The addition of after-market parts in your 5.0 liter Mustang may change the fuel pressure requirements of the engine. Improvements in horsepower and torque, especially larger injectors,may require that fuel pressure be adjusted to compensate for the added fuel volume these parts require. Fine-tuning the fuel pressure for the best performance in your 5.0 liter-equipped Mustang requires the installation of an adjustable after-market fuel pressure regulator. After you install the adjustable regulator, adjustment of the fuel pressure takes only a few minutes.
Remove the old fuel pressure regulator, located on the driver's side of the fuel injector rail near the rear of the engine in the 5.0 liter Mustang. Do this by unplugging the vacuum line from the center of the regulator and removing the three Allen-head screws that attach it to the fuel injector rail. Twist and pull the regulator from the fuel rail.
Install the new, adjustable regulator by following the reverse order of what you just did. Be sure to use the new gasket and o-ring seal supplied with the new regulator. The new regulator has an adjustable vacuum port that is locked in place by a jamb-nut. Loosen the jamb-nut using a wrench.
Install the fuel pressure gauge by screwing it onto the fuel pressure test port, which, in the 5.0-liter Mustang, is on the passenger side of the fuel injector rail near the first fuel injector. Turn the ignition key on and off three times to prime the fuel system, and then start the engine.
Observe the fuel pressure indicated on the fuel pressure tester's gauge. Turn the adjustable vacuum port using an Allen wrench to adjust the fuel pressure. Adjust the fuel pressure until the tester gauge reads 55 psi. Tighten the jamb-nut snugly while holding the vacuum port in place with the Allen wrench. Reinstall the vacuum line and check the fuel pressure on the tester gauge. It should now read 40 psi. If the pressure is too high or too low, readjust it until the pressure is 40 psi with the vacuum line installed.
