911 SC fuel pressure too low - does not start

  • Hello my name is Marco and I'll start right away that my Porsche does not start because the fuel pressure is too low

    Targa 911SC 930/16 engine MJ 1983


    The car was parked for a long time until I bought it. After the first small service (including replacing the fuel pump) the car ran. I then carried out various body and interior jobs.

    The tank had to be removed because I discovered that it was very rusty.


    Now I want to finalize everything, but the Porsche won't start. The 1 year old fuel pump no longer works. Presumably it had seen too much rusty fuel and no longer wants to. So I replaced the fuel pump and filter again.

    After a few attempts at starting, I checked the fuel under both heat exchangers


    Checked the ignition distributor and all spark plugs for a decent spark, looked good.

    So I measured the fuel pressure.

    • When I measure the control pressure, it only rises very slowly
    • Maximum system pressure was 2.6 bar (but it takes over a minute to get there)
    • Interestingly, the system pressure then dropped again to 1.9 bar

    Of course I didn't measure any further at this point, because one of the problems is already obvious.


    The pressure remains in the system for a very long time, even after 30 minutes I still have 1.7 bar (I know the level is low, I just want to say that despite the fact that the pressure drops in the meantime, it is not generally incorrect).


    Now the question where do I best start to replace?

    Pressure accumulator seems to be very old, could be replaced, but not absolutely necessary...

    Could it be that something has clogged up in the flow divider due to corrosion and that's why the system pressure is too low?

    As far as I know, the fuel filter also has a bypass, so I would rule that out, right?

    Sure, the fuel pump is new, but of course it could be defective.

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  • there are mt with inlet strainer. and warm-up regulator with inlet strainer. there may be a blockage. the further drop in pressure would also indicate this. something must be clogging up. first of all, I would measure the flow rate. then put the injection nozzles in bottles and see if anything comes out at all. the opening pressure should not go away.

    then take out the system pressure regulator and see if the piston is free - be careful not to lose the washers.

    only then think about replacing parts. it only costs money! without a real cause, it won't help!


    pressure accumulator or valve on the pump are more likely to fail with the symptoms.

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  • there are mt with inlet strainer and warm-up regulator with inlet strainer. There may be something to it.

    It should be noted that the inlet of the WLR is actually a return line from the MT. In other words - if the dirt has arrived in the WLR, then it has already passed through the MT...ergo the MT must then at least be cleaned, but in all likelihood more likely to be overhauled... It is by no means the case that the fuel filter alone retains all the dirt that comes out of the tank. Strictly speaking, the whole system has to be cleaned. Otherwise you're always chasing after some dubious problems, which in the long run can completely take the fun out of the car...as the car has been standing for a long time, this would be the only sensible measure. Correct pressures (and tightness of the vacuum system) are simply everything with the K-Jetronic. If these are not correct, the bowl will never run properly.

    The system pressure on the 930/16 must be at least 4.5 bar to approx. 5.2 bar.

    The control pressure at the WLR must rise more or less directly to the value corresponding to the temperature.

    The system pressure must naturally drop to the holding value of around 2 bar after switching off the engine/the BP and slowly drop to at least 1.3 bar or higher after 30 minutes. This seems to be the case for you so far. You must therefore look for the reason why the system pressure does not reach the value of around 5 bar.

    In this case, all parts, or rather all actuators, of the K-jetronic must also be checked for correct function. In other words, it must be verified by measurement and/or function test that each part does what it is supposed to do. Either in the car with the appropriate equipment or outside on the workbench in a "test environment".

    Greetings, Thomas

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    81er ex-US 911 SC Coupé, platinmet., eSSD, K-Leder sw., Turboheckflügel & -frontspoiler, NSW, Maxilite 7/8J, Bilstein Fw("SpoStra" - noch...), 915/61, 930/16 o.Kat, WebCam 20/21 NW, hydr.KS, Dansk 92.502SD, 123 Ignition, orig. Porsche SWV, Seine Systems Gate Shift Kit, Momo Prototipo m.Nabenerhöhung


    „Nicht das Auto verdirbt den Charakter, aber wer keinen Charakter hat, sollte nicht Auto fahren.“

    Ferdinand Porsche

  • The dirt is just an assumption, albeit an obvious one, given the course of the problem. I would therefore first remove the fuel pump again and measure the flow rate. (just to rule this out)

    As I "remove" the WLR from the measurement by closing the tap, any contamination would also be a problem, but cannot be the cause for the time being. It is clear that if the MT is clogged, I will not be able to avoid cleaning the WLR accordingly.

    If there is a strainer on the MT, then this is in the banjo bolt, right? or is it somewhere else? There was nothing about a strainer in the parts catalog, but the banjo bolt should give me an idea of what it looks like in the MT.

    Just thinking out loud... since the system pressure rose when measuring and then dropped again... and there is assumed to be no leak in the system because the accumulator was able to hold the pressure... the pressure must almost escape in the direction of the tank. So it must at least be assumed that the pressure regulator is open? Because otherwise the pressure would continue to rise even if it was dirty? or is the path through the MT towards the WLR susceptible to blockage? otherwise the pressure would continue to rise anyway, albeit slowly, or am I thinking too complicated here?

    Can the system pressure regulator be removed when the MT is installed?

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  • The dirt is just an assumption, albeit an obvious one, given the course of the problem. I would therefore first remove the fuel pump again and measure the flow rate. (just to rule this out)

    Can be done in the same way when installed, simply open the line to the fuel filter, place a sufficiently large collecting vessel underneath, bridge the BP relay or disconnect the safety switch on the baffle plate housing, switch on the ignition and off you go.

    As I "remove" the WLR from the measurement by closing the tap, any contamination would be a problem, but cannot be the cause for the time being. It is clear that if MT is clogged, I will not be able to avoid cleaning the WLR.

    I would be surprised if the system pressure increased due to the closed tap of the pressure measuring device, I suspect other causes.

    Just thinking out loud... since the system pressure rose when measuring and then dropped again... and there is assumed to be no leak in the system because the accumulator was able to hold the pressure... the pressure must almost escape in the direction of the tank. So it can at least be assumed that the pressure regulator is open? Because otherwise the pressure would continue to rise even if it were dirty? or is the path through the MT towards the WLR susceptible to blockage? otherwise the pressure would continue to rise anyway, albeit slowly, or am I thinking too complicated here?

    Is it possible to remove the system pressure regulator when the MT is installed?

    There are so many places where the pressure can escape... backwards through the BP and non-return valve, forwards through the injection valves, via the flow divider, etc.

    Yes, you can remove the pressure regulator when it is installed.

    But before you start tampering with the K-jetronic, please do your research, watch videos, read manuals, etc. so that you know the function and structure. Something can be damaged too quickly and replacements are expensive.

    Greetings, Thomas

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    81er ex-US 911 SC Coupé, platinmet., eSSD, K-Leder sw., Turboheckflügel & -frontspoiler, NSW, Maxilite 7/8J, Bilstein Fw("SpoStra" - noch...), 915/61, 930/16 o.Kat, WebCam 20/21 NW, hydr.KS, Dansk 92.502SD, 123 Ignition, orig. Porsche SWV, Seine Systems Gate Shift Kit, Momo Prototipo m.Nabenerhöhung


    „Nicht das Auto verdirbt den Charakter, aber wer keinen Charakter hat, sollte nicht Auto fahren.“

    Ferdinand Porsche

  • There are so many places where the pressure can escape...backwards through the BP and non-return valve, forwards through the injection valves, via the flow divider, etc.

    I may have expressed myself incorrectly. Fuel pump was running. Pressure rose (too slowly) to 2.6 bar and then dropped again (pump was still running). Ball valve was closed. So I assume that the pressure was at the system pressure regulator and this then opened and something was blocked between the MT and WLR inlet, which is why the pressure build-up was delayed.

    Or the pressure regulator is open too early or always open. Because even with a low flow rate, the pressure at one point (MT inlet) and the other point cannot be so different. This should equalize over time.

    Now that I'm writing like this, the idea ("forward through the injection valves") is very obvious. I'll have a look at that and measure the flow rate of the fuel pump. I'll report back...

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  • So the nozzles hold the pressure and none of the nozzles open at the current pressure, the system pressure rises slowly, only from 2.4 bar something came at one of the screws of the nozzle and fuel line, but I had only screwed them tight by hand.

    Then I wanted to have a look at the pressure regulator. I unscrewed one of the nozzles and expected the fuel to spray out. However, there was no continuous stream of gasoline, but many interruptions, as if there was a lot of air in the system. I have never heard of a venting system, is there one?

    The pressure regulator looks a little dirty, but I haven't managed to get the piston out yet.

    I then had to go up because my son had woken up again and wanted me to put him to bed.

    Tomorrow I'll check the pressure regulator and then put it back in. Once I've measured the flow rate of the fuel pump, I'll still try to bleed the air through one of the fuel lines to the nozzles.

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  • air is possible. if the system was empty, it takes a little while until all cylinders wake up. but that is not the problem. the pressure is missing. this needs to be clarified - then fine-tune with all the gimmicks. i.e. fuel quantity pump. compare injection quantity for all cylinders. co....

    but always in order: otherwise it will only get worse!

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  • Fuel pump is running. I had 500ml in just over 10s. So this can be ruled out for now.

    I removed the system pressure regulator (without the piston). This did not look worrying. I cleaned it in an ultrasonic bath. However, the fluid was then very discolored.

    Put the system pressure regulator back in.

    Pressure test: System pressure rises very quickly to 4.9 bar. Very ideal, so a step in the right direction. But now the injection valves inject directly. Not just a little but felt completely open. (Again, not perfectly even, but there is no point in adjusting this now)

    Then control pressure measured 3.2 bar (20 degrees), i.e. too high.


    Now to the question:

    Doesn't too high a control pressure tend to push the control piston down, so that the fuel distribution to the cylinders is closed rather than open?

    My assumption is therefore that the control piston is hanging in the MT. Or are there other ideas?


    Where does the excessive control pressure come from? If the system pressure regulator piston (great word) did not move, the pressure would have to rise. But then the system pressure should be closer to 6 bar if the fuel pump delivers enough fuel as tested above. Due to the fact that the injectors are clearly draining fuel/pressure, this may of course be the case.

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  • If the spool is hanging, you can feel this when you play with the flap.
    You should feel a slight but constant resistance during the movement.
    At the same time, the sound of the injection nozzles must change.

    The control pressure is basically regulated by changing the cross-section in the warm-up regulator.
    If the cross-section is additionally reduced by dirt, you will have a higher control pressure as a result.
    The fine screen in the warm-up regulator often becomes clogged.

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    911 SC USA 1980, 930/07 - mit Zyl/Kolben vom 930/10, SSI und Dansk 92.510S,

    digitale ECU & ZZP-Steuerung von AndrewCologne, anthrazit auf Füchsen 6 & 7/16, A/C delete, Heat backdate