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Thread: Fuel pressure

  1. #1
    Join Date
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    Fuel pressure

    Hello anybody i have just completed the engine and gearbox overhauling and now i miss only repainting before complete my 1991 850i restoration.
    I am driving the car since a few days and it runs very smooth and nice except that i have the feeling of not being on a 300hp car.
    The first thing i made was a INPA diagnosis and the only error i got was something with ASC missing signal for more than one second but this should not implicate loss of power.(hope to solve this too very soon)
    I focused my attention on the fuel pressure and this morning i inserted a meter on fuel line and i get 2.65BAR on both.
    I put the meter on the firewall pipes before pressure regulators.
    As far as i know the pumps should give 3.0BAR so i ve doubt that mines are giving me wrong pressure.Is that correct?I try to insert some photo of my car20181226_094435.jpg20181017_163550.jpg20190102_102825.jpg
    Thanks for any help and suggestions

  2. #2
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    The operating fuel pressure is 3 bar, usually the new/good pumps have more than 3 bar and the fuel pressure regulators then regulate it.
    operating pressure is 3 bar
    conveying capacity at 12 V 1.7 liter/minute
    capacity against counterpressure 875 cc/30 seconds

    already changed fuel filters?

    A missing ASC signal could, however, also cause limpmode = reduced power, I assume, when reading the EML information, https://www.europeantransmissions.co...MW/EML-BMW.pdf
    ASC is linked /interfaced with the EML module.
    Shogun tricks and tips for the E32 series are HERE!

  3. #3
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    Thanks for reply Shogun.
    Fuel filters seems to be new i did not change them but to make sure i can meter the pressure before fuel filters.Could it be a good idea?

  4. #4
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    Do you measure at idle speed?
    Then, this is normal.
    Press down the throttle paddle - then the pressure must go to 3 bar.

  5. #5
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    Yes i was at idle....but i increasing rpm the value was steady at 2.65

  6. #6
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    Pull off the vacuum hose on the fuel pressure regulator and test it again.
    You can suck with your mouth on die fuel pressure regulator - watch the pressure gauge.

  7. #7
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    best test is indeed pulling off the vacuum hoses.... standing still one must give a lot of rpm increase to get that vacuum away .. almost full throttle ! Not healthy for your engine.

  8. #8
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    Testing for a leaking, ruptured, fuel pressure regulator diaphragm
    That is a good analysis. How to verify the cause of an intermittent long crank and a start/stall condition. No need to have the electronics as shown first (because we backyard mechanics do not have them anyway), go to 1 minute 34 seconds, just remove the vacuum hose and sniff if there is fuel smell or even fuel coming out. Watch for fuel. Takes over a minute that fuel is coming out of the port in this vid, diaphragm cracked. https://www.youtube.com/watch?v=AKtR_yF7bi8
    To be on the safe side, I would connect on the fuel pressure vacuum connector a transparent hose and lead it out of engine bay into a container/bottle, just in case a lot of fuel is splashing out.
    Shogun tricks and tips for the E32 series are HERE!

  9. #9
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    Ok i will test pressure regulators and let you know result.Thanks for help

  10. #10
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    I ve done the test and both pressure regulators seems to be ok.....i think i will test pump pressure before fuel filters if it is less than 3bar the only thing i can try is to change both fuel pumps.Hope it will not lose of money

  11. #11
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    Quote Originally Posted by qseven View Post
    ...
    I focused my attention on the fuel pressure and this morning i inserted a meter on fuel line and i get 2.65BAR on both.
    I put the meter on the firewall pipes before pressure regulators.As far as i know the pumps should give 3.0BAR so i ve doubt that mines are giving me wrong pressure.Is that correct?I try to insert some photo of my car
    Thanks for any help and suggestions
    The is the deal!
    The Fuel Pressure Regulator operates across the "injector" or between the fuel rail and the intake manifold.
    The pressure gauge is measuring the fuel pressure between the fuel rail and the atmospheric barometric pressure.
    So to get the correct fuel pressure regulator pressure you will want to add in the intake manifold pressure?
    Fuel Pressure regulator pressure = Fuel Rail pressure + Intake Manifold Pressure
    If you have a vacuum gauge that measures in Bar it will read a negative .507 Bar for 15inHg.
    For example: 2.65 + .507 = 3.107 Bar actual fuel regulator pressure.
    2.65 - 3 Bar = -.35 bar intake manifold vacuum. A -.35 Bar is only 10.5 inHg which is a vary low intake manifold pressure at engine idle

  12. #12
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    The fuel-rail pressure regulation is bit more basic as the FPR is a two-state device, it runs at the nominal pressure (as is stamped on the body and should be 3.0 Bar) when there is no vacuum applied to the control port - but with vacuum applied (from the manifold, which is present except at full throttle) the fuel pressure drops by 0.5 Bar.

    The pressure is maintained between the fuel pump and the fuel rail with excess fuel returned to the tank - this means the pressure can be measured anywhere on that line.

    BMW have a test procedure for the FPR as below:

    https://www.meeknet.co.uk/E38/Fuel-rail-pressure.pdf

    The relevant passage for the E31 is:

    Description of operation: Fuel pressure regulator

    Depending on requirements, the fuel pressure regulator regulates a low or high fuel pressure. This requirement is set with the help of the pressure regulator.Depending on the engine's operating state, less or more fuel is needed:-

    at idle speed, less fuel
    at full load, considerably more fuel.

    So, the reading of around 2.6 Bar is quite close enough - but it is worth checking if the pressure increases when the vacuum line is removed.
    Last edited by Timm; 01-24-2019 at 11:51 AM.
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  13. #13
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    Quote Originally Posted by Timm View Post
    The fuel-rail pressure regulation is bit more basic as the FPR is a two-state device,...
    FPR is a two-state device. Not exactly!
    Two state is either On or Off (or On and not so On) i.e. binary. A fuel injector is a binary device. However, a binary device like an injector can provide a linear fuel delivery by varying the solenoid pulse width.
    The FPR is a liner fuel pressure device that is directly regulated by the intake manifold vacuum applied.

    This descriptions of the FPR is also misleading:
    ..the fuel pressure regulator regulates a low or high fuel pressure...
    The FPR has a single job, that is to maintain a constant injector fuel pressure.. not fuel rate. Fuel rate is the job of the injectors pulse width as determined by the DME from the various inputs; throttle position, RPM, AFM, coolant temp, etc.

    Therefore, the FPR's regulation always maintains a constant 3.0 Bar (or the FPR rating, i.e 3.5 Bar) across the fuel injector (one end in the fuel rail the other end in the intake manifold).
    The purpose of the FPR is to ensure the injector solenoid pulse width produced by the DME generates an exact measurement of fuel delivered.

    The document you posted is a good measurement reference, with over simplification in some of functional explanations.

    IMHO, it is also important to observe the fuel pressure long enough to run the FPR through the extremes of engine warm-up and the manifold vacuum changes, that is; cold start to warm coolant and an intake manifold pressure from 0 Bar with engine off through -.7 to -.8 Bar form a high rev-coast down vacuum to make sure the the FPR maintains a smooth transition without unanticipated pressure changes up or down.
    A Vacuum gauge is a helpful tool to use along with the FPR pressure testing as it helps to monitor the intake manifold pressure changes that cause the fuel rail pressure to change measured relative to atmospheric pressure.

    The last FPR I changed was intermittent and would loose pressure (causing a lean air fuel ratio); i.e., the internal FPR valve was sticky and did not always correctly regulate on manifold vacuum.
    Lean AFR is the stuff full throttle detonation and burned pistons are made from. Also, occasionally this engine had long crank times because the FPR was stuck open (low fuel rail pressure), but the engine shaking during start-up would cause the FPR to seat and correctly pressurize the fuel rail.

    Also, if you observe something unexpected when measuring fuel rail pressure, replace the FPR.

  14. #14
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    Here are some pics showing how the fuel pressure regulators trim fuel pressure vs. intake vacuum in my 91 850i. Sorry about the glare... sun was out in full force.

    At WOT the intake vacuum approaches zero and fuel pressure at its highest ~ 44psi.
    Coasting with foot off the gas the intake vacuum is ~ -25inHg and fuel pressure drops to -34psi.

    IMG_2289.JPG

    IMG_2298.JPG

    IMG_2299.JPG

    IMG_2286.JPG
    Last edited by BMSman; 01-26-2019 at 11:46 PM.

  15. #15
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    [QUOTE=BMSman;30191940]Here are some pics showing how the fuel pressure regulators trim fuel pressure vs. intake vacuum in my 91 850i. Sorry about the glare... sun was out in full force.

    At WOT the intake vacuum approaches zero and fuel pressure at its highest ~ 44psi.
    Coasting with foot off the gas the intake vacuum is ~ -25inHg and fuel pressure drops to -34psi.

    QUOTE]

    BMSman, that is a cool MID+ instrumentation. Techie for sure.
    The Fuel pressure (psi) measurement you are displaying is more correctly described as a psia (Pounds per Square Inch Atmosphere) measurement, reference to atmospheric barometric pressure.
    The actual Fuel pressure should have minimal fluctuation (nearly solid 3.0 Bar or 43.5 psi) irrespective of throttle position when the measurement is made PSID (Pounds per Square Inch Differential) as the injector sees the fuel pressure; between Fuel Rail to Intake Manifold vacuum.
    PSID measurements are more difficult to make as they require a differential gauge or 2 sensor (analog/digital) electronically setup that includes both measurements mathematically summed.

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