I think it is the same thermostat housing on the Sigma 1600 in my 4/4. Same symptom in mine started 3 years ago as a slight weep. New housing fixed it. Not easy to fit on a Sigma as the thermostat housing is directly beneath the alternator.
Paul Costock, UK Plus Four 75th - Furka Rouge - Pip Disco 5 Teddy - 17h1 Irish Draught cross
