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This may be related to Al Wicks reason for running with a cup of air trapped in his system, a buffer if you will.....Tim Andres

I do notice that after I have refilled the coolant system (and before the air is all flushed out), that I do not get the immediate surge of pressure to 21-24 psi on start up.  It will slowly climb to 8-12 psi depending on power settings and airspeed.    However, after several flights and the system is purged of air this phenomena starts to occur.  Again, no coolant leak associated with it.  I suspect that with the air purged, that this surge has to do with the coolant volumetric area.  With no cushion, then any movement of coolant is likely to be sensed as a pressure increase - once the block has warmed up and coolant passages, etc, have warmed up (and expanded), I think there is a slight increase in coolant passage volume and the pressure decrease as a results.  Just a SWAG of course.

Ed A

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