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The design of my Jeep's 4.0L straight-six has the fuel rail, intake manifold and exhaust manifold all stacked up together on one side of the engine, and the exhaust downpipes coming off the manifold also have a couple of small cats right below that. So, this design seems to concentrate a lot of heat under the hood which heats it like a griddle. Jeep even issued a corrective measure for some models which involved wrapping some of the injectors and/or fuel rails in insulated foil material to help prevent the fuel boiling, vaporizing and causing hot-start issues.
About using spacers to raise the rear of the hood, it does work, and it works best at slow speeds and when stopped. However, when running at speeds on the highway, I'm told it can be less satisfactory because a high-pressure site develops in the central cowl area due to wind hitting the windshield. The effect of a raised hood in the rear is said to be the engine bay air sometimes can be forced backwards, which some believe can hinder air flow through the radiator. However, I suspect that problem is more theoretical than practical.
About the additional mileage expectations: I wouldn't hold my breath about that. I doubt you'll be able to measure the difference due to hood venting. At least I know I haven't been able to.
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