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Thread: K75S fuel pressure regulator testing

  1. #1
    broadstone
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    K75S fuel pressure regulator testing

    I seem to have lost the followers on my previous thread re fuel issues so here's the question. I have reason to believe that my fuel pressure regulator is stuck closed. To try and test this I disconnected the return line at the fuel rail and tried blowing through it. I couldn't move air through it using my mouth so I applied approx - 25psi on the vacuum hose after disconnecting it from the throtle body and still could not blow air through the return hose. Does this indicate a malfunctioning regulator or am I totally misunderstanding the operation?
    Last edited by broadstone; 10-23-2012 at 10:49 PM.

  2. #2
    K75RT Keith
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    The fuel pressure regulator bleeds off any pressure above 38psi

  3. #3
    broadstone
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    K75 fuel pressure regulator

    Thanks for the reply, kkeller. I know that the regulator is supposed to control the pressure at the rail but what is the function of the vacuum line if not to relieve pressure? At the risk of sounding dumb, if the pressure is controlled simply by spring loading in the body of the regulator, why is there a vacuum line connected to it in the first place? I assume there is no good way to test the regulator except through pressure testing and/or exchanging the regulator.

  4. #4
    3 Red Bricks
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    The basic operation of the regulator IS a spring relief. Everything above about 36-38 psi gets bled back to the tank.

    When the manifold vacuum drops (as during hard acceleration) the regulator only bleeds off everything above 40-42psi, thereby enrichening the mixture. This is the function of the vacuum hose.


    I do not believe that applying vacuum to the regulator would bleed the rail to zero. If it did, when you rolled off the throttle going into a corner, it would drop the pressure in the rail to zero. Then when you went to pick up the throttle mid-corner there would be no fuel pressure and you would get a HUGE bog!


    The ONLY way to check the regulator is by doing a fuel pressure test!!!


    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
    Mine, Hers, Spare

  5. #5
    Nick Kennedy
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    Function of the vacuum line on the Fuel Pressure Regulator:
    The amount of vacuum in the line is controlled by, intake manifold pressure, which is always lower than ambient air pressure.
    This vacuum line is attached to the intake manifold, so the vacuum in this line is actually manifold pressure, which is a NEGATIVE PRESSURE, a Vacuum.
    At idle, low and moderate throttle settings this vacuum is quite high, at full throttle there is less Vacuum, but it is still a vacuum. Remember these are ALL NEGATIVE AIR PRESSURES.
    At full throttle, the injectors are under slightly higher fuel pressure and spray in MORE fuel per pulse, due to the Fuel Pressure Regulator reacting to the attached vacuum line being under LESS VACUUM
    At Idle, low and moderate throttle the injectors are under slightly lower fuel pressure and spray in slightly less fuel, Due to being under MORE VACUUM.
    In my limited experience working on the Bosch L Jetronic fuel injection system, and testing these Fuel pressure regulators the fuel pressure difference with the vacuum line hooked up or not
    was/is very slight. You do need to hook up appropriate gauge to see what is up.

    I hope this was some help.

    The confusing thing is the the phrase " manifold pressure"
    It is really a negative pressure....
    The only time the manifold pressure reaches AMBIENT pressure is when the engine is off.
    Nick Kennedy

  6. #6
    Curmudgeon At Large Bobmws's Avatar
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    Quote Originally Posted by broadstone View Post
    I seem to have lost the followers on my previous thread re fuel issues so here's the question. I have reason to believe that my fuel pressure regulator is stuck closed. To try and test this I disconnected the return line at the fuel rail and tried blowing through it. I couldn't move air through it using my mouth so I applied approx - 25psi on the vacuum hose after disconnecting it from the throtle body and still could not blow air through the return hose. Does this indicate a malfunctioning regulator or am I totally misunderstanding the operation?
    Really?!

    I posted this yesterday on that thread:

    Quote:
    "Originally Posted by broadstone
    98Lee, I quoted the wrong book in referring to relieving fuel pressure. It was on page 6.5 (middle of page) of the 1999 Haynes manual that discusses alternative methods of releasing fuel pressure from the rail. It makes sense to me that this would work but, indeed, I still wasn't able to blow air through the return line even with nearly -40 psi of pressure applied to the vacuum line at the throttle body. Because of the significant task of getting to and removing the FPR, I'm actually hoping that someone will convince me that my conclusion re a frozen closed regulator is wrong. Thanks again."

    There have been numerous recommendations telling you to do an actual check of the fuel pressure. Without that knowledge you can not diagnose any further.
    The parts are available at any box store; a brass tee, some barbed hose fittings to screw into the tee, a cheap pressure gauge from the water well section, some fuel hose and some clamps.
    Less that $20 material, less than 20 minutes to put it together and hook it up.
    If your pressure is in the 38 psi range your FPR is fine. If the pressure is high, then you may be correct in your assumption.

    No other way to confirm that than to do the test.

    Following that I would look at pulling the injectors and to see if they are frozen open. If open you should be able to blow through it. You can apply 12V to an injector and should hear/feel it click as it moves. They can be cleaned locally, look for a diesel injection service.

    I know this because I had stripped my former 89 K100RS down for paint, basically had the fuel rail open when I ruptured a disc in my back. 8 months later I was getting everything back together and couldn't get the bike to run. Pressure test showed the FPR was fine, but I found the injectors to be frozen.
    __________________
    Bob Weis
    '04K12RS - Hannigan Hack
    www.earplugco.com

    Since you just don't want to take the advice that has been presented previously, feel free to continue flailing, go ahead and purchase a replacement FPR without testing and see what happens.....
    Bob Weis
    '04 K12RS - Hannigan Hack
    www.earplugco.com

  7. #7
    broadstone
    Guest

    K75 fuel pressure

    Many thanks for your clear and pertinent explanations; they make sense now. I will not continue on this tack until I check the fuel pressure with a proper meter. I have enough other issues that I need to address anyway. One of these is that it looks like a leg of the wiring harness was eaten by the radiator fan. After a fashion I'm sure I can splice it back properly. Who knows, it might solve my problem....fat chance. Many thanks again.

  8. #8
    3 Red Bricks
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    IMPORTANT QUESTION!!!!!

    Is the wire to the temp sensor the one that is chewed up????



    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
    Mine, Hers, Spare

  9. #9
    broadstone
    Guest

    K75 fuel pressure

    Thanks for the suggestion re the temp sensor. I'm still trying to determine where the various broken wires originate. Btw, I bought a fuel injection wiring harness on eBay. Even if that doesn't solve the problem, it may be one less thing to be concerned about. One of the broken wires, in fact, could be to the temp sensor. I'll update when the harness arrived and I get it installed. Could this wire have been the Primary cause of the problems spoken about in this forum?

  10. #10
    3 Red Bricks
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    Quote Originally Posted by broadstone View Post
    Could this wire have been the Primary cause of the problems spoken about in this forum?
    Jim,

    As per our phone conversation this morning, yes the broken wires to the temp sensor and in the fuel injection harness are almost definitely the source of the problem. Please keep the forum updated with you progress. Good talking to you.


    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
    Mine, Hers, Spare

  11. #11
    broadstone
    Guest

    K75 fuel pressure

    Thanks again, Lee, our conversation was enlightening. Since we spoke I checked the fuel pressure and found it to be within specifications so that theory of a stuck closed regulator is put to rest. As I said, I ordered a used wiring harness through eBay to replace mine which has some crushed and severed wires and in following the schematic I found that one of these cut wires completes a connection between the fuel injector control unit and the temp sensor. This discovery confirms your suggestion that this was the potential source of my problem. I'm expecting the delivery of my new fuel injection harness in a couple of days so started preparing for that. The only issue I'm having now is that I'm having difficulty finding the terminus of a leg of this harness which disappears behind the air box. It has 4 conductors including a ground, white-red, white-grey, and green-red wires and I'm assuming that at least one of these is to the temp sensor which I haven't yet located. Do I have to remove the fairing to get to this? It's an S model.

  12. #12
    broadstone
    Guest
    Correction....I stated that I suspected that one of the conductors in the leg of the harness behind the air box was to the temp sensor. What I meant to say is the temp sending unit instead.

  13. #13
    broadstone
    Guest

    K75s fuel pressure, etc

    Because I bought this bike having been stored not running for several years as "pig in a poke", I had no knowledge of its mechanical history. As I earlier explained, I had major flooding when attempting to start it so spent much of my time (a couple of weeks) trying to diagnose its origin primarily suspecting fuel pressure regulator and/or injector problems. I was wrong and when I discovered that a multitude of wires in both of the harnesses were severed, I began to check schematic diagrams in an attempt determine where these conductors originated and terminated. Lee helped me with this quest for routing and predicted that wiring problems were the likely source of my fuel and ignition problems. Evidently, portions of the main and injector harnesses were compromised partly as a result of being shredded by the radiator fan. Specifically, the fan motor, temp sending unit, communication circuits between the fuel injector control computer and the ignition control box as well as both wires to the air mass sensor were compromised. I bought a used injector control harness and spliced non essential wiring for things not directly related to its ability to run such as the hazard flasher relay, horn, gear position indicator, etc. Eureka, it now runs and everything seems to work with the exception of rear turn indicators, hazard lights and the horn but I'll keep working on those thing.

    Thanks VERY much to all of you for your time and tech assistance with special thanks to Lee who should write a book.

  14. #14
    Curmudgeon At Large Bobmws's Avatar
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    Glad to hear you found and corrected the problem. Use some caution securing the new wiring so it stays clear of the fan, but is not to tight that it is stressed with normal movement, handlebar turning, forks compressing.
    Enjoy the K75.
    Bob Weis
    '04 K12RS - Hannigan Hack
    www.earplugco.com

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