It comes down to 3 things on rejecting heat. The ability of the coolant to absorb the heat, the air flow to remove it, and the time that is spent doing both. If the coolant spends mor time picking heat up than giving it off the temp will rise.

If the fan is on check to see if there is room to put a shroud around the fan to help air flow through the radiator. Air picks up speed it will pick up more heat.

Hope this helps

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86 300SDL. 250,xxx on #14 Head. One eye always on temp gauge.. Cruising towards 300K