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Thread: 12-927 Fuel Pressure Issues

  1. #1
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    Default 12-927 Fuel Pressure Issues

    I have a '67 Cutlass with a 400 ci V8 that I originally installed a Commander 950 TBI. The pump is mounted on the frame rail below the tank, and originally had the return line routed back to the feed line before the pump. I have converted the Commander to a Holley HP EFI computer & harness, plus I've ran the return line back to the pump. Since then, I've had trouble with, what seems to be, fuel pressure anytime the secondaries are opened! The self tuning map wants to add so much fuel and the car just can't accelerate!

    The fuel tank pick is new, and has a filter sock on the end. The inline provided filter from the Commander install kit before the pump and a 40 micron screen filter after the pump. The ONLY 90 deg bend in the whole system is where it connects to the throttle body. Could that 90 deg be my restriction? I'm also having trouble finding a gauge to read fuel pressure, any suggestions? Thanks for the help in advanced, Ken

  2. #2

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    Quote Originally Posted by 67w30 View Post
    I have a '67 Cutlass with a 400 ci V8 that I originally installed a Commander 950 TBI. The pump is mounted on the frame rail below the tank, and originally had the return line routed back to the feed line before the pump. I have converted the Commander to a Holley HP EFI computer & harness, plus I've ran the return line back to the pump.
    First this is a terrible idea. You're going to overheat the fuel, and overheat the pump, or create a vapor lock. Get the return back into the fuel tank.

    Since then, I've had trouble with, what seems to be, fuel pressure anytime the secondaries are opened! The self tuning map wants to add so much fuel and the car just can't accelerate!
    The fuel tank pick is new, and has a filter sock on the end. The inline provided filter from the Commander install kit before the pump and a 40 micron screen filter after the pump.
    The ONLY 90 deg bend in the whole system is where it connects to the throttle body. Could that 90 deg be my restriction?
    I'm also having trouble finding a gauge to read fuel pressure, any suggestions?
    Keep it simple. It's nice to log fuel pressure straight into the ECU with a 0-100 psi transducer that's mounted in the fuel rail or in the return regulator. But for diagnostics, just buy any fuel injection fuel pressure gauge at an auto parts store. Tee it into the system anywhere between the pump and the return regulator, and watch it as you go into full throttle (though you will probably see the pressure drop before that from the sound of it).
    No, the 90 degree bend is not your restriction! That's for sure. Fuel pumps hate restriction before them so double check for hose kinks in the supply line.

  3. #3
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    Quote Originally Posted by 67w30 View Post
    ...and originally had the return line routed back to the feed line before the pump.
    ...plus I've ran the return line back to the pump.
    I don't know where you got this idea from. Don't read internet stories; read the Commander 950/HP EFI manuals.
    As FPSeth stated, the fuel return line must be routed into the fuel tank (not into the fuel feed line).
    https://www.holley.com/document/tech...y_r10179-1.pdf (C950 TBI Manual - Page 14)
    https://www.holley.com/document/199r10552rev.pdf (Avenger/HP TBI Manual - Page 11)

    The fuel tank pick is new, and has a filter sock on the end. The inline provided filter from the Commander install kit before the pump and a 40 micron screen filter after the pump.
    If a sock filter already exists in the fuel tank, do not add any other pre filter before the fuel pump (especially that notorious plastic pre filter - Link). The post filter for EFI applications is supposed to be 10 micron, not 40 micron! (A 40 micron post filter is for carbureted applications.)
    https://www.holley.com/products/Fuel...es/parts/562-1 (C950 Post Filter)
    https://www.holley.com/products/fuel...s/efi_filters/ (EFI Billet Aluminum Fuel Filters)

    I'm also having trouble finding a gauge to read fuel pressure, any suggestions?
    https://www.holley.com/products/Fuel...s/parts/26-505 (0-30 psi, Liquid Filled)
    https://www.holley.com/products/Fuel...s/parts/26-502 (0-30 psi, Non-Liquid Filled)

    The Holley EFI fuel & oil pressure transducers are completely optional. The engine runs the same with or without them.
    The fuel & oil pressure transducers are used for Data Monitoring, Data Logging and setup Outputs based on fuel & oil pressure.

    This is not to be confused with the "Actual System Pressure" fuel parameter (in Engine Parameters),
    which must be entered accurately because the ECU does use this single value for lb/hr calculations.

    Ensure your TBI fuel pressure is properly adjusted to 21 psi, and the "Actual System Pressure" is accurately entered in Engine Parameters.
    With TBI, the Actual System Pressure is measured anytime engine is running (no vacuum reference hose), or key-on/engine-off pump running.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  4. #4
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    Thanks for the input, I really appreciate this!
    Just to make things clear, I have the return line from throttle body to tank!
    I also ordered the 10 micron post filter and the suggested pressure gauge. Thanks!!
    Do you think that the sock filter on the pick up tube and the white inline filter is restricting fuel that bad?
    Also, is the 40 screen style post filter too restrictive too? Man!
    I went back to engine parameters and check that the TBI fuel pressure value was at 21, it was!!
    I was then wondering if I have the 2-4 switch off, checked it and it seems right.
    But in the help section of the software says that Holley TB usually start at 30%, and mine comes on at 50%.
    Your help is greatly appreciated!! Ken

  5. #5
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    Quote Originally Posted by 67w30 View Post
    Do you think that the sock filter on the pick up tube and the white inline filter is restricting fuel that bad?
    Also, is the 40 screen style post filter too restrictive too? Man!
    I already answered these questions in post #3.
    The 40 micron post filter isn't filtering enough (EFI applications).
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  6. #6
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    Danny!!! Thank you man!!!
    It's running better than ever. She's been apart for years, and since I got her back, I never got to see what she was really like!
    But just one more thing, I hope you can help me out with! The pump is really noisy? You can hear it over Flowmaster 40s!
    I also got pump 12-920 instead of the 12-927, I realize it's a higher volume pump, but should it be noisy? Thanks for all of your help!!! Ken

  7. #7
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    Danny, just one more question. Can I control fuel pump voltage with the HP computer? I'm gonna use it for TCC, can I do both if I can? Thanks again, Ken

  8. #8
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    Quote Originally Posted by 67w30 View Post
    The pump is really noisy? You can hear it over Flowmaster 40s!
    A noisy fuel pump suggests cavitation to me. I'd check the fuel system plumbing, fuel pressure and pump voltage. How far away from the fuel tank is your fuel pump located?

    Quote Originally Posted by 67w30 View Post
    Can I control fuel pump voltage with the HP computer?
    http://forums.holley.com/showthread....M-Output-Setup (Holley EFI ECU Controlled PWM Fuel Pump Control)
    http://forums.holley.com/showthread....-Compatability (Aftermarket Stand-Alone PWM Fuel Pump Controller)
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  9. #9
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    Cavitation! So I found a place on the frame rail at its lowest point just in front of the rear end. It was the lowest point in the factory bent line. It's gotta be 6-7 feet of fuel line between the pump and the tank, is that an issue? Thanks for the links to fuel pump control! Ken

  10. #10
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    Quote Originally Posted by 67w30 View Post
    It's gotta be 6-7 feet of fuel line between the pump and the tank, is that an issue?
    That's too long for a fuel suction line. Mount the fuel pump much closer to the fuel tank.

    NOTES:
    Since I discussed fuel plumbing aspects, you might find this helpful.
    Forum posts by Brian Macy - TurboNova (Instructor at EFI University)
    that really parallels everything I've done with my own fuel system:
    http://www.cpgnation.com/forum/threa...06/#post-72225 (pump cavitation)
    http://www.cpgnation.com/forum/threa...77/#post-74345 (suction side)
    http://www.cpgnation.com/forum/threa...44/#post-75311 (pump fittings)
    http://www.cpgnation.com/forum/threa...10/#post-68486 (tank venting)

    TIP:
    In-tank pumps are great, however, I've never had a single problem with an external in-line pump.
    My external in-line fuel pump has never been mounted below the level of the fuel tank.
    It's more important to mount the fuel pump as close to the tank as possible, keeping the suction line as short as possible.
    Drill the Walbro pump's inlet & outlet fittings a little larger, and use the shortest possible suction hose.
    My fuel pump has been mounted near the top of the fuel tank for over 12 years and never had any problems.
    UPDATE: I now have a custom made fuel pump module mounted on the fuel tank frame crossmember like THIS.

    FYI:
    I've never used the ECU's fuel pump trigger wire to directly power the pump (even if it is less than 10 amps).
    I've always used a separate relay (shown below) because that green wire is still only 14 AWG.
    The 2nd fuel pump relay is used so the fuel injectors aren't subjected to the pump's amp draw.
    It's only a 20 amp fuse and it also feeds power to the fuel injectors (& ignition coils - LSx).
    I prefer to use 10 gauge wire due to its long length, especially for high output race pumps.
    You'd simply use the green fuel pump wire to trigger your additional fuel pump relay.

    https://www.holley.com/products/fuel...s/parts/12-753 (Holley 12-753 Fuel Pump Relay Kit)
    http://documents.holley.com/199r10059.pdf
    (Holley 12-753 Fuel Pump Relay Kit Instructions)
    https://www.holley.com/products/fuel...arts/30-199QFT (Quick Fuel 30-199QFT Fuel Pump Relay Kit)
    https://www.holley.com/products/fuel...s/parts/40100G (Mr. Gasket 40100G Sealed Relay Kit)
    https://www.summitracing.com/parts/pco-5593pt/overview/ (Pico Wiring 5593PT Sealed Relay Kit)
    Delphi Metri-Pack 630 Pull-To-Seat female terminals for retrofitting to a sealed relay socket connector:
    #12033997 10/12 AWG (3.0 mm²), #12066614 14/16 AWG (2.0-1.0 mm²), #12020156 18/20 AWG (0.80-0.50 mm²)
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

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