1968 Shelby GT500

Fuel Pressure Regulator (FPR)

The Fuel Pressure Regulator is attached to the fuel supply rail downstream of the fuel injectors. It regulates the fuel pressure supplied to the injectors. The regulator is a diaphragm-operated relief valve. One side of the diaphragm senses fuel pressure and the other side is connected to intake manifold vacuum. The nominal fuel pressure is established by a spring preload applied to the diaphragm. Balancing one side of the diaphragm with manifold vacuum maintains a relative fuel pressure drop across the injectors. As the regulator ages, the diaphragm may tear, leaking fuel into the vacuum line. Fuel pressure is typically regulated to 39PSI (270kPa) higher than manifold pressure. As idle vacuum lowers the manifold pressure, fuel pressure is lowered to match. If you have a Super/Turbo Charge, as boost increases, fuel pressure increases to match.




To test the fuel pressure you’ll need this tool. You will need to screw it onto the schrader valve on the top of the fuel rail, it looks like a tire air valve stem. After attaching the fuel pressure tester, run the fuel pump for 10 seconds. Check that the pressure is within specs, and it doesn’t leakdown more than 5PSI within 60 seconds after pump shutdown.


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