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Having a very strange problem with my 06 STI Forester.

It is a stock EJ257 with:
- a custom turbo back exhaust
- cold air pickup but still has stock airbox.
- custom tune.

The problem is that it sputters / bogs down from idle to about 2800-3000rpm. After this it revs fine. When it does this it sorted sounds like its farting and struggles to build revs

I was thinking it was the MAF or 02 sensor, but I've changed them with other ones that I had (not new ones though) and nothing changed. I've even run with them unplugged (one at a time) and still no change.

What's weird about this is the following:
- i'm cruising along at, say, 80km/h (55mph) in 4th - I then put the clutch in and let engine drop to idle. Try to rev it and engine does its bogging/sputtering carry on.
- then put gearstick into neutral and engine will rev cleanly with no bogging/sputtering whatsoever.
- put back into any gear and engine will bogg/sputter again.

So if it was a sensor issue it shouldn't matter if its in gear or not??
Does the neutral position switch alter the map that is used by the ECU?

I am still thinking that this still could be a bad 02 sensor as the engine could well be going into open-loop around 3000rpm.
But doesn't explain why it all works when in neutral. Does the engine go into open-loop when in neutral??
If its not an 02 or MAF then i'll need to get in touch with my tuner (which is a little difficult as he is in another state).

No CEL's (except when I unplugged the 02 / MAF). I can't give you a log as I have no way of recording one. All I have is freeSSM as my ECU is a JDM one and doesn't work with OBD2 readers and no one has COBB stuff here in Australia.

Things I've tried/changed:
- Crank pos sensor good.
- changed plugs & coils of a Forester XT.
- swapped MAF & 02 of a Forester XT.
- Don't appear to have any vac or turbo leaks.

I have been researching this for a few months now and I've read hundreds of posts about bogging, hesitation and misfiring etc but none have described my issue. Any help/suggestions appreciated.
 
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