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Thread: Fuel Pressure Calculation Help

  1. #1
    Senior Member jaymanek's Avatar
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    Fuel Pressure Calculation Help

    Hi all,

    When I carried out the 500TE conversion, I used the original "Wagon" fuel pump mounted at the tank which takes the fuel over the rear axle to the standard fuel pump. The Wagon had one pump at the tank and one in the normal place under the rear floor area.

    I couldnt use the single wagon pump, cant recall why now, so I transplanted the 500E dual pumps. In effect I had 3 pumps now.
    I thought it would be fine as the system has a pressure regulator up front anyway?!

    Well the first time I turned the key, the front fuel lines exploded. In reality I had forgotten about the fuel pump setup that I h ad installed weeks/months before. The fuel pipes were very brittle under the braiding so just put it down to age.
    Pipes NLA from MB so had them rebuilt locally.

    This weekend I was under the car and noticed the short rubber hose from fuel filter to metal fuel line was showing some cracking.

    I tried to measure fuel pressure with my cheap fuel pressure guage. It shot off the scale at over 10 bar.

    So decided to put it back to how the 320 wagon was set up. So in effect I bypassed one of the pumps on the dual setup and disconnected its electrical connections.

    Tried again and the fuel pressure is still off the guage. Hmm? Scratchy head?

    I cant see what could be wrong. Each fuel pump is supposed to have a working pressure of 4 bar and a flow rate of around 100-120L per min.

    I am tempted to just leave it now but worried that I may have more exploding fuel pipes and I have a longish drive next week.

    What do you guys think? It seems its less than straight forward to calculate what pressure I should be seeing from two standard Mercedes fuel pumps plumbed in line.

  2. #2
    .036 Hooniganô
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    gsxr's Avatar
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    Re: Fuel Pressure Calculation Help

    I'd either use the old-style dual pumps, or the new-style single pump, but not a combination. Really not sure what is going on here, but how old is the FPR on the fuel rail? Makes me wonder if it's stuck closed?

    Dave M.
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  3. #3
    Senior Member S70/2's Avatar
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    Re: Fuel Pressure Calculation Help

    Is the return line blocked? I can’t see how else you are getting 10+ bar of fuel pressure

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  5. #4
    Senior Member jaymanek's Avatar
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    Re: Fuel Pressure Calculation Help

    I am measuring it under the car, just after the fuel pumps. So return etc doesnt come into it?

  6. #5
    E500E Guru JC220's Avatar
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    Re: Fuel Pressure Calculation Help

    This is a great how to on measuring the fuel pressure. See the link also in post 1 about testing the function of the FPR.

    There is the possibility of a defective FPR or a blocked return line. But in reality the dual pumps or single later pump alone should be more than capable on their own. I'm not sure that they are supposed to be fitted with an additional fuel pump in line like that. Gravity feed only to the back of the pump - at least in the saloon that's how it is.

    https://www.500eboard.com/forums/showthread.php?t=5455
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  8. #6
    E500E Guru a777fan's Avatar
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    Re: Fuel Pressure Calculation Help

    The return through the regulator will get into the overall pressure you see in the line. If the return line is blocked or the regulator is broken your pumps will be essentially pressurizing a closed volume. Youíd still need to have some pretty massive pumps on there to get to 10+ bar though! IIRC, most fuel injection systems have 3-4 bar regulators. Not sure specifically about the 500.

    This video isnít the most polished, but it gets the regulator principles across pretty well.

    https://youtu.be/OBRyl2Ib-1s


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  9. #7
    Senior Member S70/2's Avatar
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    Re: Fuel Pressure Calculation Help

    Quote Originally Posted by jaymanek View Post
    I am measuring it under the car, just after the fuel pumps. So return etc doesnt come into it?
    I see, well deadheading the pumps is not really going to tell you system pressure. I recommend hooking up a gauge on the fuel rail and running the pump(s) with the car off to see what your operating pressure is.

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  11. #8
    Senior Member jaymanek's Avatar
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    Re: Fuel Pressure Calculation Help

    Quote Originally Posted by S70/2 View Post
    I see, well deadheading the pumps is not really going to tell you system pressure. I recommend hooking up a gauge on the fuel rail and running the pump(s) with the car off to see what your operating pressure is.
    Thatís what Iím doing wrong I guess.. Iíll measure at the rail and report back...

    The dual pumps from a saloon are set up exactly the same in effect. The outlet from first pump goes into the inlet of second pump which then feeds into the filter.

    The reason the pumps are separated on a wagon is due to the tank being low down and the fuel needing to be pumped up and over the rear axle..

    Thanks all...

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    gsxr (04-29-2018)

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