Booney and Felmo both spot on.
It looked good without a thermostat and took a little longer than usual to warm up....five minutes.
I did every head gasket check known to mankind and a few extra known to woman kind....it just did not show as head gasket.
I had a think about what Booney said and Flemo observation on the thermostat. I bit the bullet and got an aluminium radiator from coolantman off e-bay . Cost $269 bucks delivered to my front door. Did a buy it now at 8am and it was at my place by noon in the Disco by four pm.
Problem solved. Immediately it takes around fifteen to twenty minutes for the Disco to warm up ( still no thermostat) on a warm day . I cannot get the temp higher than the lower set of little wavy lines on the temp gauge. My thermocouple tells me that's 84 degrees C . I know if I put the thermocouple back in it will get up to the 88 degrees specified in the manual.
I will not re-install a thermocouple until I can find one that operates in other than a complete plug fashion ( I want a butterfly or rotating vane ) I also want a jiggler and some kind of failure bypass . ( a 3/16 hole for instance. )
Only slight hitch with the new radiator is the filling bung leaks on account of the supplied seal being not that good. ( the bung out of my dud radiator fits fine and doesn't leak .
I will persist in running the coolant as purified de-ionized water and drilling the filling bung to take a sacrificial zink anode.
attached a picture of the temp gauge at 82 deg C
I have measured and the lower mark in the arc measures 60 deg C half way will be 88 deg C the thermostat operating temp ( though mine was fitted with an 82 degree thermostat. ) so I am guessing but think that the top mark in the arc ...the one below the red line is 115 deg C or close to it and the red line is around 125 deg C near the boiling point of coolant at 15psi .
