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From what I've read here, the check valve only holds pressure for so long - but by the time it has allowed rail pressure to dissipate, the manifold is no longer hot, hence no vapour lock problem.
Regarding cold starts, in my experience a failed check does impact, but not nearly as significantly. I believe that even though fuel rail pressure does leak down in a few hours, a good check valve still maintains some, hence a cold engine fires after fewer revs. With a failed check valve, even though the priming of the fuel pump when you first hit the ON key position (before cranking) should pressurize the fuel rail, my experince has been that it takes longer to fire. Warm starts nowever are a more significant issue.
You can try to confirm a check valve issue on a cold engine by doing several fuel primes before the first crank attempt: turn the key to ON, listen for the pump prime, and then OFF. Repeat this a couple more times and then crank. If the engine fires faster after this then it typically does without doing this, there's a good chance that it's the check valve. I suppose other culprits under these circumstances could be leaky injectors or the fuel pressure regulator, although problems with these should exhibit other symptoms also.
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David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto
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