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2004 Forester Sti 6 MT
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727 Posts
Discussion Starter · #1 ·
A couple of weeks ago I started getting weird slight on off acceleration when cruising around town in 30mph sections.

It was quite subtle but definitely noticeable, apart from that the car drove fine.

So.. hooked up romraider and get these weird oscillations from the front lambda sensors.



Anyhow, I reckon it is the front O2 on its way out and reacting slowly (maybe too coated with gunk) which is causing the ECU to over and then under correct.

Looked on FAST for the O2 sensor and got

22641AA450 which seems to be an FSTI only part with no ITCA alternative on my version of FAST.

Phoned up Subaru parts and their version of FAST is recommending an alternative part 22641AA211 which is much easier to source as it is the 2.5STI sensor from the states.

£299 from Subaru parts but a quick search on scoobynet (and this forum)

Lambda for 06 hawk sti - ScoobyNet

http://www.subaruforester.org/vbulletin/f75/fsti-front-lambda-sensor-help-75474/index3.html

shows you can get the identical part by looking for the DENSO part number.

Just ordered it from here:

Subaru Forester, Impreza Lambda / Oxygen Sensor | World Car Parts > Store

they had it in stock and I should get it Weds, so I'll update if this fits and fixes it.

Al
 

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Emeritus Forum Staff
2015 Triumph Tiger XCx 1 Down. 5 Up.
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5,242 Posts
Seems like the most probable thing - especially as I imagine you were in closed loop based off the data. Fingers crossed a new sensors sorts it!
 

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2004 Forester Sti 6 MT
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727 Posts
Discussion Starter · #3 ·
Yeah, the CL/OL is logged in orangey colour, shows it is in Closed Loop.

I also logged stuff like battery voltage and fuel pump duty as I thought it was some fuelling issue.

However, when you look at the graph it is so regular (oscillates around every 1.3 seconds) I wondered what it could be and I couldn't find any posts where people had similar issues as it seems for most the sensor just fails.

So I'll see if the replacement sorts it out.
 

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2004 Forester Sti 6 MT
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727 Posts
Discussion Starter · #5 ·
The slight on off acceleration around 30mph is exactly what mine was doing (amongst other things) just before I replaced the fuel pump and fixed the issue. It was more noticeable on a light throttle (or coming downhill at around 30mph)

http://www.subaruforester.org/vbulletin/f75/fuel-pump-after-all-251114/

Let's hope yours is due to the sensor though and the new one fixes it
Jeez, just read the thread it might well be that!
I have already installed a Walboro pump and new fuel filter a couple of years ago but I think the Walboro pumps look less robust than the Unijecs one that was in it already.

The weird thing is that the car is fine during open loop acceleration which I thought would be more taxing on the fuel pump that the cruise (although during cruise the pump is only shown running at 33.3% duty so might be an efficiency issue.

I have a wideband installed as well but sold my old laptop with the serial connector so have ordered a different connector and can compare it then with the output from the LC-1.

One option is to swap back to the OEM Unijecs pump and see if that fixes it.
Pretty straightforward swap but I'm just not a big fan of messing around with petrol!
 

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2004 Forester Sti 6 MT
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727 Posts
Discussion Starter · #6 ·
Holy cr*p sipie,

Just seen what you went through:
Another change of plugs
New maf
New o2 sensor
New HT leads
New coil packs (not sure how many were changed)
Compression tests
New fuel pressure regulator
New knock sensor
Subaru mapper re-loaded standard map
All data sent to Subaru uk etc
...and it was a fuel pump!

OK, so this weekend... new o2 sensor in (compare it with my wideband) then if that doesn't fix it, put the old fuel pump in and if that fixes then order another walboro!
 

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2015 XT CVT.....sadly !
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1,199 Posts
Holy cr*p sipie,

Just seen what you went through:


...and it was a fuel pump!

OK, so this weekend... new o2 sensor in (compare it with my wideband) then if that doesn't fix it, put the old fuel pump in and if that fixes then order another walboro!
Luckily all the initial replacement stuff (the HT leads is an error on my part as they don't have them !) was done by the selling dealer, but the remap and fuel pump were at my cost.

Really hope you get yours resolved soon, nothing more annoying than a car than just doesn't run as it should. Good luck.
 

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2004 Forester Sti 6 MT
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727 Posts
Discussion Starter · #8 ·
Well it looks like the replacement sensor solved the problem.

Fingers crossed!
 

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2008 GRB 20th anniversary 6 MT
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6,591 Posts
Good everything seems OK Alistair!
 

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2004 Forester Sti 6 MT
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727 Posts
Discussion Starter · #10 ·
The installation wasn't too bad either, partly because I am running not undertray so easy to get to.

After trying an adjustable spanner I did bite the bullet and head on down to Halfords to buy a 22mm spanner which I think it vital to get the purchase you need.

The little plastic pegs that hold the sensor cable in place are an absolute pig to get out so I ripped them apart needle nose pliers

Most importantly I didn't need ramps or stands which makes it a lot simpler.

I would watch out for any on off throttle behaviour when cruising, if I had changed mine earlier no doubt I would have saved myself a few £ in petrol!
 
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