The brief
This 135i came in with the check engine light on and a flat spot in the power. We read it on the BMW scan tool, because a boost-related code with a flat spot on a turbo engine points at air escaping somewhere it shouldn't, and the data helps narrow down where. The turbo pushes pressurised air through hoses to the intercooler, which cools it, then on to the engine. The hose between the turbo and the intercooler runs hot and flexes with every change in boost, and over the years the rubber hardens and splits, usually at a bend or a clamp, so boost leaks out before it gets cooled and delivered. The engine feels flat, the management throws a boost or fuel-trim code, and a split hose doesn't reseal, so it gets replaced.
The diagnosis
The BMW scan tool showed a boost deviation, the actual boost not matching target, and a check found the turbo air hose to the intercooler split and bleeding boost, which is the light and the flat spot. The turbo, the intercooler, the charge pipes and the wastegate were sound otherwise. That's a hose replacement and a recheck of every intake and boost joint, rather than chasing a leak that opens and closes with heat.
The work
The split turbo air hose was removed and a new genuine BMW-spec hose fitted with fresh clamps, and the rest of the intake and boost joints checked tight while it was apart. The system was leak-tested again to confirm it held pressure with no escape, the boost fault cleared, and the adaptations reset. A road test confirmed the light stayed off, the boost coming on cleanly with no flat spot, and full power back.
The outcome
No check engine light, clean boost with no flat spot, full power back, the running steady, and the intake holding pressure. The 135i went home pulling properly again. A split charge hose only gets worse and drags the running down, so finding it on the scan tool and replacing it fixed the boost for good.
