Lack of boost, P2177 fault code

[m500dpp]

2007 Cooper S turbo, 74k miles. Occasionally won't boost over about 3000 rpm, but at other times it's fine right round to red line. Starts and runs fine. EML lit up yesterday P2177 idle mixture too weak bank 1. Turned it off and it stayed off after a short drive.

I have 2 suspects, HPFP and turbo (wastegate worn). Can anyone help me diagnose this further, I'm reluctant to sort it by replacing expensive parts that may not actually be the problem, after an extensive search I'm not finding anything really definitive on the web.

[Old_and_shagged]

Hi - sounds like your ecu is sensing a possible overboost in the turbo so is kicking in overboost protection - that stops you getting over 3000 rpm and the car drives likes there's no turbo. Switch off the ignition and restart - it all comes back as normal until the protection cuts in again. This may or may not throw a code.

Why it's thinking it's going to overboost is a bit of a rabbit hole - could be the wastegate actuator is sticking, the control mechanism may be not performing properly (not sure if yours is vacuum or electric), maybe turbo vanes are having a bad day, airflow sensors, inlet manifold lead etc - needs you to grab hold of some measuring kit and see what it tells you. If you cannot get hold of any see if you can find a someone who has some and knows the car/engine i.e. a specialist may be better than a standard garage as just chucking parts at it as a remedy can prove expensive.

(Or you may get lucky and find its a £30 part first time out - somebody has to )