A stuck injector wouldn't be able to relieve 40psi in 1 second. The only other possibillity is the pump, or part of the line in the tank being cracked. Follow the return line going from the regulator to the tank, and locate a section that doesn't have steel braid. Turn on the key, and have someone clamp some visegrips on that part of the hose, then turn off the key. Turn it on again, and after a second, turn it off again. If your pressure holds, it's the regulator.
If you can't locate a section of the line to clamp, then you are going to have to do it differently. You will have to make your own tool. You will need a replacement fuel filter. When you have one, remove your old one, and cut off the tubes that go in and out of the filter, clean up the cut ends with a file, and clamp a piece of fuel line over the ends, so that it is the same length as your new filter (in other words, you are substituting a piece of hose for your filter). Have someone turn the key on and off a couple times, and then when it's on, clamp a pair of visegrips on the rubber hoe you just installed. Turn the key off, and check the pressure. If the pressure is holding, then the problem is in the tank, and is either the pump or a cracked line to the pump. If the presure goes right to zero, it's the regulator.
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