Viscous coupling or you have air in the system, if you roll up a news paper and stick it in the fan and it stops its the viscous coupling as for bleed ing the system correctly always do this from cold, fill up with engine off and A/C on max heat but fan at lowest setting. fill and bleed you may have to do this several times, if this fails as Whitshark says possible waterpump.
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I stepped in a plate of Pasta the other day - now I have to worry about my Carbonara footprint!