Endeavour intake manifold flap control issue

I had similar symptoms with my Endeavour (though it was a petrol engine). Reduced power, rough running – sounds very much like an intake manifold issue. Have you checked the flap control mechanism itself? Sometimes they can get stuck or the actuator motor can fail. The intake manifold's flaps help control the airflow into the engine. When they aren't working correctly, it can mess with your engine's performance quite a bit.

When you say 'flap control mechanism,' are you referring to the switch that controls the manifold flaps? I've read that a defective switch could be the culprit. Is that what you experienced, and if so, were the symptoms consistent with what I'm observing?

Yes, that's exactly what I'm referring to. In my case, it turned out to be a faulty switch. The symptoms were very similar to yours: hesitation, hissing, and generally poor engine performance. Honestly, unless you're very comfortable working on engines, I'd suggest taking it to a workshop. It can be tricky to diagnose and repair properly. It is possible, but for me, getting the workshop to do it was the best option.

I appreciate the confirmation. I suspected as much, given the symptoms. I've taken your advice and booked it in to a workshop. They have identified the switch intake manifold control as defective, and quoted a repair cost of 465€. Thank you for your assistance!