I had the same problem which ultimately turned out to be a partially blocked bypass, It was holding the T'stat open with back pressure. I know you have checked it, but so had I on mine, but a flexible bit of spring steel (backing from a auto wiper blade) finally got the last bit of impeller out. I suspect that it had been sitting somewhere in the block until after I had cleared the first blockage. It has run at 160 for the last 4 seasons. The rich running is caused by the ECU thinking the motor is cold and enriching the mixture, ie a bit like a choke on a non ECU motor.
Also make sure that the water supply path to the manifolds is clear, on mine both outlet stubs on the T'stat housing were partially blocked with corrosion (salt water boat) I ran a drill through 3/8 inch I seem to think.