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What are the symptoms of a bad o2 sensor?

xj92

xj92

NAXJA Forum User
Location
Junction City, KS
Since my 2000-mile-trip a week ago, my Jeep hesitates badly, dies sometimes, feels like a cylinder is not firing or something. It smooths out at higher rpm's but has a real hard time getting past idle, especially if you give it gas quickly. The plugs have black soot on them which I believe may indicate running ruch. The plug wires are away from the block and appear to be in good condition. They were replaced a year ago. The fuel filter, cap & rotor, & air filter have all been replaced. I'm thinking of doing the o2 sensor next since I've never done it yet (126K miles on the Jeep). Anybody seen an o2 sensor cause this or know what might? TIA.

1993 Cherokee
Straight 6
 
this may help you... recently i had a rough idle, terrible exhaust smell, sooty plugs, sooty tailpipe and all around crappy running. i would periodically get an obdII code of "system too lean". i was sure it was not the o2 sensor because i had always been under the impression it was only for fine tuning the fuel curve and could not contribute to that bad a fuel dumping issue. well apparently it can, i changed it and everything's been peachy ever since. i'd say at that mileage it's about due anyway.
 
sounds like an o2 sensor to me especially if u have 126000 on the jeep. they usually need to be replaced overy 100000 miles so. check that first and if not let us know i may have some other ideas
 
I'd replace the sensor. They don't cost much. Also check the wires. I had emission issues, because a wire to the sensor burned against the manifold.
 
You can back probe the attached/working O2 sensor with an analog volt meter and test it. The meters run about $10 to $20 bucks. It must be the older style analog meter. Can't test it with a digital meter.

Except fot the rich fuel, black carbon fouled plugs I would have said the TPS is bad. Could be both going out at that mileage! The TPS can be tested with an analog meter too!

xj92 said:
Since my 2000-mile-trip a week ago, my Jeep hesitates badly, dies sometimes, feels like a cylinder is not firing or something. It smooths out at higher rpm's but has a real hard time getting past idle, especially if you give it gas quickly. The plugs have black soot on them which I believe may indicate running ruch. The plug wires are away from the block and appear to be in good condition. They were replaced a year ago. The fuel filter, cap & rotor, & air filter have all been replaced. I'm thinking of doing the o2 sensor next since I've never done it yet (126K miles on the Jeep). Anybody seen an o2 sensor cause this or know what might? TIA.

1993 Cherokee
Straight 6
Click to expand...
 
After looking at it more, I thought it was a fuel issue cause I wasn't getting hardly any fuel pressure out of the bleeder valve. I replaced the fuel pump and it's better, but still has the symptoms, just not as bad as before. It pretty much runs normal once it's warmed up now, but when you first start it it wants to die, especially if you stomp the gas quick. Any other ideas? I still haven't done the o2 sensor, but here's what's been done so far:

fuel pump
fuel filter
triple-checked plug wires for routing/tight connections
cap & rotor
new Autolite Platinum spark plugs
 
Where is the tps? Is that the one in the top of the transmission or somewhere around there? What are the symptoms if it is bad? By the way, I read the code on it also, and it only gave a 55, all clear. 1993 so it's OBD1.
 
New o2 sensor didn't fix it either, just put one in. Clogged fuel injector?
 
Check out EGR system. Could be a stuck open EGR valve.
Check the CTS (coolant temp sensor) and the MAT (manifold temp sensor); these sensors will cause a rich mixture if they're telling the ECM that temps are low, when in fact the engine is at operating temp.
 
probably wouldn't be an egr issue, as the h.o. motors don't run one. it sure sounds more like a sensor issue than a mechanical problem, i.e. clogged injector.
 
Another thing to check for is a cracked or split vacuum hose to the MAP.
 
xj92 said:
Where is the tps? Is that the one in the top of the transmission or somewhere around there? What are the symptoms if it is bad? By the way, I read the code on it also, and it only gave a 55, all clear. 1993 so it's OBD1.
Click to expand...
Its on the throttle body on the opposite side of where the throttle shaft connects to the linkage.
 
Replaced the TPS, didn't fix it. I drove 500 miles or so and am in Moab now but it has gotten worse. My computer finally threw some useful codes though:

12 Battery or computer recently disconnected
14 MAP sensor voltage below .16V or over 4.96V

So I'll be checking and or replacing the MAP sensor. I don't have my repair manual with me here, where is the MAP sensor? What is its function? TIA.
 
MAP sensor is the little sensor high up on the firewall near the driver's side of the firewall, above the intake manifold. The plug going into it should be green and there should be a vacuum line running from the intake manifold to the MAP sensor!
 
xjbubba said:
Another thing to check for is a cracked or split vacuum hose to the MAP.
Click to expand...

Checking for this common problem will be cheaper than replacing the MAP, which will not fix the problem, if the hose going from the throttle body to the MAP is cracked, an loosing vacuum. Just because the code you got says MAP voltage low, doesn't mean the MAP is responsible.
 
After replacing the MAP, returning it, etc. I had a shop diagnose it. Bad coil. Replaced it, ran great. Ran the Kane Creek Canyon trail without incident, then drove about 20 miles north of Moab before the new coil just died completely. The shop that had replaced it in the morning is a tow shop also, so they towed me back and replaced it again. Ran great again. Drove 75 miles north of Moab this time, died again completely. Had family heading the same direction that towed me into Grand Junction. Tomorrow I'll try a coil from the dealer instead. If that's not it, anybody know what would cause a coil to repeatedly go bad? I'm thinking the aftermarket brand (Standard brand) coil just doesn't cut it for an XJ. At least I'm hoping.

To throw some tech info in, they found the problem with the coil while checking the timing. When the engine would cut out, the timing light would turn off. So they checked for spark, and there was none coming from the coil. An easy way to check for spark is to simply pull off the cable from the cap & rotor to the coil and hold it a 1/2" or so away and see if there is an arc with the engine running.
 
Assuming the coils were not poor quality (which might be the problem, poor quality) then I would check the input to the coil for excess voltage. Don't know much about the later XJs like yours, so I am not sure if the input voltage between the coils is independent or not.

Said another way I would look for something that could cause the coil voltage to get too high on that one coil which would damage the coil insulation thus grounding it out, or look for a better quality coil.
 
As I understand in 2000 they went to six seperate HV coils, one for each plug, and he had just one coil of the six go bad repeatedly now.

lawsoncl said:
A bad CPS sensor will also kill spark, but I think that should also throw a code on a 2000.
Click to expand...
 
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