I believe they actually will keep the car interior a little bit cooler since they actually isolate the heater core and the tubes that are under the console area. Since you are shutting off the flow in the engine compartment the hot water won't get in those pipes either. My big concern is that the water then just sits in the pipes and/or heater core and may lead to corrosion. I asked around while I was doing my restoration and apparently there is no easy (or cheap)way to replace those pipes since they are welded in.
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