Confirming Bad O2 Sensors

Butch

Well-known member
Mar 8, 2006
459
0
52
Warner Robins, Ga
I have a 98 Disco with 123K on the clock and the gas mileage is getting worse. The vehicle is completely stock and I'm getting about 10 MPG in the city about 225-250 miles per tank. I know that Disco's are not known for their superior gas mileage, however I'm getting worse mileage than guys with lifts and racks loaded on their vehicles.

I have changed the plugs to champion and the wires to magnacore 8mm wires. I got rid of the K&N (not trying to start another debate :))and put a NAPA paper filter in along with a NAPA gold fuel filter. I have ran 2 cans of Seafoam in the tank, one in the intake and one in the oil. I run dino fluids and change them at specified intervals. There are no known fuel leaks, but the exhaust smells of fuel on start up. I have no codes present in the computer and even just passed my emissions check.

So after this long explanation, do you guys agree that the O2 sensors need to be replaced? I just want to make sure before I order these.

http://store.summitracing.com/partdetail.asp?autofilter=1&part=NGK-25044&N=700+115&autoview=sku

Thanks.
 

martinquebec

Well-known member
Jun 19, 2006
155
0
montreal, Canada
i would say no, if you passed the test your engine is ok as the O2, you seems to got right with engine i would think that it is your climat that might be the cause...in winter here in Montreal it cost more on fuel for sure but burn better than hot temperature...cant say more !!
 

Butch

Well-known member
Mar 8, 2006
459
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52
Warner Robins, Ga
The loss in gas mileage has been happening over the past year/10,000 miles, but has become more of an issue as gas prices are close to $3.75 a gallon. If I can get my city mileage up to 13 MPG at a minimum, that would be almost 100 miles more per tank (about where it was when I bought the vehicle), 15 MPG would be even better!

Are all 4 sensors the same model or are the 2 pre-cats different from the post-cats?
 

Butch

Well-known member
Mar 8, 2006
459
0
52
Warner Robins, Ga
I've done some searching on here and I have read to replace all 4 at the same time. If I only replace the pre-cat O2's, am I going to start throwing codes? Granted, saving $120 is great, but I'd rather spend the money than pull my hair out fixing codes. Although $120 could get me closer to getting new pads and rotors!

What do you guys think?
 
Oct 27, 2004
3,000
4
I say you do some testing other then the 12 guage method.

You can test them with a voltmeter.

And, no check engine light makes me think they are working.

Do you have a emission test reading?

What are your fuel trims?
 
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Butch

Well-known member
Mar 8, 2006
459
0
52
Warner Robins, Ga
After reviewing my test results, it doesn't look Alaska prints out that information on their reports. The only thing it is showing for O2 sensors is a visual inspection and the OBDII report section shows that the O2's are ready. It doesn't show any other info.
 
Jan 26, 2008
1,185
2
In the bunker
Butch, PT is correct about the post-cat O2 sensors; all they do is montor the exaust gasses after they have left the converters. In fact, if you are worried about a check engine light, you might be better off if you leave the post-cat O2 sensors undisturbed.
The best way to check the pre-cat O2 sensors is with an ocilliscope or a fast reacting graphing scanner. I'm guessing you don't have either one of those, or a buddy who does. You could go to a shop that does have that equipment, and knows how to read the signals. But that diagnosis would cost more than a couple of new sensors.
My advice: Forget about the post-cat sensors. Replace the front ones. Inflate your tires to 45 psi (on road). And if you get 13 mpg, buy a six-pack of New Castle every time you fill-up: that's what I do.
 

Butch

Well-known member
Mar 8, 2006
459
0
52
Warner Robins, Ga
Sounds like two pre-cat sensors and a six pack of beer and I'll be set! Beside a whole lot of PB and a sensor socket, is there anything else I'll need?
 
Oct 27, 2004
3,000
4
Actually, to be completely accurate... Post cat O2s on OBD2 cars can adjust Fuel trim up to 10%. Not a bunch, but it can do some.

I would love to see his exhaust readings. I bet its pretty close to Stoich (We will see), in which case, I would look at fuel trims for a vacuum leak.

However, Test....Dont guess.
 
Chris-St Louis said:
Actually, to be completely accurate... Post cat O2s on OBD2 cars can adjust Fuel trim up to 10%. Not a bunch, but it can do some.

According to LRU course TT 19.2, page 44, the rear oxygen sensors are used to "monitor performance of the three-way catalyst". and, "the rear sensors are also part of the ECM's fuel system back-up strategy. Should the signal from the front HO2S fail, the signal from the corresponding rear sensor will be used so that the vehicle can remain in closed-loop operation".

Now, get back to figuring out why my Lucas EMS truck won't accurately control the A/F ratio! I need to drive that truck to a job interview tomorrow!

PT
 

bleizit

Well-known member
Sep 27, 2007
113
0
Lawrenceville, GA
Went through the same issue with decreaded mpg and decided to replace all four O2 sensors after two of them threw codes (original O2's w/115K). My mpg bumped up only slightly and I still ran rich. I changed out the cats and the mpg jumped back up to 13-16 mpg while running it with my rack and the 2" lift OME springs provided. If you don't get the increase you're looking for, perhaps have the cats looked into. My post-cat O2 sensors were coated in soot!
 
Oct 27, 2004
3,000
4
You are right, the post cats O2s are to moniter cats, However, I just went to a seminar about 3 weeks ago, that showed me post cats can and do affect fuel trim up to 10%.

Next time you are in STL, or I am in Indy, I will show you. :)
 
Chris-St Louis said:
You are right, the post cats O2s are to moniter cats, However, I just went to a seminar about 3 weeks ago, that showed me post cats can and do affect fuel trim up to 10%.

Next time you are in STL, or I am in Indy, I will show you. :)

I find it curious that the documents that Land Rover uses to educate techs doesn't state that.

I'm still trying to figure out why my Disco won't control A/F well...
 

Butch

Well-known member
Mar 8, 2006
459
0
52
Warner Robins, Ga
bleizit said:
Went through the same issue with decreaded mpg and decided to replace all four O2 sensors after two of them threw codes (original O2's w/115K). My mpg bumped up only slightly and I still ran rich. I changed out the cats and the mpg jumped back up to 13-16 mpg while running it with my rack and the 2" lift OME springs provided. If you don't get the increase you're looking for, perhaps have the cats looked into. My post-cat O2 sensors were coated in soot!

I have notice a slight rattle coming from around the cat area, but just ignored it. What was the cost of replacing the cats?