Quote Originally Posted by rspears View Post
I agree with Justin. The difference between the two pumps is volume capacity, not a pressure difference. It should start and run OK with the lower volume pump, but might stumble at WOT on a sustained high rpm run. Can you hear the pump running for sure? If so, it sounds like you've got something blocking flow. Maybe an old in-line filter? Have you put a pressure gauge on the fuel rail? That's one of the first things I'd add, just to know what pressure you're getting and eliminate the unknown. You can pick up a little 0-60psi liquid filled gauge for about $20.
Roger, the fuel filter is new, pressure regulator is new, fuel lines are stock Mustang. I tried using a tire pressure gauge on the release valve on the fuel line (looks like a valve stem) but it didn't register on that. I'll double check the lines, etc tomorrow and see if there's a restriction somewhere....