Cadillac Repair: fuel pressure test, fuel pressure regulator, balance test

Question
1994 4.9 motor. I need to know what the fuel pressure should be. I hooked up a fuel pressure tester and when I turn the key on fuel pressure goes to 40psi while the fuel pump is still running and drops off to 18psi when fuel pump stops. I do not have a fuel leak and I already changed the fuel pressure regulator. The engine still runs but a little rough. I think the problem is my fuel pump not pumping enough pressure and not holding it.

Answer
40 psi is OK for the fuel pressure. You most likely have a faulty injector and or faulty spark plug or wire.

FIRST perform a compression test on all 8 cylinders and then perform an injector balance test.


The fuel pressure should maintain 40 psi with the engine running.