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Old 11-02-2011, 05:08 AM
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Fudman Fudman is offline
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Location: Sudbury, MA
Join Date: Apr 2007
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Mein Auto: '02 530i Sport auto
Originally Posted by bluebee View Post
The question for this thread, is:
Q: How does the BMW E39 sucking jet pump (aka suction jet pump) work & how does it fail?
RDL's .pdf file gives a good explanation. The sucking jet pump is not a "pump" at all but just a fitting that creates a vacuum for the master cylinder to boost braking power. This is a feature on all power assisted braking systems. In this caes, once the engine starts, the intake manifold vacuum draws air from the intake bellows through the fitting (jet pump). The fitting constricts the air flow, increasing airflow speed which creates a venturi effect, which then causes a pressure drop. The angled fitting connection (which is attached to the brake master cylinder) is exposed to this pressure drop which creates a vacuum in the line. This vacuum is used to boost the brake master cylinder.

Q: Why not attach directly to the intake manifold? There is probably insufficient vacuum (pressure drop) at the intake manifold to directly attach to the master cylinder. Hence, the engineers use this "pump" to increase the pressure differential to increase the brake boost without adding an electrical pump. Very clever!

Q: How does vacuum increase braking pressure? Beats the hell out of me. Haven't a clue. Start another thread.

Q: How does it fail? Since there are no moving parts to the "jet pump", failure would seem highly unlikely. One mode could be caused by clogging. Since it is downstream from the air filter, it does not seem likely that contamination will come from the air box side. Another failure mode could be material failure, but again highly unlikely given the lack of mechanical loading. Hence, the pump appears to have high reliability.
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