Fuel pressure may drop due to a leaking injector or bad check valve in the fuel pump itself. The regulator does not come into effect until the engine is running and a vacuum supply is produced to it. If the regulator is leaking internally, it could send fuel back to the tank as well. Prime the fuel line and pinch off the return line ( if possible ) to eliminate a regulator problem. If the fuel pressure drops with the return line pinched off, suspect a leaking injector or fuel pump. If injector is leaking, the vehicle should run rich and cause a hard start/rough run condition
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