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Thread: High RPM Fuel Pressure Drop

  1. #1
    Join Date
    Nov 2016
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    Moscow, RF
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    Question High RPM Fuel Pressure Drop

    Hello. I've got some problems with fuel delivery on my next configuration:
    • 360 N/A stroker Magnum, about 500 HP.
    • Holley EFI Terminator.
    • 12-890 Holley fuel pump.
    • Holley 100 micron fuel pre-filter & 10 micron post filter.
    • 12-846 fuel pressure regulator installed after fuel rails.
    • -8AN line from tank to pump, -6AN feed line to Terminator & -6AN return line from regulator.

    So at this point, FPR set to 43 psi at idle (with disconnected vacuum line). It works fine and keeps this pressure for about 2000-2500 RPM (with connected vacuum line pressure is about 38 psi). After that, at higher RPM pressure seriously drops to 15-20 psi sometimes to 6 psi. When pedal pushed abruptly on neutral, I can see the same thing.

    I've already changed the fuel pump to another brand new 12-890 from box, installed fuel pump relay, pump feed & ground now directly from battery, installed another new FPR, check & clean all the filters & fuel lines, nothing happened. I also tried to feed pump with gas directly from canister. Voltage on alternator is about 14V. At this point, I've dismounted Terminator throttle body, check fuel rail lines, but they are good. Really don't know what to do now, already have a headache from all of it. Can you advice something to me, what I have missing?

    P.S. Few months and about 500 miles ago, there was same configuration on this car with only 12-600 fuel pump, and pump feeding from Terminator. All other parts were same and it has worked good, without any fuel problems.

  2. #2
    Join Date
    Dec 2009
    Location
    Connecticut
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    Is this fuel pump cavitation? Where is the fuel pump & pre-filter located in relation to the fuel tank? How long is the suction hose? Is there a restriction somewhere in the fuel system plumbing?
    May God's grace bless you in the Lord Jesus Christ.

    '78 BRONCO: 508" stroker, TFS heads, 11:1 comp ratio, Dominator MPFI & DIS, cold air induction, Spal dual 12" fans/aluminum radiator, dual 3" exhaust/Magnaflow mufflers, Moroso vacuum pump, Accusump, engine oil & trans fluid coolers, 100HP progressive dry direct-port NOS, A/C, LenTech Strip Terminator wide-ratio AOD/2500 RPM converter, 3:1 Atlas II, modified Dana 44/60-lockers-4.10s, hydroboost/4-disc brakes, ram-assist/heim joint steering, Cage long radius arms, traction bars, 4" Skyjacker lift, 35" mud tires

  3. #3
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    Nov 2016
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    Moscow, RF
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    Default

    Quote Originally Posted by Danny Cabral View Post
    Is this fuel pump cavitation? Where is the fuel pump & pre-filter located in relation to the fuel tank? How long is the suction hose? Is there a restriction somewhere in the fuel system plumbing?
    It doesn't look like cavitation, but after ~10-15 minutes of work, the pump is really hot. Pump located at the middle of frame, it's about 4-5 feet from tank. As I say, to exclude problems from pump feeding, I connected a short hose to inlet port of pump and put other end in canister, no effect. There is no visual restrictions or clogging in fuel lines. I want to try add check valve after pump, and change feed line to -AN from pump to Terminator.

  4. #4
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    Quote Originally Posted by BBBT View Post
    Pump located at the middle of frame, it's about 4-5 feet from tank.
    This is not a good fuel pump location, especially long term use.

    As I say, to exclude problems from pump feeding, I connected a short hose to inlet port of pump and put other end in canister, no effect. There is no visual restrictions or clogging in fuel lines.
    Then I'm not sure what the problem is.

    I want to try add check valve after pump...
    No need for that.
    May God's grace bless you in the Lord Jesus Christ.

    '78 BRONCO: 508" stroker, TFS heads, 11:1 comp ratio, Dominator MPFI & DIS, cold air induction, Spal dual 12" fans/aluminum radiator, dual 3" exhaust/Magnaflow mufflers, Moroso vacuum pump, Accusump, engine oil & trans fluid coolers, 100HP progressive dry direct-port NOS, A/C, LenTech Strip Terminator wide-ratio AOD/2500 RPM converter, 3:1 Atlas II, modified Dana 44/60-lockers-4.10s, hydroboost/4-disc brakes, ram-assist/heim joint steering, Cage long radius arms, traction bars, 4" Skyjacker lift, 35" mud tires

  5. #5
    Join Date
    Nov 2016
    Location
    Moscow, RF
    Posts
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    I've done what I spoke of above; changed feed line from pump to EFI. Problem has gone at this moment. Ride about 30-40 minutes with pedal in the floor and pressure stay at 39-40 psi at full throttle. Just change 4 feet -6AN hose to -8AN hose with check valve, and the pressure drops are gone. Don't know how it works, but it does.

  6. #6
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    Quote Originally Posted by BBBT View Post
    Just changed 4 feet -6AN hose to -8AN hose with check valve, and the pressure drops are gone. Don't know how it works, but it does.
    You had a restriction somewhere in that -6AN hose/fittings. -6AN hose/fittings won't cause a fuel pressure drop. My entire fuel system is plumbed with -6AN hose & fittings.
    May God's grace bless you in the Lord Jesus Christ.

    '78 BRONCO: 508" stroker, TFS heads, 11:1 comp ratio, Dominator MPFI & DIS, cold air induction, Spal dual 12" fans/aluminum radiator, dual 3" exhaust/Magnaflow mufflers, Moroso vacuum pump, Accusump, engine oil & trans fluid coolers, 100HP progressive dry direct-port NOS, A/C, LenTech Strip Terminator wide-ratio AOD/2500 RPM converter, 3:1 Atlas II, modified Dana 44/60-lockers-4.10s, hydroboost/4-disc brakes, ram-assist/heim joint steering, Cage long radius arms, traction bars, 4" Skyjacker lift, 35" mud tires

  7. #7
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    Nov 2016
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    Moscow, RF
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    It was likely some problem with the 10 micron filter in this line, that was deleted from there too, because hose & fittings are good. It's strange, because I took it apart & cleaned it with compressed air, so it has good flow before installation checked by my mouth. I'll try to install another new one tomorrow and see what would change.

  8. #8
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    Well I'm glad you found the problem. Thanks for the update.
    May God's grace bless you in the Lord Jesus Christ.

    '78 BRONCO: 508" stroker, TFS heads, 11:1 comp ratio, Dominator MPFI & DIS, cold air induction, Spal dual 12" fans/aluminum radiator, dual 3" exhaust/Magnaflow mufflers, Moroso vacuum pump, Accusump, engine oil & trans fluid coolers, 100HP progressive dry direct-port NOS, A/C, LenTech Strip Terminator wide-ratio AOD/2500 RPM converter, 3:1 Atlas II, modified Dana 44/60-lockers-4.10s, hydroboost/4-disc brakes, ram-assist/heim joint steering, Cage long radius arms, traction bars, 4" Skyjacker lift, 35" mud tires

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