Did you check fuel pressure with a fuel pressure gauge as I recommended or
did you just replace the fuel pressure regulator? If you checked fuel
pressure, what was the pressure after sitting overnight?
Bad fuel pumps tend to be all or nothing, that is, they work or do not work,
unless the tank has been run dry a few times, in which case they can get
worn. I've never seen the inside of the fuel pump, but I don't think it has
a check valve.
A fuel pressure gauge installed in the fuel line ahead of the fuel pressure
regulator and after the fuel pump will indicate whether the fuel pump itself
is bleeding pressure back down, but I doubt if it is the fuel pump.
The car may have a cold start injector which supplies extra fuel when
cranking. If it or the cold start injector circuit (timer, wiring, coolant
temperature gauge, etc.) were bad, it would be hard starting any time the
car is cold-soaked, say after 4 hours.
Check for leaks in the intake between the air filter and throttle body,
check manifold vacuum, especially in the hoses leading to the fuel pressure
I always recommend checking stuff before replacing parts - fixing a car will
be a lot cheaper that way!
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