I have had a Magnex round backbox and a K&N panel air filter fitted to my MY00 estate turbo. It has no other mods. I noticed on the way home that in 3rd, 4th & 5th from about 3500 revs the boost seems to come in and then go away and then come back etc all the way on and off to the red line?

Does anyone have any idea what this may be due to. I thought it may be crap petrol so I filled up with BP expersive stuff (can't remember the name) but this does not seem to have made any difference.

 

The BP expensive stuff (Ultimate?) is appauling. Won't help anything... use Optimax or Esso SUL if Opti is tricky to get hold of.
A spot of clarification please - does boost come & go, or does it just feel like it's doing this? Do you have any available monitoring kit? Boost, AFR & knock would all be handy things to know about at this point!
Also - just wondering how your idle is? Another potentially helpful pointer...

 

As nom touched upon, if the boost is coming and going, it's more than likely your wastegate solenoid. If it just feels like it's coming and going (hesitant acceleration), it's probably your MAF.

 

I don't have any way of telling whether or not the boost is coming and going as the car doesnt have a boost guage. I assume a subaru garage will be able to tell?

Is this a coincidence that it happened a few hours after fitting the exhaust and filter? Or has this caused it?

Oh, and what is the MAF? Sorry to be dim.

 

No monitoring kit unfortunatley! Idle is fine, and it seems to be OK in 1st and 2nd.
Do you think it is the exhaust and filter combo or is that a coincidence?

 

Sounds like a coincidence. Personally, I'd try giving the wastegate solenoid a clean. You could then reset the ECU as well, just to be certain the MAF is OK.

MAF is Mass Air Flow sensor, by the way.

 

The exhaust & filter change will lean out the mixture a bit, especially if the ECU has been learning around a failing MAF (it's guesses for the correct amount of fuel to be added will be based upon the less free-flowing exhaust/filter, & more fuel will be needed now).
I'd reset the ECU (instructions are kicking about somewhere, or you could just leave the battery unplugged overnight!) & see how the idle is doing. My guess is that it won't be too good! Hunt out the MAF & unplug it. The idle should improve... in which case you need a new MAF!
By the way, it's very bad indeed running with a failing MAF... and your local Subaru dealer won't be able to diagnose it either.

 

Could be as simple as boost spiking due to better flow since your mods & the car is fuel cutting, This can seem like hesitency

Dean

 

Not quite so gentle, though
First time I had it, I thought I should probably check that the engine was still in the car - not quite 'hesitancy'!

 

When i fitted my decat, I was quite puzzled for a while I had boost power fluctuations, Turns out it was Fuel cut, Boost was being pulled back for a few seconds when it cut & all it felt like was hesitency, It does not always come in with such a Kick, My money is on that, Get a boost gauge fitted & you will see it draw back for a few seconds

Dean

 

Fuel cut is cut of fuel (hence the term ). The car simply stops accelerating momentarily, which, considering you will have been booting it at the time, feels like you've driven into something head on. There's a distinct "thud" - quite scary.

Boost fluctuation/oscillation/hesitancy is totally different. Fuel is still flowing, the car is still accelerating, just not with "the usual" boost pressure.

 

I had similar symptoms to you in my MY00, then got CEL. I had the MAF changed a coule of days later (5min job). Problem solved.

BTW Stick with Optimax or ESSO Super if you can, BP gets a bad press all round on the SN.