So, here goes....

I normally have a test pipe in place of my cat. Well, emissions are finally due, so I set out to put the cat back in. I removed the rear O2 sensor so the cat-back portion of the exhaust wouldn't hang from the O2 sensor cable harness. I installed the cat, installed the O2 sensor, and thought I was on my way to a clean emissions bill. I start it up, and.....the check engine light pops on. Great. I thought, oops, I forgot to remove the CEL fix for test pipes (the O2 extender) and maybe it was whining about that. So I took off the sensor again, removed the extender, and started it up again. No change. I give up for the night.

I started the EVO the next day to drive it to work and the check engine light wasn't on, so I thought I was in the clear. Nope, it returns on the way. I also notice that when I get into boost the A/F ratio is INCREASING not decreasing. Crap. So, while at work, I plug in the laptop and check the codes:

EGR Sensor
Knock Sensor
Barometric Pressure Sensor
Vehicle Speed Sensor
Fuel Pump Relay
Injector Circuit

Each time I checked the codes they changed. Sometimes they went away completely. The car drives fine, doesn't stutter, doesn't sound different, pulls hard, etc...

Knowing the only thing I touched was the O2 sensor, I decided to start there. I removed the rear sensor and checked continuity between pins 3 and 4 (heater), and got 13 Ohms (manual says this value should be between 11 and 18 Ohms). I checked the wiring for any obvious damage, did some more continuity tests, didn't find anything, and reinstalled the sensor.

Now, the check engine light isn't on, but when I check codes w/EvoScan it comes up with the same list as above. The AFR's still go up with boost. I took some logs and didn't see any knock at all. Both of the O2 sensors seem to be within range compared to some of my other logs (front O2: 0->0.92V; rear O2: 0.1->4.9V). Although, when I graphed front O2 sensor voltage versus rear O2 sensor voltage, the graph was different when compared to four of my other saved log files. The difference is that for my current problem, the rear O2 sensor never gets above 2V unless the front O2 sensor is below 0.2V. On my previous logs, the rear sensor wont get above 2V unless the front is ABOVE 0.7V.

This is strange. If it was an O2 problem, I would expect one of the four or so O2 error codes. Nothing seems to be obvious at this point, since there are so many DTC's being thrown by the ECU. I would just replace the rear O2 sensor to find out, but those are around $300 and have weeks of lead time.

Any thoughts?

 

bump for a foul day!

 

mine did the same the same thing when i put the extender on, if my trip meter hits 800 miles on it then it will throw those codes 4 my 02, then i reset my ecu and trip and there gone. so now i set my trip meter back to zero every week. i know its crazy but thats how mine works. maybe it will help

 

Unless the latest version has it working properly, Evoscan can not check OBD-II codes. It just spits out a bunch of bogus codes that keep changing. If you need to check OBD-II codes download ScanTech from aktivematrix.com.

-Paul

 

Thanks for the input, I'll try the trip meter thing, but I doubt it would make my AFR's go crazy, right?

I'll check out the ScanTech program - thanks for the advice!

The thing that bothers me the most is that the O2 voltage profile is different than any of my other log files. It appears that the rear O2 is acting exactly backwards, but the car drives fine - no stuttering or strangeness at all. Hmmm....

 

Any more thoughts anyone?

 

Reseting the trip meter didn't do anything, not that I thought it would.

The CEL isn't on, but I'm still getting weired AFRs under boost. If the rear sensor is bad, why would the AFRs go nuts when the AFR #s are coming from the front O2 sensor?

 

did you check the codes with something other than evoscan? you can get them pulled with an actual code reader at autozone for free

 

Not yet. I downloaded and installed the ScanTech program last night, but didn't have a chance to try it. Besides, the check engine light is off now, so I'm guessing it would be uneventful to check codes.

Any thoughts on non-OEM O2 sensors? I have found a couple direct replacement versions that come with the connector and the cable grommet. They are around $80, which beats the dealership price of $330, or online OEM of ~$250. I just don't want to risk the money when I'm not sure what the problem is.

 

So, after running some more logs and comparing them to old logs, I have found that the front O2 sensor voltage is diverging from the rear O2 sensor, TPS, and Load readings. Generally speaking, these all follow the same contour (and should). The front O2 is reading up to 18 AFR under boost, and I'm getting only 1 knock reading every so often. I'm thinking that the front O2 sensor may have gone south, because if I was really running that lean (due to low fuel pressure or something), I would be some some other weird behavior like lots of knock, increased injector pulse width, wild fuel trim adjustment #s, etc... Everything other than the front O2 reading looks normal.

Now that I think of it, I was down at the track and blew a hole in an IC hose. I had to drive a couple hours back to my house running wicked rich, so that might have fouled the front O2.

 

try disconnecting the battery and reset the ECU

 

So....after many long weekends, I finally have the car back together and fixed! It turns out that it was indeed the front O2 sensor - it was fouled.

When I tried to get it out to "check" it (using an O2 socket tool of course), I had to put so much force on it that I rounded the hex nut!! I ended up removing the two heat shields, high pressure AC line, exhaust manifold, bottom pastics, lower IC pipe, down pipe. I had a vise grip on it, but to no avail. I had to remove the turbo exhaust housing that holds the sensor, took it to work, put it in a vise, and used a freakin' huge pipe wrench w/heat to get it out. Then I proceeded to melt it with a torch .

Anyway, she's all back together and acting normally again!!! Plus, I just passed emissions today! Thanks for all the advice guys!