Before anyone asks, yes i've done the usual searches but I couldnt find anything definitive
Background: Stock Rb25det NEO. standard intake, exhaust, boost and ecu.
Engine shuddering or hitting fuel cut whenever WOT and high boost (bucking like a mechanical bull), other than that, it ran fine. boosted down low ok, seems to run smooth(ish). didnt run rich, was getting on average about 450k's per tank
Replaced spark plugs with NGK coppers (1.1mm). Car then had absolutely no torque down low, would backfire constantly when cold or after WOT (which was necessary to get the friggin thing to move) and wouldn't rev over 5000rpm. It felt like it hit Rich and Retard. No codes were thrown
I noticed that the boost gauge was reading a little high after the plug change, and found a vacuum line from the charcoal canister had frayed and cracked causing a vacuum leak. Replaced this and boost reading went back to normal.
I then replaced the Airflow sensor (2nd hand from wreckers), Coilpacks (splitfires), Coilpack lead (Genuine) and Sparkplugs (NGK BKR6EIX-11).
This made a HUGE difference. the old coilpacks were likely from the factory and were just producing a weak spark.
Reset the ECU, still ran rich when cold (but not as rich), low-down torque came back but still cant rev over 5000rpm (feels like the engine just goes back to the R&R state)
It does backfire a little when cold or after WOT (more like a few pops instead of a full blown backfire). Still no codes thrown
Does the ECU lock out the top 2000 RPM when its 'learning'? or is it something else?
What should I be looking at next?
O2 Sensor?
Replace/check all Vacuum and Intake lines?
Cam angle sensor?
Really running out of ideas here