I feel like this is user error but I can't sort it. Maybe y'all can help...
1976 BMW '02. The Thermostat in the vehicle isa stuck open, aftermarket temp gauge show the car runs at the bottom of the gauge's range (130Fish). Replace with a new 176F Tstat from Bimmer World. Engine runs hot, I check the tstat in a pot of water, opens at 202. Folks at BW are super nice and just send another. This time I check it in the pot before installing. 190ish... still a bit high but maybe the boiling pot test isnt perfect and SURELY I didn't get (2) bad tstats. So in it goes, drive for a mile or so, same result. Engine gets up to 220. Not sure if its heat related but at this temp it 'diesels' when I shut it off. I should mention that in the time waiting for the second replacement tsat I ran the car with the old (stuck open) tstat with the same cool-running (130ish) engine, no issues. Radiator was replaced, the core is 14 x 15 which appears close to the replacements I've looked up.
Any suggestions on where to look from here? I feel like I must be overlooking something...