fuel pressure test

Discussion in 'Engine, Fuel and Exhaust' started by nascar7613, Mar 17, 2011.

  1. nascar7613

    nascar7613 Active Member

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    97 Road King fuel injected will start and run fine when it is cold. After an hour or so of riding it will stall if I let off the throttle. I can keep it running by revving the engine, but if I release the throttle it will die. It won't restart immediately, but if I let it cool off it will. I did a fuel pressure test by connecting a fuel pressure gauge to the end of the hose that comes off the fuel pump and goes to the fuel filter. The result was when I first turn on the key, the fuel pressure jumps up to 60psi, but immediately begins to fall off. Within 15 minutes it is around 10psi. Could the fuel pump be losing pressure internally?
     
  2. Hoople

    Hoople Account Removed

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    You really need to "T" the fuel pressure gauge into the system and check the psi with the engine running. The fuel pump is not running if the engine is not running. Your just reading the 2 second prime shot, so "bleed off" can be expected.
     
  3. nascar7613

    nascar7613 Active Member

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    Thanks for the reply. I gather from what you said that the system doesn't hold pressure for very long if the pump isn't running.
     
  4. Hoople

    Hoople Account Removed

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    Well it does and should hold pressure, you just have to a make a judgement call for the moment at hand. If the pressure fell off in 30 seconds, that really would be bad. If the pressure never fell off, that would be Great. You have to look at readings and judge if they are the reason for the ROOT cause for you original problem.

    As long as the pump can overwhelmingly delivery more pressure and volume to overcome any leakdown, then I would not consider it the cause for your bike issue. Since liquids don't compress, a static pressure drop with that small volume of fuel (1/4" hose 12" long), can't be more than a few of CC's over the course of 15 minutes.

    Yes, going from 60 psi to 10 psi over 15 minutes certainly does mean there is some leakdown present. Can it be improved? I am sure it can. But is it the reason why your having a problem with the bike running poorly, probably not. To be on the safe side of the judgement call, I would run a dynamic gauge pressure test with the bike running. If it stays at the proper value (* see below) while giving it throttle, I would look elsewhere for your problem at hand.

    Now once you find your root problem and correct it, and you want to make things perfect, I am sure you could improve the leakdown in the fuel system.

    ***One very important note: Before going any further, read this thread below starting at entry point #18. I was under the belief that all our FI systems ran at about 60 psi. It appears they do not on the early year bikes. I would confirm (per service manual) what the psi really should be for your exact bike. On the late bikes, it is 60 psi but it seems from reading this thread below that the early models are not the same.***

    My back cylinder just quit working!!! - Harley Davidson Community
     
  5. TQuentin1

    TQuentin1 Well-Known Member Staff Member Moderator

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    Check this:

    Engine Dies And Runs In Spurts - Harley Davidson Community

    TQ