Mercedes M Klasse Diesel Engine Power Loss

I had a similar issue with my M Klasse. I also noticed the check engine light and the engine felt really weak. The whistling sound, I had that too. It turned out to be related to the boost control system. Have you checked the throttle body for any leaks or damage? A small crack there can cause a big loss in boost pressure. The wastegate is definitely a suspect. It's responsible for regulating the boost. When it fails, it can cause all sorts of problems. Reduced engine power is a classic symptom. If it's stuck open, your turbo won't generate enough boost; if it's stuck closed, you could overboost, which is also bad. Hopefully that helps!

Thanks for the info! When you say 'throttle body', are you referring to where the air intake connects? And how would I check the wastegate? Is it something I can visually inspect, or do I need special tools?

Yep, the throttle body is where the intake connects to the engine. Visually inspect it for cracks or loose connections. As for the wastegate, it's tricky to diagnose without proper tools. It's usually vacuum-operated, so you'd need a pressure tester to see if it's holding pressure. Honestly, with those symptoms, especially the stalling, it might be best to get it checked by a workshop. Mine turned out to be a faulty wastegate actuator. It was causing the boost pressure to fluctuate wildly. They replaced the whole unit, and the problem was solved.

Appreciate the advice. Taking it to the workshop next week. Seems like chasing the wastegate issue myself might be more trouble than it's worth. Had it checked, wastegate valve defect was the culprit. Repair cost was 860€. Runs like new now!