Fuel My Car Needs To Much Fuel Pressure... I Think?

Discussion in 'Fox 5.0 Mustang Tech' started by sen2two, Aug 22, 2014.

  1. so I was doing some testing on my car tonight and my normal fuel pressure of 40 psi was way to lean on the wideband (first time I checked this car with it). Even when I turned it up to 46-47 psi, the car was still at 14.0afr at 5500 rpm in second gear.

    my upgrades:

    Stock block and heads (91 Fox)

    GT40 Upper and Lower intake manifold
    BBK 65mm Throttle body
    BBK Throttle body/EGR spacer
    C&L Cast aluminum intake
    Pro-M 75mm MAF
    E303 Cam
    BBk Fuel pressure regulartor
    M.A.C. Shorty Headers
    Dynomax 2.5" H pipe and exhaust (dumps at angle just before rear axle)
    *has resonators before H on each exhaust tube
    MSD 6a and blaster Coil
    *Timing is set at 12* on 93 octane

    Could it be reading leaner than it actually is because I am getting my reading at the tail pipe?

    The fuel pressure gauge use to read 2psi less when I had it on the fuel rail at the Schrader valve location. The gauge now sits on the BBK regulator.
  2. The stock fuel pressure 37-41 PSI will work just fine for your engine and modifications.

    Check fuel pressure:
    The local auto parts store may rent or loan a fuel pressure test gauge if you don't have one.
    Disconnect the vacuum line from the fuel pressure regulator. Check it for evidence of fuel present in the line by removing it and blowing air through it. If you find fuel, the fuel pressure regulator has failed. Reinstall the line; leave the fuel pressure regulator end of the vacuum line disconnected. Then cap or plug the open end of the vacuum line and stow it out of the way.
    Connect the fuel pressure test gauge to the Schrader port located just behind the alternator.
    Turn the ignition switch on & start the engine. Observe the pressure: you should see 37-41 PSI at idle.
    Turn the ignition off; reconnect the vacuum line to the fuel pressure regulator. Then disconnect the fuel pressure test gauge. Watch out for squirting gas when you do this.

    The 14. 0 A/F is just about perfect. The perfect A/F ratio is 14.7, which is what the stock tune in the computer tries to achieve.
  3. While 14.7 is what cars are "normally" set to. This is not desired for optimum power or safety. 12.5 is a far better goal for AFR.

    The problem I noticed after a few more tests was, going from 40 psi to 50psi had very little change in AFR. I saw no sign of fuel in the vacuum line from the FPR, so I do believe my regulator to be functional.
  4. Should read 38-40psi with vacuum line disconnected.

    How many miles are on your fuel system? Probably wouldn't hurt to have your injectors professionally cleaned and if you haven't already, change your fuel filter. It could be plugging up, slowing your fuel delivery. Might not hurt to swap out your original 88lph fuel pump for something a little more robust as well (155lph)? Not mandatory, but it couldn't hurt...especially if you've got the original 24-year-old one in the tank now.

    Old oxygen sensors can throw it for a loop too. Generally, if they've got 60K on them or better, they're probably due for a change. They may be on their way out, but not so much that they'll trip a code.
  5. Couple of things:

    Reading at the tailpipe isn't as accurate. Also, like J said, the computer tries to achieve something in the 14.7 range. Bumping fuel pressure will not change that. If the computer doesn't think the engine needs more fuel, but you turn up the pressure, then the computer will just shorten injector pulse width. This can be changed through a tune.

    If a car is running lean due to too small of an injector, pressure can be increased to make the fuel spray faster though the injector during the pulse width, in effect, providing more fuel. Some people may decrease pressure to "starve" the injector and cause the motor to lean out.

    With your combo, 19lb injectors and stock pump are adequate for what the computer wants to do. Bumping fuel pressure will just cause the computer to shorten pulse width of the injectors, which is why you saw no change in AFR.

    jrichker likes this.
  6. theres absolutely nothing in your listed combo that would warrant any change from the factory 39-40 psi of fuel pressure. How are you obtaining those afr numbers?

  7. :confused:
  8. The 12.5 A/F is good if you are trying to cool the engine's combustion temps because there is pressurized induction or extremely high compression ratio.

    Best power is more like 13.5 A/F, which will also prolong engine life due to fewer deposits in the cylinders and spark plugs and less pollutants in the engine oil.
  9. agreed, an afr of 12.5 is ideal in a boosted application where intake temps are higher and you need a little more safeguarding from detonation , little less than ideal for an n/a car