Correct answer
By initially closing the main radiator circuit while allowing internal flow.
It initially keeps the main radiator circuit closed while coolant can circulate through the internal bypass. This limits heat loss through the radiator during warm-up; at the specified temperature it opens that circuit to dissipate heat. It does not mean stopping all circulation or manually blocking pipes. Do not rev a cold engine highly to warm it; follow manufacturer instructions.
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By stopping all coolant circulation in the engine.
By initially closing the main radiator circuit while allowing internal flow.
By requiring the driver always to rev the engine highly.
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