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Been trying to rectify the cooling system and I'm stumped. A side issue: The temp gauge is pegged high at all times probably due to it's sensor which is somewhat inaccessible and not my current focus. Cluster test shows the gauge is ok. It's a 2000 M43 (1.9L 4cyl) 318i auto.

Symptoms: Top radiator hose gets way hot, bottom hose remaining relatively cool and pressure builds up in the system. Hoses will be solid after a 10 min drive. I had thought it may be a head gasket leak, but there's no combustion leak to coolant causing the pressure build. No oil in water and no water in oil. No white smoke from exhaust. Seems the pressure build up is due to no movement and coolant boiling.

I replaced the water pump, suspecting that was the issue. Still no circulation. Removed the brand new thermostat and replaced it with just an old thermostat housing, so no thermostat is involved for the following testing.

Checked the coolant flow for blockages. Removed top and bottom radiator hoses. Used a garden hose into the top cap of expansion tank and there was easy flow through to bottom outlet of radiator. Ok, so no blockage there. Garden hose into thermostat housing, easy flow out to top and vice versa of garden hose into top hose and out the thermostat. When I pulled the pump out, I blocked off the thermostat housing and blew water into pump bore, and water flowed freely to the top hose and blowing water into the expansion tank, water came freely out the pump bore. Disconnected the top hose from the radiator and drove water directly into the radiator, the water came out the top hose.
I have filled and bled this system at least a dozen times during all of this. I've removed the new water pump to check it was ok and reinstalled it.
Am I missing something?
 
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