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10 MPG.....O2 sensor test

C85D4x4

C85D4x4

NAXJA Member #1311
Location
Ohio, Brecksville
Ok guys I'm still getting 10mpg on my 96 with 34s and 4.56 gears even after replacing
-new O2 sensor
-cap & wires & plugs
-RP syn oil

I'm reading that ppl are getting 20mpg on 35s.

SO.............................

How do I test the O2 sensor?
What wires out of the 4 do I test and what should the voltage be?
 
Last edited:
C85D4x4 said:
Ok guys I'm still getting 10mpg on my 96 with 34" tires even after replacing
-new O2 sensor
-cap & wires & plugs
-RP syn oil

I'm reading that ppl are getting 20mpg on 35s.

SO.............................

How do I test the O2 sensor?
What wires out of the 4 do I test and what should the voltage be?
Click to expand...

what gears you got in her? I can see 10-12mph with stock gears and around town driving.
 
I don't think that an O2 sensor would be responsible for a "big" gas mileage hit. I assume that there is no check engine light? What temp is your XJ running at. Does it seem to be running rich? It could be a combination of things....
 
-Crap forgot..........4.56 gears

-No check engine light and Temps is normal (210).

I have read over the years on many ppl fix there mpg problem with a new O2. Im just wanting to know if my New Bosch O2 sensor is working correctly.
 
The O2 sensor can definitely be checked with a good meter or an o-scope. Do you have a field service manual? Or I believe even Haynes and Chilton have a procedure for testing your oxygen sensors in there.
 
Use an analog (needle, not digital) volt meter. Using ohms, power off, find the 2 ground wires first. Then with power on, the fat wire should read about 13 volts to ground. The fat wire is supplying voltage to the internal resitor / heater in the O2 sensor. That leaves the 4th wire which should read an oscillating 0-1 volt that swings back and forth across .45 volts rapidly. That signal is the O2 sensor reading.
 
did you replace the speedo gear in the transfercase? maybe your traveling further than the odometer is reading?

have you reset the computer?

cloggged air filter?

have you cleaned the throttle body, iac, and CCV lines and grommets.

how did the old plugs look?

what type of tires(at, mt)?

rear brakes dragging?

20mpg seems really high with 35's.
 
Top two items that can have a huge negative effect on gas mileage (other than a tune up, plugs, wires, cap, rotor, injectors), is a worn TPS and bad O2 sensor.

Marginally Bad TPS and bad O2 sensor will not always throw an OBD-II code, and cut gas mileage by 50%. They can both be tested with an old style analog multimeter (ohms, volts).
 
89xj said:
did you replace the speedo gear in the transfercase?-----Yes

maybe your traveling further than the odometer is reading?-----No

have you reset the computer?-----Yes

cloggged air filter?------No Its new

have you cleaned the throttle body, iac, and CCV lines and grommets.------I have cleaned the throttle body.

how did the old plugs look?------The gap was large on them all b/c the points (the white thing) were worn out.

what type of tires(at, mt)?----LTBs but my MPG was this bad on 32 BFG a/t.

rear brakes dragging?-------I dont think so.

20mpg seems really high with 35's.-----I know, thats why Im looking for 16mpg
Click to expand...
 
Ecomike said:
Top two items that can have a huge negative effect on gas mileage (other than a tune up, plugs, wires, cap, rotor, injectors), is a worn TPS and bad O2 sensor.

Marginally Bad TPS and bad O2 sensor will not always throw an OBD-II code, and cut gas mileage by 50%. They can both be tested with an old style analog multimeter (ohms, volts).
Click to expand...


Thanks man...........I have a digital, so I guess Im in need to go out looking for a analog multimeter.
 
You're better off using a digital volt meter to test the O2 output. You need a "high impedance" meter to keep from loading the ECM, causing an incorrect reading.
"they" do sell high impedance analog meters, but last time I checked, they were $$$.
So use your digital meter, they are "high impedance" by design.
The output should be crossing over .45V > 1 time/second, and should vary between .3 and .8v constantly, when the sensor is operating.
If the heater circuit is not working, the O2 sensor is not likely to get warm enough to operate correctly. So check the heater circuit first.
 
xjbubba said:
You're better off using a digital volt meter to test the O2 output. You need a "high impedance" meter to keep from loading the ECM, causing an incorrect reading.
"they" do sell high impedance analog meters, but last time I checked, they were $$$.
So use your digital meter, they are "high impedance" by design.
The output should be crossing over .45V > 1 time/second, and should vary between .3 and .8v constantly, when the sensor is operating.
If the heater circuit is not working, the O2 sensor is not likely to get warm enough to operate correctly. So check the heater circuit first.
Click to expand...

IMHO digitals are useless because of the slow display response time. Just looks like noise on the display. High impedance analogs are cheap and easy to find, Ratshack, Wallyworld, Harbor Freight, seen them as cheap as $10.
 
fwiw i got a analog meter from wally world for 9.89 because my digital fluke was not able to keep up with the varying signals from the o2 sensor, it was a lot cheaper than a new sensor, imho easier to use and read fwhen trying to check for varying signals hth
 
I'd be interested in finding a $10 analog multimeter that has internal resistance of at least 50,000 ohms. Even at 50K ohms, it's not really high impedance, but would probably be OK for most automotive sensor testing applications. Most digital meters are 50,000,000 ohms/volt.
Most analog meters (cheap ones) I've seen are at the 2,000 ohms per volt range, which is too low to be using on digital circuits.
I do use analog ohm meters to test the TPS.
 
:wierd:




Man, I'm having a hard time understanding this stuff you guys are talking about..........I just want better mpg. I know its not as hard as it sounds, but I'm guessing that this is why most people have such a problem getting better mileage in there jeep. I knew I should have studied to become a electrical engineer. :gee:
 
Digging this back up as i'm running into the same issue.

99 4.0, 3.55's with 31x10.50's.

I used to get 19mpg on the highway...didn't matter if the AC was on or off, and I don't have cruise control so it's all foot. I also used to get around 16-17 in the city, again AC on or off didn't make a difference. All of a sudden lately it's dropped to about 12 around town and 14-15 if i'm lucky on the highway.

I tested my o2 sensor today according to the instructions in my FSM, which called for a reading of 4.5(+/- 0.5)-7ohms. The way I read that is anywhere from 4-7ohms or 5-7ohms is acceptable. The sensor tested 7.5ohms, which to me is out of spec, if i'm understanding it correctly. I plan on testing one off the shelf when I get to work Saturday to see if there's a big enough difference.

The one thing stopping me from changing this sensor for sure is the fact that I do have a small crack at the header, and what I suspect is a somewhat bad exhaust manifold gasket, based on the ticking noise that would come and go towards the end of this summer.

Need to get the mpg's back up as I do a lot of driving, and primarily highway driving. Took it on a trip this weekend that I normally could make in 3/4 of a tank round trip and used 1 and 1/4 tanks....so something's definitely up.
 
Exhaust leaks don't help mileage one bit. They change the amount of oxygen read by the sensor which is a huge player in the scheme of fuel economy. Fix the leaks.
 
I have been fighting this since I bought the jeep. Currently on 33x12.50s w/ 3.55s and by the odometer I am getting about 11.5mpg. However my speedometer reads about 5 mph slower than what I am actually traveling so I just did a rough estimation by the percent the tire size has changed and figured I am getting around 13mpg but that still seems low to me.

Things I have done:
air filter
plugs
rear 02 (had CEL)

Things I need to do:
fuel filter
front o2 sensor
possibly TPS

All I want is to get 15mpg! Doesn't seem like that much to ask.
 
JJacobs said:
Exhaust leaks don't help mileage one bit. They change the amount of oxygen read by the sensor which is a huge player in the scheme of fuel economy. Fix the leaks.
Click to expand...

Yeah, i'm thinking that's the way to start right now. However i'm still going to test a new sensor off the shelf Saturday when I get to work and if it's drastically different I might grab it and throw it on. I figure even if it doens't make a huge difference now, any improvement will help and at 130K it's a bit overdue anyways. Plus I can get it for about $50 with my discount for a Bosch Exact Fit.
 
The test you ran, and resistance you got is fine. That part of the O2 sensor is OK. You need to check it live for 12-14 volts across that same resistance to see if you have a bad relay or wiring problem, and check the other 2 wires for an oscillating .1-.9 volt signal at idle and a .4 to .5 volt swing at 2000 rpm.

Exhaust leak is going to make it run rich and waist gas even with a working O2 sensor system.
 
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