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High throttle setting at low rpms puts a lot of heat into piston head and cylinder driving the temperature up. Mechanically driven water pump is also moving slowly not pulling hot coolant out. A lot of manufacturers derate the engines output by very conservative ignition timing during the "break in period" for reasons of warrantee. This is programed into the ECU. After 10,000 miles the spark curve moves closer to optimum thus improving power and fuel mileage. Biggest reason almost all new vehicles use drive by wire throttling now is to keep the engine operating outside of conditions of self destruction.
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