The fault is the starting point.

BMW 335i

Intermittent loss of power

Symptom

Sudden loss of boost under load, roughly once a week. No warning light most of the time.

Testing

Logged boost pressure, wastegate duty and intake temperature over a 40-minute road test with the fault provoked under sustained load.

Cause identified

A hairline split in the charge-air pipe that only opened above a certain pressure and temperature.

Solution

Charge-air pipe replaced, boost system pressure-tested, adaptations reset. Re-tested over the same route. No recurrence.