Bosch K-Jet/CIS Metering Plate is Wide-Open at idle, cause?? | FerrariChat

Bosch K-Jet/CIS Metering Plate is Wide-Open at idle, cause??

Discussion in 'Technical Q&A' started by Sandahl, May 9, 2021.

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  1. Sandahl

    Sandahl Rookie

    Dec 1, 2006
    10
    #1 Sandahl, May 9, 2021
    My 1988 Mondial 3.2 is in a state right now where it can be started to a rough idle and it has very little response to the throttle opening at low RPM and no load but nowhere near enough to be drivable. I can idle it with or without the frequency valve active and opening the throttle mostly just leans it out.

    The most obvious symptom is that the metering plate immediately get sucked wide-open even with the low airflow requirement of simply idling. This, along with the fact that the engine can actually idle and doesn't flood, leads me to believe that I must also have very low fuel pressure (or else a WOT-amount of fuel would be pouring out of the injectors as I understand it anyway).

    Prior to replacing the fuel pump yesterday the car would not run at all, but shortly before the old fuel pump completely failed the car had already reached the state that it's in now. I had optimistically thought that the problem was solely due to the failing/failed fuel pump.

    My question is where to look for a failure that would cause low fuel pressure AND low control pressure? I can't completely rule out the new fuel pump but it seems unlikely.

    The accumulator and filter are passing fuel since it is able to (roughly) idle. If these were simply constricting the flow, I would still expect the control pressure to reach something close to normal and the metering plate to have some resistance when pressed when the fuel pump is run without the engine running, but that is not the case: the plate requires almost no effort to open even with the pump forced on and the engine stopped.

    Since the frequency valve has very little effect on the situation, I think that leaves just the warm-up regulator and the fuel distributor itself. The symptoms don't change even after idling long enough to expect the WUR to raise the pressure.

    Any ideas on what's failed in this case or how to determine if it's a failure in the fuel distributor vs. warm-up regulator (or other)?

    A relevant question to help me understand it better might be "can a failed warm-up regulator bleed off enough fuel to drop the fuel pressure to almost nothing?"

    Thanks!
     
  2. raemin

    raemin Formula 3

    Jan 16, 2007
    1,858
    Lyon (FR)
    Full Name:
    R. Emin
    #2 raemin, May 9, 2021
    Last edited: May 9, 2021
    There is system pressure and control pressure that are counteracting to stabilize the plate. Based on your explanation I would say the control pressure is close to zero, could definitively be a warm-up regulator. Also if your car has some anti-polution device, this feequency valve could be the culprit as it modulates the pressure in order to achieve better emission.
     
  3. spicedriver

    spicedriver F1 Rookie

    Feb 1, 2011
    3,859
    #3 spicedriver, May 9, 2021
    I would start by checking the DC voltage @ the fuel pump to make sure it's getting the full 12 V.

    It is also possible to have a clogged fuel filter. These should be changed every year.

    This is a case where the test gauge setup would tell you what your fuel pressures are.
     
  4. raemin

    raemin Formula 3

    Jan 16, 2007
    1,858
    Lyon (FR)
    Full Name:
    R. Emin
    #4 raemin, May 10, 2021
    Here are two documents quite handy. First a general description of the system, and second a usefull step by step procedure on how to restart a stranded Porsche 928.

    Please note that there a many filters on these systems. Small mesh filter on the WUR inlet, (sometimes) small mesh filter on the pressure valve (distributor inlet), fuel filter after the pump (the one we usually change), and filter in the tank (i.e before the pump) that we usually forget. A failed pump is usually the sign that the filter before the pump was not properly serviced. I have no knowledge on your car, but check on your manual if you do have such a tank filter.
     

    Attached Files:

  5. Sandahl

    Sandahl Rookie

    Dec 1, 2006
    10
    #5 Sandahl, May 10, 2021
    Thanks for the replies with all of that additional knowledge and documents! I will dig into it further over the next weeks and post the eventual cause and solution to help out anyone else in the future.
     
  6. Sandahl

    Sandahl Rookie

    Dec 1, 2006
    10
    #6 Sandahl, May 11, 2021
    TEST: When looking at the K-Jet/CIS pressure testing procedure, I noticed that one of the tests simply has the supply line to the WUR closed off with a valve in the line after the gauge and the pressure on the gauge (before the valve) reads the "system pressure" (i.e. the higher pressure) that is regulated by the spring in the fuel distributor itself (please correct me if I don't have this right!).

    Since I don't have a gauge kit on-hand, I duplicated this to take the WUR out of the equation by removing the supply line from it and blocking it with a bolt through the banjo fitting along with the crush washers. Also, I disabled the frequency valve.

    As I understand it, with no WUR bleed-off, this setup then would set the pressure to "system pressure" and I would be able to observe that as a significant increase in the metering plate's resistance to movement if the WUR was the culprit.

    TEST RESULTS: when performing this quick test, I'm still seeing virtually no fuel pressure and no control pressure holding back the metering plate; the car will roughly idle as described earlier but no change in the control pressure and fuel pressure. The metering plate still goes wide-open at idle. I also hear a slight squeaking sound (not buzzing like the frequency valve) that seems to be coming from the fuel distributor when I force the fuel pump on and the engine is not running.

    That leaves me with either a non-working brand new fuel pump (which seems unlikely) or a 33 y/o fuel distributor with a failed internal pressure regulator.

    I still need to test the pressure of the fuel pump to be sure, but if this is all correct reasoning and I have a failure in my fuel distributor, any suggestions on what to do next with the fuel distributor?
    Is there anything I can do with the fuel distributor in-place like opening the port where the shims and spring are and perhaps even replace the spring without removing the whole thing?

    Thanks for any feedback!
     
  7. spicedriver

    spicedriver F1 Rookie

    Feb 1, 2011
    3,859
    #7 spicedriver, May 12, 2021
    You still should check the DC voltage across the fuel pump while it's running. These cars are known to have fuse box problems. Meaning the pump may not be getting enough juice to produce the correct pressure. Unless you're living in Antarctica, you should be able to pick up a voltmeter in less than an hour. Also, I would advise getting the proper fuel pressure meter setup before tearing apart the fuel distributor. That should only take a few days shipping. Otherwise you could end up creating more problems than you already have.

    Also, I would add a few gallons of fuel to make sure you're not running out. The fuel gauges on these cars aren't always reliable. Do this first.
     
  8. raemin

    raemin Formula 3

    Jan 16, 2007
    1,858
    Lyon (FR)
    Full Name:
    R. Emin
    #8 raemin, May 12, 2021
    The control pressure is basially system pressure minus whatever the WUR lets escape back to the tank plus whatever your frequency valve prevents from escaping through the WUR.

    If you remove these 2 components, control pressure = system pressure (i.e control pressure is set to its maximum). This higher the control pressure the less the plate raises, the less fuel is sent to the injectors.

    As @spicedriver said, you could also have a faulty electric power supply.

    When it is faulty the problem is not pressure control, but hot start problem.

    No you just have to check the flow that is returning to the tank: When then distributor plate is low, the pressure regulator sends back most of the gas to the tank. In order for this to append, the pump must be in a position to provide enough pressure so as to allow the gas to pass trhough the pressure valve. So if you do have 850cc/30s on the return line, you know the pump does work properly.

    Don't mess with the distributor until the car is somehow working.
     
  9. Sandahl

    Sandahl Rookie

    Dec 1, 2006
    10
    #9 Sandahl, May 12, 2021
    Wow, thanks for the additional insight! I will pursue the fuel pump pressure, return flow rate and supply voltage next.
     
  10. Andy 355

    Andy 355 Formula Junior

    Jun 2, 2013
    434
    Sydney
    #10 Andy 355, May 21, 2021
    Cis is very reliable. I would check the fuel supply to the pump. Nothing can happen if the main pump starves.
     

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