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E34 Cooling System Troubleshooting

I have an E34 with and M50 (1995).
The temperature slowly rises to the red zone.
There is no heat (blows cold air always).
Upper radiator hose is hot, large lower hose is cold.
Opening the bleed valve releases steam.
Upper radiator hose appears to pressureize it gets
hot.
Loosening the radiator cap causes the fluid level to
rise.  Would spill over if I let it.

I installed a new radiator 600mi ago because the old
one had a slow leak.  No problems until this
ovedrheating occurred.  When the trouble occurred, I
found the water pump had a freewheeling impeller and
that pieces of the impeller were also broken off.
I found all pieces except one small one, maybe 1/2
inch square.
Installed new water pump and thermostat.
Same symptoms persisted.

I am thinking the missing piece of impeller may be
clogging a passage or small diameter hose somewhere. 
It is not obvious what the complete system is or how
it works.  My Haynes E34 manual has a "schematic," but
it has no explanation.
Does anyone know the E34 M50 cooling system well
enough to tell me what to check next?  How to reverse
flush?
Any EDUCATED ideas are welcome.
Dan Z
Annapolis, MD
BMWCCA since '86

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Dan Zeitlin
Annapolis, Maryland
<[email protected]>
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