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Thread: Fuel Pressure Question - 1995 530i

  1. #1
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    Fuel Pressure Question - 1995 530i

    Car is a 1995 530i. This car is a project/ something to tinker and learn with for me. Anyway, I've been diagnosing for a hot start problem (cranks, but no start after shutting down from driving at operating temp.). I'm now doing a fuel pressure test.

    My actual question - Is the fuel pressure suppose to immediately show 51 PSI bar when the key is in position 2? Mine does not and stays at zero, but still starts (cold start). It jumps to 45-50 PSI, for a second, as it starts. Drops and stays at 42 PSI during idle. Once it's running, it's fine. It does not sputter or stall out.

    Other random findings during the test so far to share.
    1. Shut off engine - pressure drops to zero within 30 seconds. Used to drop faster before the fuel filters were changed. Not sure if that was the reason for the change. If I clamp the supply line, the pressure holds.
    2. When vacuum hose is removed from the fuel pressure regulator - pressure goes up.
    3. Clamping the return line - pressure goes up.
    4. Checked spark plugs and they were dry. No signs of fuel. No fuel smell either.
    5. When doing my forum search, it seems like I saw several M60B30s and 40s where people say their PSI also stays in the low 40's instead of 51 PSI. According to the Bentley manual 42 PSI is low and not optimal.

    It has another hot start problem. Unrelated to the above (at least I think it is) - Car has a starter heat soak problem when the weather is really hot. When I try to start after shutting down for a few minutes, nothing happens. No click, no sound, but the gauge lights are on. It eventually starts after it cools off. So the starter and associated wiring is next on my "to do list".

    Both the problems actually happened a few days ago. After driving a a few hours, I turned off the car for less then 5 minutes. When I tried to start it would just crank. I did this about 10+ times and then that's when the starter decided to quit and stop cranking. Guess all that cranking made it hot????? Car sat for just under 1 hour then it started right up.

    One thing I have found out is that the battery is working great. I thought it was going to die after cranking without starting a bunch of times, but it didn't.

    Thanks for reading my ramblings.
    Last edited by mazzman; 05-16-2019 at 11:55 PM.

  2. #2
    moroza's Avatar
    moroza is offline MORΩN ΛABIA BMW CCA Member
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    Where is the gauge connected? Are you sure you don't have the supply and return lines mixed up?

    Pressure in the rail should be maintained overnight, so yes, you should see full pressure before and after turning the key to ON. The pressure regulator prevents fuel from bleeding down the return line, and a checkvalve in the fuelpump keeps it in the supply line. Both components, especially the latter IME, can fail gradually. Sounds like your checkvalve isn't working at all.

    If pressure is not maintained, it should take roughly 15 seconds (I'm working from memory) to fully pressurize from zero. If you're seeing zero pressure and it starts, something in your test circuit isn't right.

    My M62 (running M60 electronics and fuel system) idled at 56-57psi according to an Autozone gauge. When shut down it immediately dropped to 44psi.

  3. #3
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    I don't have the Bentleys fuel pressure spec in front of me. But if it is 51psi that will be the pressure with the engine stopped and the fuel pump relay jumpered. The engine doesn't need 51 psi at idle because there is a vacuum. So the FPR reduces it. As a rule of thumb, it will drop about 1/2 psi for every inch of mercury vacuum. A good engine will have a vacuum of about 20 inHG. So 41 psi at idle would be about right. (It is also affected by altitude but I can see you are pretty much at sea level so you can ignore that) So if you pinch the vacuum line, the FPR will send the pressure back up again.

    The fuel pump runs very briefly when you turn on the ignition. After that, it will only run when cranking or the engine is running. So the pressure needs to be up in order for the engine to start.

    All of that is pretty much background. In practise, I would follow Moroza's advice and look at the check valve in the pump first.

  4. #4
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    spec is 3.5 bar ~52 psi with the return clamped it will rise dramatically with a healthy pump, at least 70 psi.
    Between the low pressure and the bleeding off I'd first suspect the regulator is no good. One failure mode is a ruptured diaphragm which allows fuel to drawn into the vacuum hose. Easy to check for fuel there.
    Tired pumps also allow the pressure to bleed off after shut down.
    You mention that it will crank over for a very long time without wearing down the battery. Does it crank really fast?

    If you can leave two black stripes from the exit of one corner to the braking zone of the next, you have enough horsepower. - Mark Donohue

  5. #5
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    morosa - I have the gauge before the fuel rail on the driver side (non fuel pressure regulator side). I believe this is the supply side??? I didn't say where I clamped on my first post. See below.

    whiskychaser - I'm not using the jumper method. I have the engine running. So that explains the 41 PSI at idle. I didn't realize there would be a difference.

    ross1 - Engine cranks normally. I don't notice anything unusual.

    When I clamped the return line, the gauge jumped to a higher PSI. I forget what the reading was though. I didn't leave the clamp on there long.

    I haven't clamped both sides yet. I need to get the proper tools tonight. And also do the jumper method. instead of running the engine.

    After shut off.
    If I clamp between the rail and gauge, it holds pressure.
    If I clamp between the gauge and fuel pump, pressure immediately drops. So it seems like it's bleeding at or after the rail. Injectors are new, but they wouldn't leak that fast anyway. Would they? I will check the FPR. This is also new, but could have failed already. Are there any other items to check beside the injectors and regulator?

    I don't smell fuel or see any external leaks either.

    Thanks a bunch for the replies. I will keep you updated. Maybe I'll post a youtube video with it cranking and showing the gauge.
    Last edited by mazzman; 05-13-2019 at 12:13 PM.

  6. #6
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    Regarding the fuel pressure regulator - RealOEM shows two part numbers for the regulator with one specifically stating 3.5 bar.
    Online stores also list two different FPRs with the different part numbers.
    Just though it was interesting. Anyone know if they are interchangeable?

    Part numbers:
    13531729319
    13531436110

  7. #7
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    moroza is offline MORΩN ΛABIA BMW CCA Member
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    Quote Originally Posted by mazzman View Post
    If I clamp between the rail and gauge, it holds pressure.
    If I clamp between the gauge and fuel pump, pressure immediately drops.
    Ah, this suggests that the checkvalve is fine but either the FPR or injectors are leaking. I doubt it's the injectors, and you could isolate further by clamping the return line after shutdown.

  8. #8
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    I clamped the return before shut off. It jumped to 135 PSI and it held the pressure. Also wondering if doing this is safe with that high of a pressure???

    Under "residual pressure, checking" the bentley manual reads - If the fuel system does not maintain pressure, there is a leak in the system...Check the pump check valve by repeating test, but before turning the fuel pump off pinch off the return line at the fuel rail. If the pressure is now maintained, the fault is most likely the fuel pump check valve.

    Is that written correctly? It doesn't make sense to me right now. Of course I've been inhaling gas fumes Does that mean that by pinching the return line, it pressurizes the fuel system and thus prevents any backflow from the supply side?

    I didn't have everything I need for the jumper wire, so I was doing the test with the engine on.

    And I still don't see signs of fuel in the fuel pressure regulator hose. It's dry.

  9. #9
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    moroza is offline MORΩN ΛABIA BMW CCA Member
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    No, that's another Bentley error. The checkvalve is on the supply line. Clamped return (but not supply) and it held? Bad FPR. 135psi... I wouldn't recommend holding that for any length of time, but it shows a very healthy fuelpump.

    Jumper wire can be nothing more than a ~3-foot length, one end of which you touch to F23, and the other to the B+ terminal. There may be sparks so make sure there's no fuel vapor around.

  10. #10
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    The check valve is inside the fuel pump at the outlet , in case that is bad, you can install on top of the tank and new check valve as shown here http://www.bimmernut.com/forum/showt...ht=check+valve
    Shogun tricks and tips for the E32 series are HERE!

  11. #11
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    The FPR is leaking pressure slowly when the car is shutdown. That easy, replace the FPR and you should see the pressure be somewhat maintained after the car is shut off. (The valve at the pump is ok because if you pinched the line after the gauge and the pressure did not drop...And since the injectors have been replaced I doubt that theyre at fault.)Not the first one to have this problem and it does not necessarily mean that it will leak into the intake because if it gunks up it may not fully close and leave a little gap for pressure to leak when the engine is off. And it only should start building pressure when the engine is being cranked as it does not prime the fuel pump before that. And the Pierburg FPR costs like 35-40$?

  12. #12
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    Quote Originally Posted by shogun View Post
    The check valve is inside the fuel pump at the outlet , in case that is bad, you can install on top of the tank and new check valve as shown here http://www.bimmernut.com/forum/showt...ht=check+valve
    That check valve has been NLA forever.
    The device on top of some pumps is a dampener, or so I'm told, not a check or no return valve. I believe the pump itself is the check with tired ones leaking back.
    I don't believe the regulator is responsible for holding pressure after shut down.
    as always I stand to be corrected.

    If you can leave two black stripes from the exit of one corner to the braking zone of the next, you have enough horsepower. - Mark Donohue

  13. #13
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    Thanks for the help! It was the FPR. I bought the bosch FPR part # 13531729319

    After shut down, it only dropped about 6 PSI after 20 minutes. I'll go for a drive tomorrow and double check if it starts.

    Now on to the heat soak starter...



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