I've noticed that my 06 JCW's boost is surging when between 3500 and 4500 rpm at medium throttle, between 5 and 10 psi. Lesser throttle or full throttle or rpm range it doesn't.

The only mod on the car is an oil catch can kit.

Can anyone explain to me what is happening and what to look for?

Thanks...

 

mine does the opposite at light throttle. it un-surges, and Drops from 10 to 5 psi, unless i give it more gas.

so, you may be having the opposite effect, where the bypass valve is closing and the air fuel mixture is "better", resulting in said surge.

 

Ok. What controls they bypass opening/closing then? I've got an odbc reader, so is there anything in the ECU I can monitor that would tell me what is happening?

 

oooh wait! you have the JCW, duh. do you have the "flap mod"? with the stock airbox? bc, if you Dont, your airbox's flap is opening at 4200 rpm, gushing more air into the engine, and its probably causing the BPV to close.

as for how a BPV works, google it, and go to wikepedia. theres a good animation / drawing that shows the concept.