Methods and systems are provided for regulating coolant flow in a vehicle cooling system during an engine startup event. In one example, a method may include, during an engine startup event, controlling a flowpath of an engine coolant in a vehicle cooling system via a passive valve and an actively regulatable valve, and responsive to an engine coolant temperature below a threshold at the engine startup event, isolating the flowpath of engine coolant to a subsection of the cooling system to enable rapid warming of the engine coolant without stagnating the engine coolant at an engine. In this way, engine coolant may be rapidly warmed at an engine startup event, via coolant flow isolation, rather than coolant flow stagnation, which may decrease uneven heating of engine system components, and which may thus prolong a functional lifetime of the engine system.