C70 Boost Pressure Issue

Hey, I've got a C70 as well, and your problem sounds awfully familiar! I had almost the exact same symptoms a while back – the whistling, the hissing, and the engine just feeling completely gutless. Did you check the hoses connected to the turbo? I remember mine were cracked and leaky. Also, have you tried manually actuating the wastegate arm on the turbo? It could be seized up, preventing the boost from building correctly. Just a thought!

Thanks! Could you elaborate on the hose inspection? Were the cracks readily visible, or did you employ any specific diagnostic techniques to identify them? Also, with respect to the wastegate arm actuation, is there a particular range of motion or resistance I should expect? The actuator linkage seems intact and moves freely (with engine off), but I'm not sure what normal looks like. I'm trying to avoid misdiagnosing.

Honestly, the cracks were pretty obvious once I looked closely - felt like an idiot for not seeing them earlier. They were on the underside of the hoses where I couldn't easily see. As for the wastegate, it should move smoothly without feeling too tight. If it's sticking, or doesn't move at all, that's a bad sign. Seriously though, with all those symptoms and your mileage, it might just be worth taking it to a shop to get it properly diagnosed before you start throwing parts at it. A failing turbo can cause a whole mess of issues.

Your counsel regarding a professional assessment proved prescient. Following your recommendation, I submitted the vehicle for evaluation. The diagnosis indeed confirmed a defective electrical bypass valve, necessitating replacement. The attendant cost amounted to 250€, a sum I deem reasonable for the restoration of proper engine function. Thank you for steering me toward an efficient resolution.