A fuel pump cannot function without exactly the same kind of check valve(s) as has been suggested.
Remove the fuel pump inlet hose, connect a vacuum pump to the fuel pump. Find out if the check valve leaks, and if so, how much/how bad is it?
Would be worthwhile to determine if the intake manifold is being over-heated by an exhaust restriction on one side leading to too-much exhaust blowing across the intake manifold exhaust crossover. A seized-shut (or partially-shut) heat riser (EFE) valve, for example, or the inner wall of a double-wall exhaust pipe having collapsed. Crimped exhaust tubes, etc.
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