Hope someone can halp...? Basically bought my my99 impreza turbo a couple weeks ago, I filled up with sainsburys premium fuel and started having poor boost etc.. Bought some millers octane boost and filled up again with shell optimax and problem seemed to go away until tank got low again? Filled up again with optimax but still not right! Not every time but most of the time under load it will judder and missfire and it wont boost to normal psi. It feels like a huge power loss then every now and then it will drive like it should.
It's over due a service and is now booked in next week but i'm after a answer for this? It's running a k&n induction kit so could it be- Maf sensor, fuel filter, plugs etc? Any help on this one please!!

 

sounds like plugs mate, get it checked out

 

To be honest thats what i'm hoping with it due a service! Booked in next Monday so may buy some quality plugs before hand to get fitted. Any recomendations? Cheers

 

If your car had been previously mapped on say v-power, filling up with optimax would give you the issues you're referring to. Try v-power or Tesco99 (Momentum) and see if problem goes away

 

Previous owner only ever ran it on shell optimax? I may give the tesco fuel a go though anyway. Thanks for opinion.