Paul - nice picture of the head.
Taken another shot at this.
- Coolant cap - is it a 1.4 bar cap? This equates to 20
lbs. approx. They used to be 14-15lbs. Do you have a
1.4 bar cap? I know the cap is new, but....
- Air in the system remains a possibility.
- Is the radiator healthy?
- The T-stat has an air valve that must be positioned at
the highest point. Is it?
Check out the following posts. While they may not address issues in 104 motors, I think the basic ideas carry over to just about any car. Comments made by JeffSr, MBDoc; Benzmac
are especially worth considering.
300D Cooling system diagnosis
Cooling system "burping"