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I don't think the fuel pump check valve is your problem. It has no effect on cold starting, but IS intended to ease Hot Starts by preventing the vapor lock that can occur when fuel boils in the fuel rail, due to heat soak from a hot engine. This is why the pressure holding spec for the check valve is measured in minutes—not hours. [Spec is 14.5 psi after 20 minutes.]
A normally functioning pump will bring up full pressure (from 0 psi to ~35psi) in barely one (1) rev of cranking.
But you may be having problems with your Cold Start Injector. With six 240s, you must have a Bentley manual. Check page 241-18. I'd do the test on a cold day, rather than trying to fake out the ECU with a resistor, at the virtually inaccessible ECT nsor.
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Bruce Young '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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