I just picked up an s5 gxl with a s5 turbo 2 swap in it from a forum member a couple of weeks ago. As far as I can tell the ecu is a stock t2 n370 no stickers or labels anywhere on it other then the factory labels. It has an aftermarket defi boost guage on it and under boost it spikes up to 14 psi and then winds down to about 10 psi and I can't see a fcd wired in anywhere.

It has a cone filter and catback but stock downpipe. The main issue is when under WOT under 3000 rpm it sputters and can't boost. Then above 3000 rpm when boosting you can hear the turbo spooling hard but the car feels like its hesitating and then at 3600 or so rpm the car starts to pull and run normally?

I'm thinking maybe the tps is out of adjustment? I just dont see how its running 10 psi and not hitting fuel cut? The boost gauge goes to a solenoid, is it possible that the guage is reading incorrectly?

 

So I got around to checking my tps last night, and it was at 2 volts at idle, after adjusting I found that the previous owner had attempted to adjust the tps without adjusting the idle speed. The car now idles perfectly without any missing or idle hunting. It still didn't fix my boosting issue though. I'm thinking that maybe its a boost leak somewhere?

 

Check the AFM and MAP (or I think as Mazda calls it, the boost) sensor.

Also sounds like your secondary injectors may not be operating. Get a multimeter that has %duty cycle and hook it up to one of your injector wires and +12v and see if the ECU is telling it to operate. They aren't supposed to operate until around 3800 rpm, but the tach might not be as accurate after 20+ years.

 

I think the previous owner may have messed around with the adjuster screw on the maf, it's turned clockwise all the way?

 

Adjuster screw? I didn't know the S5 AFM has an adjuster screw. The BAC does, but I don't know a thing about S5's.