Looking for advice on an odd issue. I've spent a lot of time googling and reading up on posts that seem similar, thought I'd come here and ask for advice before trying anything.
I don't have a garage (on street parking, on a steep hill) so limited a bit in what I can do to the car, but happy to pay a mechanic to do a fix if I can point them in the right direction, but I don't want to throw money at it without a steer.
A friend suggested replacing the fuel filter - I haven't had that done yet.
Problem is that the boost "surges" or cycles when under load. As an example in 4th or 5th gear going up a gradual incline and accelerating, the boost will cycle from a strong pull to less then back again, with a cycle time of about 1sec on/off.
When under full throttle at lower speed it is nearly impossible to notice, it could be surging but I can't tell. There's no surging at idle or normally, just when you're boosting under full throttle at higher speed.
The car is an 04 legacy GT BP5 spec b 5eat.
The problem goes away temporarily if I reset the ecu. Sometimes it comes back within 100km, sometimes longer. The IAM is typically about 0.900 but has been lower if I leave it for a long time (months) without an ecu reset. The worst I've seen it was a year ago at 0.560.
The turbo was replaced about 6 months ago with a new one. I've replaced the o2 sensor on the down pipe. Neither of these made any difference to the surging, but the new o2 sensor did help with turbo lag off the line, and the new turbo of course has no whine.
Everything else is stock, aside from when I bought the car a few years ago it had a pod filter, but I've since replaced this with the factory intake (fortunately provided by the seller). The surging happens with the pod filter too - I've tried putting the pod filter on, resetting the ecu and waiting to drive several 100km - the surging came back.
I run it on 98 most of the time, only dipping into 95 if on a holiday somewhere really remote for awhile. Tbh, I don't notice a huge amount of difference between the two, although I track my mileage and I get a bit better mileage out of 98 (~600km open road before the fuel light comes on). I thought 98 by npd made the car feel slightly rough so I've since avoided that when on holiday down south. I live in Wellington.
I have btssm so happy to do any logging if that would help.
Thanks a heap in advance for any advice!