View Single Post
      11-23-2012, 11:20 PM   #6
Imperator's Avatar

Drives: 2010 E92 335i
Join Date: Mar 2010
Location: Plano, Texas

iTrader: (8)

Originally Posted by bimmer35 View Post
Can anyone else chime in on this please?
Almost all vehicles with "power brakes" use a brake line pressure "booster" that is supplied with manifold vacuum. When supplied with vacuum, the brake pedal is easy to depress and assists with force to the brake hydraulic system. When that vacuum is lost, the brake pedal becomes stiff to depress as your leg is doing all the work--the power assist is lost through the brake booster.

The N54 maintains several other vehicle controls that run off of engine vacuum, especially the turbocharger wastegates. Turbocharged engines don't supply a steady amount of manifold vacuum and often employ vacuum canisters which hold a reserve amount of vacuum for operation while under boost.

Somewhere along this system, the vacuum is lost. It might take several minutes or several hours, depending on how well the system is sealed up. This is what you experience when pressing the brake pedal in the morning on a cold start. As soon as the engine starts up, it begins supplying vacuum and immediately restores the brake booster function.

I posted this up just to see if my experience aligns with others and if several known sources of vacuum leaks exist that might be worth checking into. It seems this behavior is actually normal with the N54.