N54 Limp Mode: 30 kW limit

I recently discovered something interesting while debugging a 2008 BMW 135i with N54 overheating issue using an OBD-II scanner. When the engine crossed into its over-temperature protection state, the DME dropped the car into a very aggressive limp mode, and the OBD-II power output PID showed the engine being limited to roughly 30 kW (about 40 hp).

I couldn’t find any documentation online describing a specific power cap, but the behaviour was repeatable: the car felt extremely slow, produced no boost, and held a flat, minimal power output until temperatures came back under control.

Oddly, this limit seemed to disappear above roughly 105 km/h, where the engine briefly regained more normal power delivery—likely because higher road speed improves airflow through the radiator and reduces thermal load. I’m sharing this because I wasn’t able to find any references to these exact numbers or this speed-related behaviour, and it may help others diagnose similar overheating-related limp mode issues.