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Old 09-30-2012, 07:12 PM   #1
sublinux
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Default Low rpm bucking

Please read, i did alot of research before posting.

Here my story. I bought a 2.5i 2008 2 month ago. Everything was fine until a week after a bought it. When idling the RPM is stable but is not constant and when acelerating until 3000rpm it start buckling like a missfire but after that is fine.

What i did:
New spark plug
Cleanning TB and MAF/IAT
No CEL
Reset ECU and lets the car idle
Unplug first o2

When the o2 was unplugged yes there was a cel on but the car was running so good. So i was wondering if it was a bad o2 ? The car have 106 000km
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Old 09-30-2012, 07:17 PM   #2
Counterfit
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Could be that, even though it's a bit expensive to try without knowing for sure.
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Old 09-30-2012, 09:48 PM   #3
GrundleJuice
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The symptoms of poor performance below 3000 rpm or so and normal after points to the o2 sensor. The test of unplug going out just confirms that it is almost certainly the o2 sensor sending bad data to the ecu.
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Old 10-01-2012, 03:13 AM   #4
sublinux
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I am surprise that only after 106 000 kilo i got a bad o2.. And what the second o2 (after the cat) do ? Confirm the first one ?
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Old 10-01-2012, 03:17 AM   #5
Counterfit
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I think it's there to verify that the cats are working.
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Old 10-01-2012, 07:04 AM   #6
sublinux
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Would the ecu will not see that the first one isnt working properly?
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Old 10-01-2012, 09:40 AM   #7
Bababou
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Quote:
Originally Posted by Counterfit View Post
I think it's there to verify that the cats are working.

^Correct


Havent tried myself but there are options for cleaning the o2. Has anyone had any luck cleaning one and having it work again?
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Old 10-01-2012, 10:40 AM   #8
GrundleJuice
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I've never heard of cleaning the sensor actually working. Usually it's not that it's dirty or contaminated.

The rear o2 monitors the cat and has some secondary (for lack of better term) input on fueling.

Last edited by GrundleJuice; 10-01-2012 at 09:56 PM.
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Old 10-01-2012, 04:56 PM   #9
sublinux
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Ive just bought a new sensor on RockAuto for 100$. Its a Denso 234-9123 (Same as OEM). Ill give you some feedback next week.
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Old 10-01-2012, 07:56 PM   #10
Bababou
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Quote:
Originally Posted by RaceFaceXC View Post
I've never heard of cleaning the sensor actually working. Usually it's not that it's dirty or contaminated.

The rear o2 monitors the car and has some secondary (for lack of better term) input on fueling.
Good to know, thanks.
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Old 10-01-2012, 09:54 PM   #11
GrundleJuice
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Quote:
Originally Posted by Bababou

Good to know, thanks.
Just FYI, the rear O2 only has input on fueling in the 32-bit OBD-II N/A ECU and the CAN N/A ECU (05-07, 08+) as far as I know.
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Old 10-09-2012, 05:47 PM   #12
sublinux
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Hello,
i received my new O2 and ive just removed the old one. Not so dirty but just been driving for 15 minutes and the problem is still there.. Im out of idea..
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Old 10-09-2012, 08:25 PM   #13
GrundleJuice
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Quote:
Originally Posted by sublinux
Hello,
i received my new O2 and ive just removed the old one. Not so dirty but just been driving for 15 minutes and the problem is still there.. Im out of idea..
Did you install the new one? If so, does unplugging still it fix the problem? Did you reset the ecu? If not, try that then start it and let idle for 5+ mins without touching the gas, a/c or anything electrical that might put a load on the alternator like power windows or headlights or stereo turned up loud. Then take it for a drive and see if anything changed.
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Old 10-09-2012, 10:39 PM   #14
rjh2pd
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I am having the same problem, but mine only happens at. Mine only does it every once in a while mostly under a decent load at low rpm (less than 2000) and when it gets to about 4000 rpm it starts bucking again.I have an 03 legacy with 170,000 miles. I replaced the spark plugs and it seemed to help for a week or so. Only one of plugs was in real bad condition though. I already have an o2 sensor just need to put it in. (got a cel for low heater current on the front o2 sensor). any help would be greatly appreciated.
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Old 10-20-2012, 04:19 PM   #15
sublinux
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Quote:
Originally Posted by RaceFaceXC View Post
Did you install the new one? If so, does unplugging still it fix the problem? Did you reset the ecu? If not, try that then start it and let idle for 5+ mins without touching the gas, a/c or anything electrical that might put a load on the alternator like power windows or headlights or stereo turned up loud. Then take it for a drive and see if anything changed.
Update: The new one is install, reset ECU, adding some chassis to battery and engine to battery 8 AWG. The problem is still there. On idle you can feel it very well the RPM Gauge is constent but you can see the motor shaking and the shifter too. Here a video of the shifter

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Old 10-23-2012, 07:36 PM   #16
sublinux
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Bump!
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Old 10-23-2012, 07:43 PM   #17
easterbran
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I had very similar bucking and hesitation issues with my 05 Sti...turned out to be a bad fuel pump at 155,xxx miles. I then replaced it with a Walbro 255lph pump and good as new ERZ Performance on NASIOC has a thread with a discount code and free shipping if you're interested
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Old 10-23-2012, 07:51 PM   #18
GrimmSpeed
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I think you would have realized it when checking the plugs but it sounds like it could be the symptoms of a bad valve cover gaskets around the spark plugs. It's common for them to go bad and when they do oil can pool up around your plugs and cause a misfire. I've had this happen on a couple of motors all with the same symptoms and all were fixed after replacing the gaskets

Mike
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