RamAir95TA

If the fuel pressure drops while the engine is running, you either have a dying fuel pump, a stuck open injector, or the regulator has failed. You can try removing the fuel rail with the injectors still attached, then turn the key and look for any leakage. You can also check the fuel pressure regulator. With the KOEO, pressure should be between 41 and 47psi. With the engine running, fuel pressure should drop 3 to 10psi. If it doesn't, either the regulator is faulty or the vacuum hose to it is plugged.

Like Fred mentioned, the check valve doesn't do anything once the engine has started and is not the cause of the problem.