BP9 Legacy 2.5i stuttering on heavy acceleration

[Colourfyre]

Hi,

 

I have a 2005 Legacy 2.5i- its got the 2.5l SOHC engine and has done about 240k. Recently I have found that if I am going down the road at 80-90kph then try and accelerate hard (pedal perhaps 90-100% down) (to pass a slow car for example) the car just stutters and gives no power. The rpm needle flutters somewhere in the 4000-5000 range and the only way to get power is to take  my foot off the throttle and slowly accelerate (even like this there is sometimes a hint of roughness). This condition makes it sometimes difficult to pass slow cars- especially in places where passing lanes are uphill etc and all power is needed.

 

Other then this odd time the car is fine, acceleration from a standstill is fine (although I haven't tried hard down). Driving the car normally- its only sign of fault is hard acceleration from a decent speed. I have checked for codes and there is nothing, check engine light has never come on. Fuel consumption, according to the car is approx 7.4l/100km, this is what I normally get (before this problem started).

 

The air cleaner is a new and the front oxygen sensor is about 20k old. I have checked the air intake etc (when replacing the filter) but nothing. 

 

I have been going through the manual with not much luck yet- I thought someone might have an idea.

 

Any suggestions would be greatly appreciated.

 

Cheers,

[SpeedySub]

Manual or Auto?

Also have you tried cleaning the Air flow meter with the correct cleaner?

 

[Colourfyre]

Hi SpeedySub,

 

Its an auto- the 4 speed tiptronic box. That was one of my first thoughts (the MAF)- but it doesn't seem to have one, from what I have read its got the manifold absolute pressure sensor instead, I had a look at this sensor and it looks fine, I am trying to find a diagnostic page for it in the manual now.

 

Thanks for the reply,

 

Cheers

[SpeedySub]

Where are you located. I have a wifi OBD2 reader, only shows codes but would be what you need to see if theres anything wrong.

[JGM]

Try the air fuel ratio sensor... I had similar symptoms in my old 2.0R.

[Colourfyre]

Thanks for the offer Speedysub, I have FreeSSM and have just spent the last hour looking through data- no trouble codes or stored codes at all unfortunately. 

 

Thanks JGM- I will have a look in the book and see if there is any diagnostic info for that sensor.

 

I did notice that the learned air fuel ratio was sitting at 13.3% on idle and wondered if maybe the old air cleaner was quite bad and so the car had learned to compensate for it- then on changing the air filter, it hasn't re-learned yet. I have no idea if this is even possible but- did notice an improvement in fuel economy on the air filter change. I cleared the cars memory and took it for a spin (with a couple of restarts(I think I have read something about it using three runs to learn fuel trims etc) and notice that the learned air fuel ratio is sitting close to 0 on idle. 

 

I guess this is probably barking up the wrong tree but it will be interesting to see if it returns to anything like 13% again.

 

Cheers

[Colourfyre]

A bit more info:

 

The learned air fuel ratio has returned to 13.3- but ti took a while. The performance has deteriorated and so I pulled the ignition leads to test (I figured it was an easy test) and the one on the front passenger side cylinder was completely poked. Apart from obvious signs of corrosion and black powder falling out of the sockets (at both ends) the lead shows nothing in testing with a multimeter- no continuity at all (tested right up to the highest resistance range).

 

Does anyone have opinions on spark plug leads?

Options seem to be the NGK ones (SPO2461935) from super cheap for $157 or the Bosch ones (B4777i) from partzonline for $177

 

I figured spark plugs might as well be done at the same time (I don't know when they were done last, likely not recently, but i will pull one and see what its like) the supercheap website recommends an NGK one for $52 each- partzonline recommends the same but also the much cheaper Bosch double platinum for a much more reasonable $16.09 each. The workshop manual (as far as I can see) recommends Champion RC10Yc4 which are only $8.21- at this price though replacing them 4 times as often doesn't seem bad at all.

 

What's the go on this? I have read a lot of stuff that suggests staying away from the Bosch ones in Subarus... but at 52 each the NGK ones seem extremely pricey! 

 

Thanks,