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Old 10-25-2004, 03:49 PM   #1
ShawnRS
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Default MY02 eratic rpm while drving

Hello

MY02 Impresa
TWE headers, High cat, custom piping as magna flow to the tail.
air/Fuel sensor replaced two months ago.

I have had a P0136 "Rear oxygen sensor circuit malfunction code" for about two months. The shop I deal with says don't worry about it


This just started this morning on my way to work. Driving along at a constant rpm (Doesn't matter, all though the band)). The rpm changes 500rpm +/- (up and down) or feels like it wants to stall. This while my foot is in the same position (HWY driving)

I pulled the neg off the battery to see what code if any pops up next. The car still drives like above mentioned.

Any other ideas? Anyone think the rear 02 has something to do with it?

Did a 5 hour drive yesturday everything was fine. Nice easy driving, nothing fun.
Thank you much.

SHawnrs
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Old 10-26-2004, 01:34 PM   #2
darioc
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if the car is getting all crazy and you have a cat and your getting that code, the o2 sensor is fouled, which means you will have to replace it. a failed o2 sensor would cause the car to run very odd.
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Old 10-26-2004, 03:11 PM   #3
Section 8
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The rear "oxygen sensor" has nothing to do with how the engine runs. Make sure that you are checking the FRONT sensor.
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Old 10-26-2004, 04:01 PM   #4
ShawnRS
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I'm so confused.

50% of the "experts" including mechanics say that the O2 effects the cars performance and the other half say it doesn't. Does anyone have anthing I can read that proves which it does?
I have talked to a lot of people outside of the forums.

PO 136 is the rear sensor( after the cat) right? Plus I just replaced the Front sensor (before the cat).

Hopefully I can keep driving it for a couple of more days. Then I'll have time to check things out.

Thanks all
Shawnrs
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Old 10-26-2004, 04:08 PM   #5
Section 8
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Quote:
Originally Posted by ShawnRS
I'm so confused.

50% of the "experts" including mechanics say that the O2 effects the cars performance and the other half say it doesn't. Does anyone have anthing I can read that proves which it does?
I have talked to a lot of people outside of the forums.


Probably because there are 2 sensors, and 1 of them does something. The front one feeds the ecu with information to adjust fueling. The rear one checks the condition of the catylist, nothing more. Cars with 1 O2 sensor, will depend on it for fueling. Cars with an O2 in each cylinder bank, probably depend on the first two (Mustangs have 2 rear O2 sensors, so I would imagine that they have 2 front ones as well).

By taking readings after the cat, the oxygen left in the exhaust from the engine is affected by the cat as it burns left over remnents of the combustion process. Thus reducing the ammount of oxygen in the exhaust further than it was when it left the engine, making you belive that the car is running a heavier mixture than it actually is.

cheeRS,

Greg

Last edited by Section 8; 10-26-2004 at 04:21 PM.
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Old 10-26-2004, 04:16 PM   #6
Section 8
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If it makes you feel any better about it...

Either my car was built by God, or the rear sensor is useless to the ECU for anything else than cat monitoring, because I have been driving my car for a few months now WITHOUT a rear sensor.
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Old 10-26-2004, 04:17 PM   #7
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Better yet, call TWE and ask them about your rear O2 sensor. THey are the ones who built my header with out a rear O2 bung.
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Old 10-31-2004, 02:38 PM   #8
ShawnRS
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Okay so I changed out my spark plugs. They looked normal when compared to the pictures. The car ran fine for about a 50km (my round trip commute). The code reads the same as before (po136). On the way into work, the next day, the car ran okay but not as well. The return home was the hesitation nearly stalling session that I experiecned earlier in the week.

Of course when I took it to my mechanic (he read the code) the car was running normal.

So I will be hunting down a y-pipe. Taking the headers off, for now. I'll clean the 02 sensor (not likely to work). This will be my 2nd o2 sensor in the 10 months since I bought these headers.

Section 8: What year is your 25rs? Maybe my02 ecu doesn't like not running with a malfunctioning sensor? My car ran well for about ten months with different Front and rear o2 codes. Had to replace the front recently. Then I was getting this po136 code and my fuel useage increased and now the car is misbehaving. Three weeks ago I had my 72k maintance performed on my car.

Shawnrs
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Old 10-31-2004, 03:07 PM   #9
Jejunum
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I had this problem. my timing belt tensioner went bad (noise!) and was replaced. problem gone. related or unrelate dunno...
http://www.gtv6.org/gtv6faq.htm#tensioner
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Old 10-31-2004, 05:46 PM   #10
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Quote:
Originally Posted by Section 8
If it makes you feel any better about it...

Either my car was built by God, or the rear sensor is useless to the ECU for anything else than cat monitoring, because I have been driving my car for a few months now WITHOUT a rear sensor.

I don't have the front OR the rear in...
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Old 10-31-2004, 06:05 PM   #11
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Sounds like when my Knock Sensor went bad...... And I didn't get a CEL code for 2 weeks. Then I got the code, changed the sensor, and has drivin perfect ever since. (20k miles).
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Old 10-31-2004, 06:06 PM   #12
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Quote:
Originally Posted by jasona
I don't have the front OR the rear in...
And it runs ok? WOW!
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Old 10-31-2004, 06:08 PM   #13
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Question

Try tightening your knock sensor down to 17ft lbs. if its looser than that. Might help.
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Old 12-01-2004, 09:55 AM   #14
ShawnRS
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My Mechanic final gave up and replaced the Front (AF) O2 sensor and the spark plug wires. The car is now running but still idles rough, in my opinion.

So we'll see what happens.

Thanks all
Shawnrs
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Old 12-09-2004, 06:10 PM   #15
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Quote:
Originally Posted by ShawnRS
My Mechanic final gave up and replaced the Front (AF) O2 sensor and the spark plug wires. The car is now running but still idles rough, in my opinion.

So we'll see what happens.

Thanks all
Shawnrs
Your shop's initial statement about the P0136 is correct. The rear O2 sensor is not the cause of your problem. Concerning the P0136 code, your rear oxygen sensor is either bad, you have an exhaust leak, or something is wrong with the wiring. For the WRX, the rear oxygen sensor does not affect the driveability of the car. It is there to monitor proper operation of the catalytic converters. A MIL eliminator/resistor fix will not fix the P0136 code because it requires a working sensor and more than likely you will have to replace the oxygen sensor to fix the CEL. The rear sensors cost ~$110-120 new from a dealer.

Quote:
Originally Posted by Section 8
Better yet, call TWE and ask them about your rear O2 sensor. THey are the ones who built my header with out a rear O2 bung.
All headers are built without a rear oxygen bung because the rear oxygen sensor is not located in the header. The first bung in the header itself (at the collector) is the front oxygen sensor, and the bung in the uppipe is for the egt sensor. The rear oxygen sensor is in the downpipe/midpipe (depending on your exhaust setup), after the 3rd cat, or where its supposed to be if you dont have one. You can still drive the car without the egt sensor and the rear oxygen sensor but you will throw a CEL for them.

Based on the erratic rpm description you gave in the first post, I do not think the front oxygen sensor was the cause of it. A gunked up or bad front sensor would throw its own CEL. Also, a bad front sensor would render the car virtually undrivable; it'll stall as soon as you start it unless you rev like crazy to keep it running but you wont be able to move very far and you'll be blowing thick black smoke out your exhaust.
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Old 12-14-2004, 12:31 PM   #16
Section 8
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Forgot about this thread.

I have a 2001 RS. So in all reality it could be different, but most people believe that the 2000 to 2004 are equlivant.

Jasona likely is using another form of EM other than the stock ecu or it isn't running (if he is even talking about his 2002 Subaru). I have had a car with out an O2 as well.


Quote:
All headers are built without a rear oxygen bung because the rear oxygen sensor is not located in the header. The first bung in the header itself (at the collector) is the front oxygen sensor, and the bung in the uppipe is for the egt sensor. The rear oxygen sensor is in the downpipe/midpipe (depending on your exhaust setup), after the 3rd cat, or where its supposed to be if you dont have one. You can still drive the car without the egt sensor and the rear oxygen sensor but you will throw a CEL for them.

This is where I will caution you to avoid using exclusive words. Using exclusive or inclusive words leaves no room for anomolies (you have heard the expression, "never say never", or "there is an exception to every rule", I imagine).

You used the word "all". Meaning that there is not a single header that would be made with two O2 sensors, because the second O2 sensor is on a second section of pipe, that would not be part of a header. Have you ever seen a header that is long enough to connect the heads to the midpipe eliminating the cat section (with the second O2, requiring the placement of a second 02 bung if it were necessary)?

I have because I own it. Many other peope on this site have seen it, because I have posted it. You may have also and just forgot.



1 O2 sensor in the collector which ends exactly at the midpipe (I had to lengthen the front O2 sensor ~3ft for it to reach). THere is only provisions for one 02 sensor in the entire exhaust.

Also, with your description of the exhaust it sounds like you own a WRX. There is not a up or down pipe in a naturally aspirated Subaru. There is also only 2 cats, not three.

Last edited by Section 8; 12-14-2004 at 12:37 PM.
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Old 12-20-2004, 03:40 PM   #17
nqwan
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Quote:
Originally Posted by Section 8
Also, with your description of the exhaust it sounds like you own a WRX. There is not a up or down pipe in a naturally aspirated Subaru. There is also only 2 cats, not three.
DOH! I forgot what forum I was in so now I must apologize and retract my statements. I was indeed referring to the WRX.

Now that I think about it, I dont even recall how I came across this thread.
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