04 Cobra - dual ford gt pumps, voltage, only 30 psi fuel

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  1. #1

    Default 04 Cobra - dual ford gt pumps, voltage, only 30 psi fuel

    Ok, new member. Purchased an 04 Cobra 2 years ago and hasn't been driven due to a loss of fuel pressure under boost. Fuel pressure is 29 lbs at start up, new FPS sensor. Pressure never raise above 30-35 psi and actually zeroes out with boost. Has dual ford GT pumps with fore hat and 60 lb injectors. Also has stage 1 modified FPDM. The voltage on the upgraded gauge wires coming out of the FPDM has a fluctuating 3 volts at idle on one and fluctuating 12 volts on the other. Do I have a fuel pump issue with one bad pump? How does the FPDM see the pumps? Does it only see one and is commanding full cycle at 12 volts to increase pressure but that pump isn't working and the car is only running on the other pump at 3 volts? Should the tank be dropped and each pump jumped? Thank you.

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  3. #2

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    The OEM ECU looks for a differential pressure across the injectors of 39.15 psi. At idle with normal idle vacuum the apparent rail pressure will look like less than 39.15 psi because of the engine's idle vacuum. As the engine comes off idle and goes into boost the fuel rail pressure should be the manifold pressure (boost) plus the 39.15 psi Ford wants to see as the differential pressure across the injectors.

    You are going to need to do some data logging to ferret out the gremlins. Start by data logging engine speed, manifold pressure, and fuel rail pressure. Run the car up through two or three gears to get several looks at what is going on. Post up the data logs and lets see what we can see.


    Ed

  4. #3

    Default 04 Cobra no fuel psi

    I was hoping to just get some basic info as to how the modified FPDM works with the dual pumps. I didn't think the voltage of around 3 volts on one pump and 12 volts going to the other pump is correct and that both pumps should both have roughly the same voltage going to each one. I do not have any equipment to data log and not sure where there is even a tuner shop. The fuel pressure never increases (gauge in car coming off of fuel rail) and actually drops to zero and car falls on its face as you try to boost it. The previous owner said it should have 50 psi on the fuel gauge around 4500 rpms with boost and he didn't have a problem before it was sold. So I was hoping the problem is with one of the pumps?

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  6. #4

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    Without data logging you are basically guessing, shooting in the dark if you will. You need to know what is happening to begin unraveling the ball of fishing line before you.

    If you are going to own one of these cars you can either choose to get the tools (diagnostic and service) and learn how to do the work yourself or alternatively you can choose to take the car to a service provider and pay them to do the work for you. The challenge with the service center is first finding one familiar with modified Cobra's and then communicating to them the modifications that have been made to your Cobra. The second path is usually more costly.

    I would suggest that the first step you might want to explore is getting a handheld from HP Tuners, SCT or DiabloSport to data log with. Of course you can always just take the car to a performance shop and hope you have chosen wisely. The last resort, of course would be to just shoot in the dark and hope for the best.

    As a starting point, you might want to look at how your pumps have been wired to the FPDM(s) and compare it to the wiring diagrams on the site here or elsewhere to see if the wiring has been done correctly.

    Not to sound like a broken record but these cars tend to be a hands on sort of experience. If that is not something you are comfortable with, you can farm out some of the more esoteric tuning stuff to tuning shops. In the end these cars are still hands on vehicles that will demand increasingly more owner participation the more modified they become.

    If the car is essentially stock you can usually find decent support at knowledgeable dealers. Of course the downside is the service provider that does not know how your car has been modified and is unknowledgeable about how to service a car that has been modified as yours has been modified. These sorts of experiences frequently have unhappy endings producing expensive repairs to correct the damage that the unknowledgeable service provider has wrought and maintains is your responsibility just before he refuses to pay for the damage he has done to your car.


    Ed

  7. #5

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    Yep, I hear what your saying. I believe its wired right with the 12 volt to the pumps upgraded and after checking wiring diagrams i now realize the other upgraded wire is the ground which pulses to increase the cycle of the fuel pumps running. I'm thinking of just putting full ground and full 12V at fuel pump plug by the tank to max pumps out and see if I get a large increase in fuel psi. Is this a safe hack just to make sure both pumps will increase psi? If psi doesn't increase then I will drop tank and check pumps and hoses inside of tank. If pressure does go up, then it gets into finding out why the increase in fuel is not happening from a communication stand point. Thanks for the responses.

  8. #6

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    You are shooting in the dark. This is not a recommended way to fix the problem you are faced with.

    Just as an example, you have something called a fuel rail pressure sensor (FRPS) that is a differential pressure sensor used to monitor the differential fuel pressure across the injector. It's diaphragm will typically burst somewhere above 70 psi. It is improbable, but not impossible, that you could create a fuel spike that would go that high. If it did it could rupture your FRPS and potentially damage other expensive parts of your fuel system. A replacement FRPS is about $140.

    Your capacity to damage various parts of the fuel system either mechanically or electrically is greater than you might suspect. Do the right thing get the right tools and importantly get the right knowledge before you begin to randomly see if something helps. This is not like swapping parts on a carbureted n/a engine until you finally replace the right bad component.

    If you do not know how to diagnose and repair your fuel system, as it appears is the case, then try to get the list of modifications from the previous owner and the name of the shop and tuner he used if he didn't maintain the engine himself. Go to them and ask them to fix your car. In the just in case scenario, post up a pic of the wiring diagram you used to determine that your pumps and FPDM(s) are wired correctly.


    Ed

  9. #7

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    OK. I'll be gone a few days. There is a scanner with the car that has two tunes in it. I'll try and learn what that scanner will do but it will be a while before i can get back to it. Thanks.

  10. #8

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    Quote Originally Posted by KDcarman View Post
    OK. I'll be gone a few days. There is a scanner with the car that has two tunes in it. I'll try and learn what that scanner will do but it will be a while before i can get back to it. Thanks.
    Thats good. Depending on who manufactured it, the scanner you are speaking of is likely the tool you will need to datalog.

    In the mean time, post up a pic of the wiring diagram you used to validate the fuel pump and FPDM wiring.


    Ed

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