensor will definately cause poor fuel economy. The TPS, is located right at the throttle body, and I really don't recomment d/c the o2 sensor...just my .02
 
Thanks 89xjgrey, I'll take your .02 the reason being im is im still getting the same mileage right now, and someone else had mentioned the TPS, but I dont have much experience troubleshooting sensors, so I am gonna have the TPS checked out at my buddys shop. I should mention I have the 5-speed manual. Would that make any difference?
 
89xjgrey said:
A bad o2 sensor will not necessarilly cause a hesitation, but it will cause you to run rich, or lean depending on how it's reading. Earlier xj's only use one oxygen sensor to monitor emissions and adjust fuel trim. With access to a scanner with 'live data' capabilities, you can monitor the o2 sensor volts and make sure it's switching properly, but a lazy o2 sensor will definately cause poor fuel economy. The TPS, is located right at the throttle body, and I really don't recomment d/c the o2 sensor...just my .02

The proposed O2 disconnection was only offered as a short troubleshooting test since he seemed to think it was the O2 sensor causing his hesitation. It was not proposed as a permanent fix.

I know from my own tests that these jeeps (at least the older Renix jeeps) run fine with out an O2 sensor, but who can afford the gas! They seem to use twice as much gas (they run rich, open loop mode, at least the Renix ones do) when the O2 sensor goes bad or is disconnected.
Not sure about the O2 sensors disconnection effect on the later OBD-1 models and his is a 94, OBD-1 model.

TPS is definately the "Usual Suspect".

Yes, the 5 speed manual does have a TPS, but it has only one 3 pin connector. The Automatics have an extra connector on the TPS that goes to the auto-tranny.

He should probably take it to someplace like Autozone to get a free scan since his has the OBD-1 scan capability!
 
Ecomike, they are able to test these at Autozone? I didnt know that. Do you know how much do they charge to scan? Will it find other issues as well? I apologize for all the questions. Im thankful for all the input sofar
 
I "believe" the '94 XJ had the Chrysler Engine controls, this will show you any fault codes stored by the engine computer, a bad O2 sensor being one of them.

When I say turn the key "on", I mean turn it to the run position, NOT the starter position and actual start the motor.

Turn the Key On-Off-On-Off-On within 5 seconds. Watch the "Check Engine Light" and count the flashes. It will pause between numbers, meaning a new number is coming. Arrange the numbers in 2 digits codes as they are flashed out.

Code 21 is usually for bad O2 sensor on Chrysler Vehicles.

Ignition is often a cause for hesitation as well. Not only fresh spark plugs, but the ignition wires themselves breakdown and leak the super high voltage, as well the cap and rotor needs periodic replacement. Cap/Rotor and Spark Plug wires should be replaced every 66k miles, if it hasn't been done since 141k miles on the vehicle, I would try that.
 
I had random hesitations/stalling issues when my 02 was on the fritz.

At low rpm and throttle positions the engine would lose all of its low rpm torque (almost like someone suddenly put on the parking brake half way) then the engine would backfire a few times and then the torque would instantly come back throwing me back in my seat.

New Bosch 02 sensor, no problems since.
 
Looks like cap, plus and wires all replaced recently. Ive owned the Jeep since November, and have no maintence records showing what has/hasnt been done. I do know the previous owner took meticulous care of it, and in my opinion the plugs, wires, cap all look to have been replaced within the last year or so. Im not sure about the sensors though. After work im gonna drive straightb to Autozone right by my house and have em check it out for me. Is the TPS sensor hard to replace (im an XJ newbie mind you)?
 
TPS hard to replace? No, just mildly annoying to "set" corectly for the needed readings. You need a digital multi-meter to set the thing. Try searching for TPS replacement, to learn how to set it. Also, get a good service manual, as it will work wonders for you and the XJ.
 
Well, took the XJ to Autozone and they said they only scan cars newer than '96 they had only a ODB2 scanner. I then took it to checker and they let me use theirs and it said there were no codes, or data. Soo, im thinkin fuel pump, or filter now......? Thanks
 
Follow the key on-off method mentioned above and see if you can get the code yourself.

On the o2 sensor causing a rich/lean topic, the upstream o2 sensor is used to control air/fuel mixture and will make you run rich/lean if it goes bad. The downstream o2 sensor (the one after the CAT) is used only to monitor the cat. converter and make sure it's doing its job and doing it properly. The reason I say this is a lot of people get these two confused, or think that the downstream o2 also controls air/fuel mixture and will replace it in an attempt to boost milage or solve a problem.
 
I don't think a '94 will have a downstream O2 sensor. My '95 2.5L 4cyl does NOT. OBDII was mandated in '96 model year, and the requirement to test the Catalytic Converter for Proper Function brought on the Upstream/Downstream O2 sensors to compare In/Out of the CAT.

But there were pre-1996 vehicles that did NOT have 2 O2 sensors, and it could be different between 6cyl and 4cyl.

So, if you do have 2 O2 sensors, remember, the O2 sensor between the engine and the CAT is the Upstream, it is used to measure the A/F ratio for the engine computer. The Downstream O2 sensor is after the CAT and is only there to check if the CAT is working correctly, it does NOT effect how the motor runs at all.
 
A bad TPS is still a high probabilty here. A bad spot or two on the carbon surface will send bad data to the computer making it hesitate and then speed up or rush with out throwing any OBD codes!!!! The TPS is a rapid wearing item from what I have read here!!!!

It can be testing with a multi meter for bad spots!

Sounds like your O2 sensor may be OK. If it is random, like you said originally, I would say the fuel system is all OK.
 
As well, if there is 60k miles on the Spark Plugs, Ignition Wires, Distributor Cap and Rotor, I'd look at a tune-up and replace all those. Bad Ignition from those worn out items can cause hesitation and stalling, if its not the problem, you just knocked out a tune-up you needed anyway.
 
I would also look into the crankshaft position sensor (CPS). I had a 2.5L YJ that would randomly lose power, it would practically shut off then come back to life. Problem when away when it was replaced. I never had an engine light come on to indicate the problem, but the problem was there. But then again, that old beast probably never had a check engine light, haha.

Good luck with your tech. Keep on figuring it out, you'll make it.
 
Mr Anderson,
I beg to differ...so what you are saying is that the rear oxygen sensor has NO bearing on how the engine performs and that both STFT and LTFT are adjusted by the front oxygen sensor, which you say measures the A/F ratio? Well, since I seriously doubt his 94 has a wide band oxygen sensor, the front o2 sensor is not actually measuring his air fuel ratio, merely INFERRING the a/f ratio with it's given inputs. In a typical upstream/downstream o2 sensor configuration, the rear o2 sensor is pointless only until it reaches Long Term Closed Loop Status, then the rear o2 sensor adjusts accordingly to keep the front o2 sensor at it's ideal stoichiometric conditions. Correct me if I'm wrong... Nevertheless, xylonjay, let us know what you find out. Also, verify that your EGR system is working properly because on a Speed Velocity (MAP) injection system, a good egr system is required for good drivability. Again, just my .02.
 
Are we speaking about the same type of injection system? I'm talking about a Speed Density System, perhaps Speed Velocity is just another name for the same thing.

In a way your correct, other than a wide-band, the O2 sensor doesn't really measure the A/F ratio, the gradient of voltage vs O2 is too steep and too variable with temperature to make a calibrated or accurate measurement. But, the voltage always crosses over right at the magical 14.7:1. Thats why its often called a switching O2 sensor. The engine computer looks at the rate the O2 sensor cycles back and forth between the switch over point to get a close approximation of the A/F ratio. The engine computer plays a constant bracketing game, bouncing the fuel amount slightly up and down, and moving that richer or leaner, according to how much time the O2 sensors stays on one side of rich/lean compared to the other.

Never heard of a Long Term Closed Loop Status, most speed density system operate at either Open Loop or Closed Loop. Again, maybe the same thing, just different terms from a different implementation.

Now perhaps the Jeep system is different, most of the Chrysler systems from this point on (which Jeep was well inside the Chrysler Fold in '94, but not totally converted over to Chrysler technology). The single O2 sensor provided feedback to the engine computer to how close the fuel input was too the stoichiometric point, and it adjusted accordingly. But in conditions when an A/F ratio was needed other than 14.7:1 (warm-up and Wide Open Throttle (WOT)) or the O2 sensor had NOT reached operating temperature to accurately respond, the system would be in Open Loop, meaning feedback from the O2 sensor was NOT read at all, i.e. the feedback loop was open. In open loop, the fuel was adjusted purely off a stored table with engine speed and air density and several other parameters as input, without any fine tuning from an O2 sensor. Chrysler, as well as others, system is adaptive, so the corrections from the feedback would be stored as a corrective table, usually an indication of volumetric efficiency, to apply corrected values to the stored fuel table when in Open Loop mode.

It was with the OBDII mandate, that all emission equipment on the vehicle had to be tested by the engine computer itself, including the CAT, did Chrysler add a 2nd downstream O2 sensor. The idea being, if the CAT was working correctly it would change the oxygen content in the exhaust as it completed the chemical reactions to break down some of the pollutants. Thus if you used an O2 sensor after the CAT and compared its output (rate of switching) with the O2 sensor already in use before the CAT, a correctly working CAT would show a difference between the 2 O2 sensors and NON-Working CAT would show NO difference between the sensors. I believe Chrysler holds a patent for this.

My '95 XJ, 2.5L 4 Cyl has ONLY ONE O2 sensor, because OBDII wasn't mandated to start until the '96 model year. I don't know for sure, but I thought the '94 and '95 power trains were the same. Like I said, it may have been different between 6 cyl and 4 cyl.

As well, I'm sure the implementations of fuel injections systems is different in all sorts of vehicles. Perhaps I'm wrong, I'm talking about the system I've learned about that has been on the Neon and Mini-Vans for years (my other cars). Maybe Jeep had a different way of implementing the O2 sensor then. Would you be talking about the Renix system? Wasn't that long gone by '94 and '95?

As well, I'm not saying your wrong, but I don't understand how 2 switching O2 sensors would work together to correct one or the other. They should switch at the same rate, unless the O2 content changed in the exhaust (only with a CAT inbetween), and how does that help determine the A/F ratio better? If its a matter of correction until the O2 sensor reached proper temp, I'm not sure how an O2 sensor further down the exhaust helps, it would reach operating temp later, not sooner. Besides, that is what the heaters in the O2 sensor are for. A single heated O2 sensor has to be cheaper than 2 non-heated O2 sensors.

I've heard of other pre-'96 vehicles that had 2 O2 sensors, always assumed it was just the manufacturer had implemented OBDII early in that particular car. Perhaps these vehicles run off the implementation you speak of, but I don't really understand.
 
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