[pbanders]

Lack of protection against overpressure is a basic design flaw of the MPS. In part-load operation, the diaphragm is supported by the part-load stop plate, to prevent mechanical stress on the bronze diaphragm. In full-load operation, the diaphragm is not subjected to mechanical stress (other than the flexure required to move the core), as there is still about a 30 mbar negative pressure difference across it.

When the MPS is subjected to positive pressure, due to rotating the engine backwards (i.e. when spinning with the clutch out), the full-load diaphragm moves to the full-load stop position, but there is nothing supporting the bronze diaphragm. As the diaphragm is already likely to be experiencing some degree of metal fatigue due to normal flexing, this high stress often blows out the diaphragm. Once this has happened, the MPS is operating in full-load mode all the time, and you get a very rich mixture. Amazingly, the car will still run, just very poorly.

A quick check of vacuum integrity can determine if the MPS survived a spin. Someone clever who takes their 914 to the track regularly should be able to rig an overpressure protection pop-off into the vacuum hose that leads from the intake manifold to the MPS.