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    Fuel pressure check setup

    I am chasing down a strange problem that only exists on starts after sitting for a long time at least 8h. Fuel pressure is one of my suspects. Can anybody post the set up they used for either a temporary or permanent under hood fuel pressure check?

    Current thought is to add a schraeder valve on the regulated (supply) side using a Tee with 2 barbed connections, one threaded, and Schraeder, then checking with this or similar https://www.fcpeuro.com/products/fue...facturing-3445

    kinda the right idea: https://www.bimmerworld.com/Gauges-D...iABEgLiSPD_BwE
    Last edited by r-mm; 06-19-2019, 09:38 AM.

    #2
    I use a 5v pressure transducer—but the fuel pressure rental kit from AutoZone works just fine. Hook up a tee before the rail and rest the gauge on the windshield.

    Can you describe the issue? What makes you think it's fuel pressure?
    '84 318i M10B18 | 93whp/90ftlbs | 147- Safari Beige | MS2E w/ LC, 2-Step

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      #3
      If you're running into an extended crank problem after sitting for a while, I'd pull out each spark plug and smell for gas. You might have a leaky injector.

      If those are fine, your fuel pump may be leaking pressure back to the tank.
      Originally posted by wholepailofwater
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        #4
        Thanks for the tips guys. The fuel pressure test ran on the supply line before the rail would identify pressure lost to an injector or back into the tank would it not? Does the pump have a check valve in it which fails?

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          #5
          Originally posted by Jaxx_ View Post
          I use a 5v pressure transducer—but the fuel pressure rental kit from AutoZone works just fine. Hook up a tee before the rail and rest the gauge on the windshield.

          Can you describe the issue? What makes you think it's fuel pressure?


          Can you elaborate on how you plumbed the transducer and what gauge you use?


          Sent from my iPhone using Tapatalk

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            #6
            Fuel pressure check setup

            To elaborate on the issues I’m having it’s relatively mild but I would still like to solve it. After the first 90 seconds or so of idling there is absolutely no issue from Idle to redline whatsoever. However when I start the car after it has sat for maybe 6 to 8 hours the following occurs

            Car cranks a few sec longer than usual. Still entirely acceptable and healthy but i note it for forensics.

            Car idles beautifully.

            If i give it any gas it revs nicely but then idle wont catch nicely on the way back down which leads to hunting.

            This used to be merely a quirk but after light flywheel install it often leads to stall.

            The reason i suspect fuel pressure is that I struggle to see what other component or assembly would ‘warm up’ in 90sec and never ever present again.

            Car just had every single engine seal replaced and then some but symptom was present before.
            Last edited by r-mm; 06-22-2019, 08:31 AM.

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              #7
              My point was that they're plenty of ways to measure fuel pressure. If you are simply trying to identify a fuel pressure issue, I would just do a temporary tee during you troubleshooting. Unless you plan on modifying the engine heavily, there's not a real need to continually monitor fuel pressure. I would prefer oil pressure and/or oil pressure if I had one gauge.


              With that said, I think you're on the right track. Yes, where you plan to Tee the line is perfect. Don't tee before the fuel filter(unless you suspect it to be the issue), and don't tee after the regulator. If you decide for a permanent solution, put the pressure sender/transducer in the same spot.

              You should inspect the fuel pressure regulator as well—make sure there's no gas in the vac line, and that removing the vacuum line at idle raises fuel pressure. I have no idea on the check valve. Personally, I would wonder if it's possible for your fuel lines to "empty" without a leak in a line. I presume you have an in-tank pump and in-line? I would inspect both to make sure they're working.
              '84 318i M10B18 | 93whp/90ftlbs | 147- Safari Beige | MS2E w/ LC, 2-Step

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                #8
                I just had this happen to me. It was the fpr. If i let it sit and then turn key directly to start, it would not start... just crank. But if i turned it to position 2 and waited 2-3 secs, it would start every time. The fpr was not completely failed, but was failing. Swapped it out and fixed the problem.

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                  #9
                  I have a new Pierpoint FPR. I also understand that the fuel pump on late motronic cars only runs when the car is cranking.

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                    #10
                    i added a mechanical fuel pressure gauge.

                    on the hose going to the fuel rail i added an adapter and a fuel gauge there.

                    i was also troubleshooting some problems.

                    Untitled by ALEXANDER RENGIFO, on Flickr
                    Last edited by iwantspeed; 06-26-2019, 03:07 PM.


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                      #11
                      Originally posted by iwantspeed View Post
                      i added a mechanical fuel pressure gauge.

                      on the hose going to the fuel rail i added an adapter and a fuel gauge there.

                      i was also troubleshooting some problems.

                      Untitled by ALEXANDER RENGIFO, on Flickr
                      I'm just getting around to doing this. Can you share what parts you used for your Tee and Gauge?

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                        #12
                        plumb in a pressure gauge, and restart as normal and monitor the pressures. do the test again after priming the pump prior to cranking.

                        with the factory setup the fuel pump does not run with ignition on.

                        Also the ICV is important while the engine is cold and warming up
                        89 E30 325is Lachs Silber - currently M20B31, M20B33 in the works, stroked to the hilt...

                        new build thread http://www.r3vlimited.com/board/showthread.php?t=317505

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                          #13
                          I found the following (also posted on Bimmerforums)

                          42psi fuel pump jumped, engine not running

                          32psi Idle (engine running)
                          43psi idle, vac line to FPR removed
                          90+psi idle, with return line clamped

                          38psi engine OFF
                          32psi 5min later
                          26psi 10min
                          18psi 15min
                          11psi 20min
                          5psi 25min
                          0psi 30min

                          Restarts very well, pressure back to 32psi

                          FPR is new Pierburg 3.0 Bar (stamped on side). Fuel pump is unknown presume original 123k miles.

                          Bentley calls for 40-46psi idle.

                          Not sure how to interpret these results but am wondering if there isn't too much vacuum at idle = greater reduction in fuel pressure from nominal 3bar

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                            #14
                            FPR are checked with the vac hose disconnected. 43PSI pretty much 3 bar.

                            32 seems low at idle.

                            How old is your fuel filter, Any lines pinched?

                            Road test it and see if it stays at 42~ at full throttle

                            The drop of fuel pressure could be the pump or leaking down at the injectors - Extended start, rough start is probably injectors. You can upgrade to BOSCH Gen 3 injectors.
                            Zach@Zakspeed.us

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                              #15
                              Because I have 3bar/43 psi with vacuum disconnected or fuel pump relay jumped and can build over 90psi with return line pinched I tend not to suspect the pump or supply lines. Since more vacuum = less fuel pressure wouldn’t one suspect there is too much vacuum at idle, or that the FPR is over reacting to vacuum? Bentley calls for 40.6 psi idle, ‘more’ with vacuum line disconnected.

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