What is the black stuff in radiator?

system chemical corrosion due to the mixture of metals
copper, brass, steel ,aluminium its called (Magnetite), no inhibitor was put in the system originally, or constant refilling has diluted it, maybe able to salvage if all rads work
do a complete drain down to get rid of as much black crude as poss, insert a (Fernox) chemical cleaner into the system , now refill up the water system leave this in for a fortnight/month then do a second complete drain down ,this time put the(Fernox) inhibitor into the system , then refill

wirecutterman
March 2010