Hi, I have a problem with my BMW 318i. It can't go beyond 40 km/h. Once I step

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Hi, I have a problem with my BMW 318i. It can't go beyond 40 km/h. Once I step on the accelerator peddle it can't go beyond 40 km/h, it will just start raving itself. I took it to the garrage and was told to replace the airflow sensor but it couldn't respond. Please advise. sammy mulenga