Transit Boost Pressure Loss

Sounds familiar. A pressure drop leading to turbo lag can feel like reduced power. Check the vacuum lines and the boost solenoid. A faulty vacuum switching valve can cause these symptoms. Inspect the intake manifold for leaks too.

How can I reliably test the vacuum switching valve? And are there specific areas on the intake manifold that are prone to leaks on the Transit?

Proper valve testing needs diagnostic equipment to check its electrical operation and vacuum holding. Visually inspect the intake manifold for cracks or loose connections, but a smoke test is ideal to pinpoint leaks. Given the symptoms, a workshop visit might be best to diagnose the issue and prevent further damage.

Took your advice and visited a workshop. Diagnosis confirmed a defect in the vacuum switching valve. Repair cost 140€, and engine performance is back to normal. Appreciate the assistance!