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  • Fuel Pressure Measurement - Sticky

    Somebody asked me to do this... Can anyone spot any problems with how I have captured things ?

    Note I have shown use of two filters as typical for aftermarket and modified vehicles. S2/RS2 has single fuel filter under rear of the car near the tank whereas S4/S6 has it up in the engine bay.

    Fuel pressure regulator is always at end of the fuel rail so that it controls the pressure applied to all injectors evenly.

    The return side of audi fuel pressure regulators is always at the bottom of the device.

    I shall add some notes to this thread on how to 'hacktivate' the fuel pump relay so that its possible to test behaviour of fuel rail pressure in response to a variety of control/manifold pressure without the engine actually running.

    I just realised that I haven't included a non-return valve on the feed... our fuel pumps incorporate that but others may not. This (along with the diaphragm in the regulator) ensures there is some pressure stored in the fuel rail after shutdown to make for swift restarting... vs the pump having to push fuel all the way up the pipe to engine every time the motor is restarted.
    Attached Files
    Paul Nugent
    Webmaster http://S2central.net
    Administrator http://S2forum.com

    1994 S2 Coupe ABY - aka Project Lazarus
    2001 A6 allroad 2.5TDi - family tank
    2003 S4 Avant 4.2 V8 - daily burble

    Purveyor of HomeFries and Exclusive agent for Samco hose kits (S2/RS2)

    There are only 10 kinds of people that understand binary - those that do, and those that don't

  • #2
    Nice diagram Paul. As for activating the fuel pump during a non-engine test, on the C4s you can power the fuel pump through the (+) post on the firewall scuttle to the downstream side of the No. 17 fuse. I don't know the equivalent B3/B4 fuse number.

    To test the fuel pressures under different manifold pressures, it is possible to hook up an little air pressure pump to the vacuum/pressure connection on the regulator and simulate the various boost pressures. This is explained in this procedure to directly relay the fuel pump on a C4 UrS4/S6 to the battery (under the back seat).

    http://forums.quattroworld.com/s4s6/msgs/24327.phtml
    RS2'd 93 UrS4 5 spd sedan
    94 UrS4 V8 6 spd manual avant

    Comment


    • #3
      I know how to do that ok... Have mityvac and know how to use it... Just not found time to write it up.
      Paul Nugent
      Webmaster http://S2central.net
      Administrator http://S2forum.com

      1994 S2 Coupe ABY - aka Project Lazarus
      2001 A6 allroad 2.5TDi - family tank
      2003 S4 Avant 4.2 V8 - daily burble

      Purveyor of HomeFries and Exclusive agent for Samco hose kits (S2/RS2)

      There are only 10 kinds of people that understand binary - those that do, and those that don't

      Comment


      • #4
        i am interested in this mityvac solution...can you ellaborate please paul...will the gauge read what ever pressure the fpr is running(max)

        Comment


        • #5
          interesting information, when I tested mine I was getting about 2.6 bar on a 3B and 0.5 bar vacuum which seems to correlate.
          1991 Audi S2 Coupe

          Comment


          • #6
            So - on the subject of fuel pump relay 'hacktivation' - hopefully the picture explains it well enough... but here are some words to clarify some stuff.

            1 - The jumper bridges the two fat relay pins in position 10 to make the pump run without the key or ECU involved. It effectively connects unswitched 12V to the live terminal of the fuel pump.

            2 - This is safe because the output side of the jumper goes immediately into Fuse 13 en route to the pump

            3 - You need 3/8" spade terminals to do this, not the normal 1/4" jobbies...
            Attached Files
            Paul Nugent
            Webmaster http://S2central.net
            Administrator http://S2forum.com

            1994 S2 Coupe ABY - aka Project Lazarus
            2001 A6 allroad 2.5TDi - family tank
            2003 S4 Avant 4.2 V8 - daily burble

            Purveyor of HomeFries and Exclusive agent for Samco hose kits (S2/RS2)

            There are only 10 kinds of people that understand binary - those that do, and those that don't

            Comment


            • #7
              I cant help myself with a soldering iron and heatshrink... enter 'the hacktivator' with lil box and a nice momentary button to make it easier for any timed tests of flow from the pump.
              Attached Files
              Paul Nugent
              Webmaster http://S2central.net
              Administrator http://S2forum.com

              1994 S2 Coupe ABY - aka Project Lazarus
              2001 A6 allroad 2.5TDi - family tank
              2003 S4 Avant 4.2 V8 - daily burble

              Purveyor of HomeFries and Exclusive agent for Samco hose kits (S2/RS2)

              There are only 10 kinds of people that understand binary - those that do, and those that don't

              Comment


              • #8
                So I used the hacktivator today and its a great assistant for this kind of testing...

                Firstly a top tip... to minimise fuel spillage when you open the feed pipe joint. Run the engine and then pull out the fuel pump relay to stall it. The ECU holding relay keeps the injectors alive and trying to squirt until the fuel supply dries up i.e. very little fuel in the line & rail.

                Next - a word of warning to anyone interfering with the fuel lines - it took me five attempts to seal the feed joint correctly - it hasnt been moved in ~18yrs so I guess that it didnt like being disturbed. The joint is just a flare connection and it needs to be surprisingly tight. You need a 17mm spanner to hold the flexi joint and a 14mm on the flare joint - a brake pipe style spanner would be better here than open ended.

                I used pipe sealant on the threads too and that took a few attempts until it was properly nipped down. Phew.....

                First picture shows the fuel feed line opened.

                I then calibrated a 1liter bottle with water from a measuring jug, emptied it out and used it as a receiver for fuel at the feed line for FIFTEEN seconds while I hacktivated the pump.

                Second picture shows I got ~650cc fuel in that time for just over 2.5l per minute or 150 liters per hours. That is the original factory ABY pump with 12V battery ==> less at pump terminals and free flowing without any restriction from the fuel rail.

                According to the manual, my result is a good one as 10-11V at the pump terminals should yield 500-600cc in the same 15 second test.

                I will do further tests under pressure and perhaps with a relayed power feed to the pump that comes from closer to the battery (less voltage drop).
                Attached Files
                Paul Nugent
                Webmaster http://S2central.net
                Administrator http://S2forum.com

                1994 S2 Coupe ABY - aka Project Lazarus
                2001 A6 allroad 2.5TDi - family tank
                2003 S4 Avant 4.2 V8 - daily burble

                Purveyor of HomeFries and Exclusive agent for Samco hose kits (S2/RS2)

                There are only 10 kinds of people that understand binary - those that do, and those that don't

                Comment


                • #9
                  Does anybody knows where to get a t-connector to screw in a fuel pressure gauge at this place?
                  [__]=oooo=[__]

                  Comment


                  • #10
                    there is separate thread on that exact question... seemingly 034 have one for $tupid money... I havent pursued my search for a cheaper one this week
                    Paul Nugent
                    Webmaster http://S2central.net
                    Administrator http://S2forum.com

                    1994 S2 Coupe ABY - aka Project Lazarus
                    2001 A6 allroad 2.5TDi - family tank
                    2003 S4 Avant 4.2 V8 - daily burble

                    Purveyor of HomeFries and Exclusive agent for Samco hose kits (S2/RS2)

                    There are only 10 kinds of people that understand binary - those that do, and those that don't

                    Comment


                    • #11
                      thx
                      [__]=oooo=[__]

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