02 Sensor Operation FYI
 

This seems like the appropriate place to post this.

After many months of horrible gas fume smells and 12 MPG being the peak MPG I could attain (going down hill with winds to my advantage with 400 lbs of tools in the rear), I have finally found why my engine stalls down to 400 RPMs when slowing down and smells like a fuel farm anytime I turn it on. Below is my lesson-learned story.
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Both 02 sensors work together. If the downstream isn't operating like it's supposed to, you won't get a code for the front one.

I've gone through 2 upstream sensors (miswiring I had done destroyed them). I finally decided to replace my rear 02 sensor (which had previously been submerged in water). I figured after I rewired them (the front 02 sensors), properly, if it was bad I would get a code for it. I didn't.

As soon as I put in the new downstream it told me the upstream was too slow, and I could smell gas fumes within 10 feet all around it. Then I put in the other 02 I had previously swapped. I didn't get a code for it, but it would buck, surge, and gas fumes still pervaded the area. The hesitation and surging was so bad it was undrivable.. yet I never got a code for that 02 sensor. I found that odd. I think it was jumping from one extreme to other, but staying within the boundaries of OBD code thresholds.

Then I put in a new 02 in the front (3rd one). Now I smell no more gas fumes, don't have any codes, and the economy indicator indicates that I am actually getting regular XJ milage (previously could only get 12 going down a hill---now I get up to 32 when I let it coast down the same hill). Average at 1500 RPMs doing 55 on a flat road is 17-19 MPG. Mind you it's stroked, 3.5 inches lift, has an ARB bull bar, and 31x10.50's with stock gearing.


A retired chrysler mechanic I had previously talked to (a few months back) told me that the front 02 (via the computer) uses the rear 02 signal as a reference. If you have a faulty 02 in the rear, then the one in the front won't operate properly. I thought nothing of it since I didn't have a code for the downstream 02, AKA 1/2 02 sensor.

It appears that the PCM compares one to the other, and as long as they are operating similarly (I think?) (refer to the link) it wont throw a code for either, unless it goes above or below the extremes of the ranges in which they are supposed to operate. As you can see on the link it can be worn out and still not cause a code to appear.

http://www.aa1car.com/library/o2sensor.htm


My recommendation is if you ever have to replace one, replace both.

My vehicle had been running rich and had a lumpy idle for months. I didn't drive it regularly due to the richness. Since I know the computer had learned to operate this way I reset the adaptive memory on it.

To do that:

1. Disconnect the positive cable.

2. Touch positive cable to a ground such as the AC (not the battery ground obviously) for at least 30 seconds (leave negative connected)

3. Reconnect positive.

4. Turn ignition to run position (not the start)

5. Turn head lights on for about 10 seconds, then turn them off.

6. Turn key to Off position (not accessory) and you're done.

Now the computer, for the first 50 times it operates at "operating temperature" will memorize data collected about driving. Most importantly in today's economy of high gas prices, fuel efficiency curves. Now would be a good time to do the IAT sensor relocation if you have intentions of doing that.


Coffee Commando~

Thanks on this bro.

makes sense...but if your jeep has four and your having the exact same issues then replace four for best affects?

Sent from my droid using Cherokee Forum

I would say yes. If you replace one half of a V8's 02 but not the other half, and that other half is operating improperly, then you have a fluctuating difference in 02 readings and your reference in the rear will constantly be changing for the fronts.

The bottom line is if you replace one, and the other (however many you have) are old, or a different model altogether, I recommend replacing all.

Of course if you know 1 is just messed up you can replace that initially and see if you like the results.

In my case I went through 2 before I replaced the rear, which ended up with me buying a 3rd front. That was 4 02's. If I would have just bought 2 at the same time (and hadn't messed up the wiring) I would have been best off.

One old one operating slower than the newer one causes the ECU to constantly fluctuate between richening and leaning.

If you have that problem, you will certainly smell it. Like I did. Even with the windows rolled up doing 20 I could smell it. I don't understand how. Other than horrible aerodynamics.

Funny...just came from messing around with my newly aquired 95 and experiencing the same issue. Gas smell is horrible and after reading the post, it all makes sense. I hate you went through the pain and multiple o2 sensors, but that just means that I don't have to!

so I need 2 for my xj and 4 for my gf's wj...we both have cel's with o 2 reference...

the upstream and downstream O2 sensors share a common ground and use a .5V reference signal. when one goes out, it usually backfeeds through the common ground and takes out the second O2. If you can get ahold of a scantool that shows voltage for the O2 sensors, you can determine which ones have failed.

That makes sense. I had the Ground and the Signal to the ECU backwards. I think my 02 ckt is actually faulty on the ECU now. It's wired correctly now and when I put in a new 02 sensor it works for about all of 10 minutes then the gas milage reverts to 12 at best.

Something I have found though is that the factory default programming for the ECU isn't bad at all. I still get 18-19 with it unplugged (since it isn't plugged in it uses the factory defaults for that circuit).