I have fitted a Magnex downpipe to a MY96 Impreza and now I am experiencing a slight hesitation just above 4000 revs. The boost drops off slightly then comes back. I have seen no check warning light etc, I was woundering could it be the lamda sensor as this is the only thing I have touched and does the lamda sensor register a fault code on the ECU ?
The problem is intermittent but started occuring more frequently and only happens in 4th and 5th and once or twice in third and always just over 4000 rpm.

I am pulling my hair out over this so any information or advice will be appreciated.

 

I have fitted a magnex twin dump downpipe to my 95 WRX no propbs at all goes like stink the only thing i can advise is if the engine check light isnt on check spark plug gaps as once i had Nippon DENSO istalled with a 0.6mm gap and had similar problems so installed NGK pfr 7B and no probs dont know weather yours has coil packs on top of each plug? if so worth checking them also when out pull back rubber and check the plastic it usually breaks
hope this gives you a starting point
mark

 

It sounds abit like your car is overboosting but only very slightly.
This will occur in 3rd, 4th and 5th gear in and around the 4k area (where you get your max boost).
The best way i have been told about (and it does work) is take your car on a long slightly up hill road and floor it in 3/4 or 5th gear.
If you get the hesitation (slightly more than normal) then its overboosting.

Tony