Internal combustion engine with brake system

A direct injection engine (10) is coupled to a vacuum brake booster (130) wherein vacuum created from engine pumping is used to supplement driver braking force. The brake booster is coupled through a check valve (134) to the engine intake manifold (44). A method is disclosed for estimating pressure in the brake booster based on operating conditions. A method is also disclosed for estimating operating parameters based on measured brake booster pressure. Further, a method is disclosed for diagnosing degradation, or monitoring, a brake booster pressure sensor based on operating conditions. In addition, a method is disclosed for diagnosing degradation in other vehicle and engine sensors based on measured brake booster pressure.