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Thread: First time start up, too much fuel. 350 chev 7747,

  1. #1
    Carb and Points!
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    First time start up, too much fuel. 350 chev 7747,

    Hello all,

    We are working on a 350 chev that came out of a 91 burb with the 7747 ecm that was transplanted into an off road crawler. We have the orig ecm and stripped off some sensors from the wire harness like egr, esc, knock sensor, vss. What we have left is MAP, TPS, IGN mod, IAC, coolant sensor and oil sensor. We tried to fire the rig up with the stock ecm and the injectors were delivering way too much fuel. In the process of finishing other modifications to the truck I modded the ecm with a zif socket and emulated a dloaded bin and tried to fire again. Same results with too much fuel. We swapped injectors, TBI, and even tried a variable pressure regulator in the throttle body. Still too much fuel. (even did a redneck regulator by pinching the fuel line with vice grips to the same result) The fuel pump is 40psi capable.

    So, with the stock bin this thing just dumps the fuel and won't start. It sprays a big amount on crank but as it tries to fire and revolutions quicken so does the amount of fuel to the point it floods and can't start. At this point we can disconnect the injectors and crank and the engine will actually stumble for a few seconds while burning off all that extra petrol.

    A couple of questions, what modifications to the bin do you see off hand that need adjusting with the deletion of all those sensors? And, can you think of another reason why it's delivering so much fuel?

    Thanks for the help guys and gals!!

    Jon

  2. #2
    EFI GearHead ! EagleMark's Avatar
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    What does the Check Engine Light do when key is turned on?

    What is fuel pressure?

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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  3. #3
    Fuel Injected! jim_in_dorris's Avatar
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    ESC and knock sensor control spark advance, and VSS tells ECM your speed. Without them you can indeed tune it, but it will be a challenge. What is the timing set at with the brown/tan wire disconnected? That much fuel isn't normal. What BCC are you using? Some of the real tuners will be along shortly with a better idea of what to check.

    LOL Mark posted while I was typing
    Square body stepsides forever!!!

  4. #4
    Fuel Injected! gregs78cam's Avatar
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    If your pump is capable of 40psi, you really NEED to know what the fuel pressure is. It is entirely possible for a higher flowing pump to over power the regulator spring and force more fuel through than the return line can flow. That will increase pressure seen by the injectors. This happened to me when I converted my truck. Gotta check pressure.
    1978 Camaro Type LT, 383, Dual TBI, '7427, 4L80E
    1981 Camaro Z-28 Clone, T-Tops, 350/TH350
    1981 Camaro Berlinetta, V-6, 3spd
    1974 Chevy/GMC Truck, '90 TBI 350, '7427, TH350, NP203, 6" lift, 35s

  5. #5
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    Pressure gauge is @18psi and 0, supply and return respectively. Is that high enough to push that much extra fuel?

    Edit Swapped in another regulator and got 16 psi and 14 while cranking. Have actually installed 3 different regs and same result with all of them as to too much fuel.
    Last edited by live2wheel; 07-01-2013 at 04:10 PM.

  6. #6
    EFI GearHead ! EagleMark's Avatar
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    That's pretty dam high for a stock motor and stock chip that needs about 13 PSI.

    What is your CEL doing when key is turned on? Should come on, blink, stay on till engine starts and then go off! If something other then that is happening you may be in Limp Home Mode...

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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  7. #7
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    Double post
    Last edited by live2wheel; 07-01-2013 at 04:49 PM.

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    Cel blinks once when ign is turned on and then stays lit. Haven't reached full fire yet because it floods out and won't start.

    We salvaged the hard fuel lines from the doner suburban. Do you think these are too restrictive and are building pressure on the return side?

  9. #9
    Super Moderator Six_Shooter's Avatar
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    Quote Originally Posted by gregs78cam View Post
    If your pump is capable of 40psi, you really NEED to know what the fuel pressure is. It is entirely possible for a higher flowing pump to over power the regulator spring and force more fuel through than the return line can flow. That will increase pressure seen by the injectors. This happened to me when I converted my truck. Gotta check pressure.
    I've never seen that happen in a bypass type system, like GM uses. A friend of mine has used Ford Port injection fuel pumps from pick ups for years in TBI swaps, without issue.

    Quote Originally Posted by live2wheel View Post
    It blinks once and stays on. Haven't got it to fire yet, just floods out. No codes either.
    You're in limp home mode.

    Check all connections to the ZIF socket, I would have used a DIP socket. I will check for continuity between the DIP socket itself and the trace as far away as I can trace it to be, to ensure that the connections is good.

    Swap in a stock not modified ECM, if you can't find any problems with the ZIF socket connections.

    There is ZERO reason that you should have that much fuel being injected, from the ECM side of things. The omitted sensors will not cause this issue at start up, probably cause drivability issues, however, issues that could be difficult to tune around without VSS.

    The high fuel pressure will cause more fuel to flow, but shouldn't be so much that the engine will not start. Start and run rough yes, but not a no start condition. Check timing also, that can cause hard to start situations, that are more akin to what you describe, especially in limp home mode.
    The man who says something is impossible, is usually interrupted by the man doing it.

  10. #10
    Fuel Injected!
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    At key on (fuel pump prime), not cranking, is fuel coming from the injectors ?

    TOM
    1994 3500 Dually , 502 (509) , 264HR , Edelbrock MPFI , PFI '7427
    2006 Forest river-Sierra 37' 5th wheel
    1999 Astro Van, L31 Vortec , 4x4 conversion
    1994 Pontiac Sunbird "Work ride"
    1993 K-3500 "Ex-diesel" 454/4L80E - '7427
    1992 454SS "Eh Stock" , Blower & '7427 swap .
    1992 S-10 434 SBC/Tremec - '7427
    1984 S-10 , SAS, 496/700R4/205 , D44/14BFF -'7427
    1980 Z-28 454/700R4
    1977 Olds 98 Regency 403/700R4

  11. #11
    Super Moderator Six_Shooter's Avatar
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    Quote Originally Posted by Nasty-Z View Post
    At key on (fuel pump prime), not cranking, is fuel coming from the injectors ?

    TOM
    Ohhh... great question.
    The man who says something is impossible, is usually interrupted by the man doing it.

  12. #12
    EFI GearHead ! EagleMark's Avatar
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    Quote Originally Posted by live2wheel View Post
    Cel blinks once when ign is turned on and then stays lit. Haven't reached full fire yet because it floods out and won't start.
    Quote Originally Posted by Six_Shooter View Post
    You're in limp home mode.
    I don't think so, that is what it's supposed to do... but he's never got it to start and go off. The blink once is a good sign.

    Yes a small restrictive return line can raise pressure on feed line.

    You could also be one of the millions out there with a fuel pressure gauge that sucks! If you can run the return through a 3/8th line to a gas can and check pressure that would tell us if the return has a restriction?

    There are many fuel tanks with what looks like a return line... only to find it is a pin hole vent for CCP.

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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  13. #13
    Super Moderator Six_Shooter's Avatar
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    I didn't read it as blinks the first time around.

    Just to verify, at jey on, the CEL, will turn on, then off, then back on again, correct?
    The man who says something is impossible, is usually interrupted by the man doing it.

  14. #14
    Fuel Injected! gregs78cam's Avatar
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    Quote Originally Posted by Six_Shooter View Post
    I've never seen that happen in a bypass type system, like GM uses. A friend of mine has used Ford Port injection fuel pumps from pick ups for years in TBI swaps, without issue.
    When I first swapped to TBI in my truck, I had a nice Bosch inline pump (rated 50gph@50psi) that I thought would be easy to put on it, and work good. It was very rich, way over 13psi on the feed side. Changed to a standard TBI pump and everything was fine.
    1978 Camaro Type LT, 383, Dual TBI, '7427, 4L80E
    1981 Camaro Z-28 Clone, T-Tops, 350/TH350
    1981 Camaro Berlinetta, V-6, 3spd
    1974 Chevy/GMC Truck, '90 TBI 350, '7427, TH350, NP203, 6" lift, 35s

  15. #15
    Carb and Points!
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    With key on and not cranking no fuel emits from injectors. SES blinks once then stays on. I turn on ignition then start to crank and immediately engine tries to start and revolutions increases and as that happens injectors are pulsing and as revolutions increase the injectors REALLY start to dump fuel squashing any chance of fire and revolutions slow down, stop cranking at that time.

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