O2 sensors and mpg

ChuckstrPT

NAXJA Member #791
Do bad O2 sensors have the potential to cause really bad mileage? If they are bad, or are not performing appropriately, will it cause a fault code? Or do I actually have to get a multimeter in there to see if they are putting out correctly?
The Truck: '01, 95k, 4.0, AW4, 242, Mobil 1 and synthetics in diffs, 3 inch lift, 30's, 3.55's, and a ton O steel protection, resulting in 10-12 mpg round town. I do have a heavy right foot too. But is does seem to be signficantly less even when not in a hurry! If I concentrate real hard, I can get up to 13-14 round town. But I gotta REALLY concentrate.:sunshine:
That '01 4.0 actually has 4 O2 sensors hanging on the exhaust. At 95k on it, it's likely to need them, but I'd rather not go to that expense if they are working propperly.
Any suggestions?:viking:
 
1.) Yes. 2.) Not necessarily. 3.) 90K I consider the limit of their functional life though some will argue with that. I would start with the "upstream" two as those are most directly responisble with fuel mix. But, at 12 mpg I would look for other engine problems. Exhaust of vaccuum leaks, worn sensors, filters and igntion components, underinflated tires, etc.
 
bajacalal said:
1.) Yes. 2.) Not necessarily. 3.) 90K I consider the limit of their functional life though some will argue with that. I would start with the "upstream" two as those are most directly responisble with fuel mix. But, at 12 mpg I would look for other engine problems. Exhaust of vaccuum leaks, worn sensors, filters and igntion components, underinflated tires, etc.
Yeh, I'm with you on most of that. I am of the mind set that most "wearable" items should be fairly worn 'round 100k, and should be fair game, ie. money well spent, if replaced. The '01 has the distributorless ignition, so there really isn't much to "tune up" is there? I replaced the plugs maybe 20k ago, when I got it. I'm thinking if those coil packs go bad, or have a problem, I'll have more issues than just bad mpg. Got a K&N in the air filter. The tires were far from underflated, was 30psi, just upped to 33, but I'm not expecting much from that. I guess I'm assuming if there was something else, sensor malfunction, or vacuum leak, shouldn't it throw a code? I actually thing one of the biggest problems is the steel armor: Bulletproof rear with swing out tire carrier, Skid Row gas tank skid, CRS tcase/tranny crossmember skid, T&J chassis stiffeners, double tube rock rails, and ARB front bumber..I'm thinking #500?
Off Topic- are those aftermarket Code Readers, the $100-200 variety, worth it? of just run it over to Autozone for a quick scan?
Thanks!
ps. what are O2 sensors going for these days???
 
Don't forget the CAT it can be bad and still pass emissions test.
The passages inside start to break up causing back pressure the exhaust that does come through is still clean.
 
I have a stock (w/ upcountry package and 235 75 15 tires) and I was getting around 9.5 mpg around town. I changed oxygen sensor, fuel filter, distributor cap, rotor, plugs, and ignition coil and I am getting around 13.5 mpg in town. I still have the crack in my header right before the oxygen sensor so it may be causing it to run lean - I'm not exactly sure until I get that fixed and see if I notice a difference in MPG.

By the way, O2 sensors are around $55-$60.00.

Don't forget, look at the fuel mileage of a Grand Cherokee with the same engine and around the same year. If I am not mistaken, they do not have unibodies and therefore weigh more....so you are probably closer in weight to one of those. All in all your fuel mileage should be closer to that of a ZJ than a stock XJ is my guess, with your added weight.
 
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O2 sensors prices seem to vary quite a bit. I bought a Bosch OEM one for nearly $60. They also had a generic which did not have the factory connector for $20.

As far as the scanner goes, it depends. If you have several cars that you can use it on, look on eBay. I got one that pulls codes, resets MIL, etc, for about $60. It supports all protocols except CAN. The only thing is, I have to look the codes up in a booklet or FSM. The more expensive ones "translate" the error codes, can auto update and some even have battery back to keep the codes once you disconnect the scanner. These scanner run typically over $100 (more like $150 and up).

If it's a rare occasion and your AZ is nearby, I'd just go there.
 
I still have the crack in my header right before the oxygen sensor so it may be causing it to run lean - I'm not exactly sure until I get that fixed and see if I notice a difference in MPG.

It can cause you to run rich not lean.
 
First, "lazy" O2 sensors normally wont set a code, but will result in excessive gas mileage. The sensors on the back side of the cat sense cat effectiveness and do not affect fuel mixture. A leaking or cracked exhaust manifold will cause the O2 sensors to tell the ECM the mixture is lean, resulting in a rich mixture to offset the perceived lean mixture, resulting in poor gas mileage; however, with OBDII I believe a code would be set if the O2 signal was biased too far lean for an extended period.
Basically, WEIGHT is the primary cause of lower gas mileage, followed by FOOT activity! I believe when you add a lot of weight to the 4.0L equipped Jeep, you're pushing the engine to operate where it wasn't designed to operate , resulting in poor mileage.
By example, my 4.0L '88 got ~20MPG when I first bought it. After adding suspension mods, 33's, 4.56's, Atlas, skids, bumpers, winch, other "toughening" mods, etc. she weighed in arround 4100LBS. Mileage dropped to ~16 on the road. After adding another 100LBS via NV4500 and MPFI Chevy 350, mileage is back up to 18-19MPG on the road while fully loaded; the V8 is not being "pushed" like the 6 was to give me the same performance.
 
So, at nearly 100k, I guess run to Autozone, check the codes, and regardless, replace the 2 O2 sensors in front of the cats? Do I need to worry about the 2 sensors behind the Cats? and I agree, I think more than how the engine is working, my foot, and truck weight are having more of an effect. Since I'm not changing those, figured I try to make sure the engine is operating optimally!
Thanks!
 
Blaine B. said:
I thought it was 1 sensor before and 1 after the cat...
My '01 has 2 before the 2 cats, and 2 after the cats. and those upper sensors are right up under the intake manifold. I'm not even sure I can reach them from underneath, and I've got long arms! I can't see them from above. Surely this isn't a situation the manifold needs to be removed for??? Maybe just reaching under, blind, feeling for it?
 
my 97 has one sensor about a foot in front of the cat, one sensor stuck in the back of the cat and a third mystery sensor back in the tailpipe past the muffler- anyone heard of this?- I thought it was either two or four, not three.
 
Why is it people throw all sorts of "information" around with OBDII 1996+ vehicles? I have heard all sorts of odd things mentioned such as non serviceable fuel filters (1997+) having to do with OBDII. I don't think theres a huge difference in how the systems work, just more "functions" and how they report codes is different. An OBDII system will not necessarily throw an O2 sensor code unless the sensor readings are so far off (deviate too far from typical readings). My 1996 (first year of OBDII) never said anything about my SEVERE manifold leak. In fact, I had bad sensors and it still thought the readings were in-spec, no code was thrown. Though I agree later model computer systems are smarter and more "sensitive" to recognize abnormal situations.

But seriously, replace your cracked headers, check your tire wear patterns, take out excess weight and obstructions (roof racks) if you don't need them and think about how your driving style impacts gas mileage.
 
OBDII="Though I agree later model computer systems are smarter and more "sensitive" to recognize abnormal situations."
 
UPDATE:
So I priced the O2 sensors first....after finding no codes...they are expensive, one was $114?? So took it to mechanic, and put it on the computer for $50. All 4 O2 sensors are working as they should be. All parameters were within spec. Mechanics conclusion: For 95k the engine is running well. The excess weight, and heavy Right foot are to blame for poor mpg. Sucks. I guess I need bigger tires, and gears/locker!:woohoo:
 
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