96Cherokee113

Oh sorry, I was referring to the post by djb383. Should've quoted it, oops.

djb383

A little further down in the article it explains OBDII and the relationship between the upstream and downstream O2 sensors. Again, we are temporarily running without a downstream O2, the system is operating in closed loop (due to info received from the upstream O2) and our mpg's are good.

96Cherokee113

I don't see how you could possibly get 21 mpg. I get 12 mpg flooring it everywhere with the downstream. Without it I get about 10 mpg driving easy. In a similar thread I was told that the downstream O2 measures the air/fuel ratio to automatically adjust the fuel curve. When I go to the junkyard, I'm probably going to get the upstream O2 sensor also to see if I see any gains in MPG. It's probably the 193,000 mile stock O2 sensor.

OAS

It COULD be just a bad sensor.
Has the cat ever been replaced?
How many miles are on the jeep?

When i bought my '97 there was no cat (test pipe was installed) and the sensor was wrapped in tape and hung from the wire bracket.
Oddly there was no cel on.
After getting a cat and a new sensor installed i read this in the service manual............

quote:
"MIL 64—CATALYST MONITOR
To comply with clean air regulations, vehicles are
equipped with catalytic converters. These converters
reduce the emission of hydrocarbons, oxides of nitrogen
and carbon monoxide.
Normal vehicle miles or engine misfire can cause a
catalyst to decay. A meltdown of the ceramic core can
cause a reduction of the exhaust passage. This can
increase vehicle emissions and deteriorate engine
performance, driveability and fuel economy.
The catalyst monitor uses dual oxygen sensors
(O2S’s) to monitor the efficiency of the converter. The
dual O2S’s sensor strategy is based on the fact that
as a catalyst deteriorates, its oxygen storage capacity
and its efficiency are both reduced. By monitoring
the oxygen storage capacity of a catalyst, its efficiency
can be indirectly calculated. The upstream
O2S is used to detect the amount of oxygen in the
exhaust gas before the gas enters the catalytic converter.
The PCM calculates the A/F mixture from the
output of the O2S. A low voltage indicates high oxygen
content (lean mixture). A high voltage indicates a
low content of oxygen (rich mixture).
When the upstream O2S detects a lean condition,
there is an abundance of oxygen in the exhaust gas.
A functioning converter would store this oxygen so it
can use it for the oxidation of HC and CO. As the
converter absorbs the oxygen, there will be a lack of
oxygen downstream of the converter. The output of
the downstream O2S will indicate limited activity in
this condition.
As the converter loses the ability to store oxygen,
the condition can be detected from the behavior of
the downstream O2S. When the efficiency drops, no
chemical reaction takes place. This means the concentration
of oxygen will be the same downstream as
upstream. The output voltage of the downstream
O2S copies the voltage of the upstream sensor. The
only difference is a time lag (seen by the PCM)
between the switching of the O2S’s.To monitor the system, the number of lean-to-rich
switches of upstream and downstream O2S’s is
counted. The ratio of downstream switches to
upstream switches is used to determine whether the
catalyst is operating properly. An effective catalyst
will have fewer downstream switches than it has
upstream switches i.e., a ratio closer to zero. For a
totally ineffective catalyst, this ratio will be one-toone,
indicating that no oxidation occurs in the device.
The system must be monitored so that when catalyst
efficiency deteriorates and exhaust emissions
increase to over the legal limit, the MIL (check
engine lamp) will be illuminated."

Sounds like you could do without it, unless you live in a state that checks for it. (insections)
But that would be bbaadd.........right?

OAS

Mine has always gotten decent milage.
Before and after replacing BOTH upstream and downstream o2 sensors.
I get about 12 to 14 city, and have gotten as high as 25 driving 90% highway.
Lately i get a max of about 19, but i blame it on the crappy cold weather.

JerseyJeeper

Ok so it's a 99 with just under 118000 miles on it and I live in New Jersey so yea there is inspection every two years. Luckily it went off after my inspection so really I have two years to fix it lol. I'm pretty sure it's the sensor because I pulled it out at it looks pretty bad. I'm going to swap it out probably tomorrow because I have no time today to do it. I think it finally just gave out because it is the stock sensor. The cat is also stock. Thanks for all the help everyone!!

96Cherokee113

So are you recommending replacement of both sensors to get MPGs up? My downstream is ripped off so I'm getting another on Tues when I have a free day, and probable grabbing the upstream too.

djb383

The auto parts stores here list two different O2 sensors for our '98 XJ. One sensor/part number is unique to the upstream O2 and a different sensor/part number is unique to the downstream O2 sensor. You can't mix-n-match O2 sensors and their respective locations. That thread you read is either wrong or you read it wrong.......the downstream O2 DOES NOT control the air/fuel ratio, the upstream O2 sensor does that.

96Cherokee113

I never planned on mix-n-matchin the O2 sensors. I know that each one is specific to their location. I said I was going to get an upstream O2 sensor also while I'm at the JY to see if it changes my gas mileage. I'm replacing the downstream with the downstream O2 from the donor Jeep and the upstream with the upstream O2 from the donor Jeep.

djb383

.........and you can be positive that O2 sensors from a junk yard donor jeep are the correct sensors, in their correct locations and functioning properly?????????????

96Cherokee113

Well, considering I know the Jeep was driven in, has 70,000 miles, is the same year, engine, and sub-model and was immaculate until it rear-ended somebody, I am 99% positive they will be fully functional. And I have no downstream O2 right now, so anything is better than what I got now. Also, if upstream O2 affects gas mileage and I'm getting 12 MPG, I don't think it would hurt to try an O2 from a donor. I'll be pulling them myself, so I know they're from the correct locations. I'll be getting them for free anyway. I guess with your lifestyle, this would be considered "taking a chance".

djb383

We're not even going to mess with a downstream O2........can you say O2 simulator? Since the downstream sensor has no effect on mpg or engine performance, all we want to do is turn off the CEL.

96Cherokee113

Okay, well then I have the thing for you. Now instead of arguing, we can make friends. I'm not sure if you've heard of spark-plug nonfoulers but that's how you do this mod. When I got my Jeep it had no cat so I needed some way to get the light off for inspection and I did some research and found this. Basically you need a set of these which can be found in the Dorman HELP! section at AutoZone. http://www.audiworld.com/tech/pics/eng114_pict00110.jpg You want the 18 mm ones so the O2 sensor screws into the threads. You basically drill one of them out with a 1/2 inch drill bit and screw that on top of the second one which you leave alone. That screws into the O2 bung so it makes the distance farther away so it reads the cat is doing it's job (whether it is there or not). Then your CEL goes off. Mine passed inspection like that catless. Here's a thread about it http://www.355nation.net/forum/how-t...ight-pics.html . Not sure if this applies to your specific situation, but it works if you have a faulty or missing cat.

djb383

With regard to the plug non-foulers, been there, done that. Favorable results were spotty at best. Going to try a simulator.

96Cherokee113

Yea, those are expensive, though. I'm always going as cheap as possible seeing as I'm working 2 jobs just trying to keep the damn thing on the road.