intermittent fuel pressure issue
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intermittent fuel pressure issue

  1. #1
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    Default intermittent fuel pressure issue

    So I finally got the engine running good, timing was dead on, fuel pressure was pretty steady, steady idle, things were good. I go out to the river, get one good holeshot and then by the time it got out of the water it was dying on me. My fuel pressure went down to about .5 psi. We let it sit like 10 minutes, and tried it again, and it started, and had a pretty steady 6.5 psi. Go to make a run on it again, and the same thing happened?

    Its a mechanical pump, if it was a pushrod problem, the problem shouldn't be intermittent.

    So, think its just crap in the tanks clogging the pick up tubes?

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  3. #2
    Senior Member EVILFORCE's Avatar
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    Running out of fuel is one of several things.
    1- Could be a bad fuel pump
    2- Low float levals.
    3- Dirty fuel filter.
    4- To small of fuel lines.
    5- Crap in the tanks.
    6- Cam wear or even the rod that drives the pump could be worn down and not giving it a full stroke.

    I would put a Vacuum/presure gauge on it. Put a t in the supply side to the pump. If it goes into a high vacuum it will prove you have blockage on the supply side. That is where I would start.
    No Fool Like A Old Fool is there?

  4. #3
    Senior Member Oldsquirt's Avatar
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    Let's add restricted tank vents to the list. If suspected, make a run with caps open. If it now runs well, check out the vents for blockage.

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  6. #4
    Senior Member GT Jets's Avatar
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    Also should add collapsing inlet fuel hose. It could just be sucking the fuel hose closed.

    And just for drill, an air leak on the suction side will do this too..
    GT


    Quote Originally Posted by Quickjet View Post
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    Quote Originally Posted by Oldsquirt View Post
    Let's add restricted tank vents to the list. If suspected, make a run with caps open. If it now runs well, check out the vents for blockage.
    So...Its interesting you add that point. I took the fuel vents off thinking it was the supply hose (shut up...I was didn't look at it close enough, I was pissed and in a hurry.... ). and after we took them off and put them back on is when it started running right again. I thought that maybe a clogged vent was the problem, but quickly dismissed it before I thought it all the way through, Now that I think about it more though, I see that if the tanks don't have atm. pressure, there is no "suction"

    So maybe I should look there first. That would make me very happy!


    I can rule out pretty much 1,2, 3, 6 and 4 I kind of doubt.

    I'll take a look at it this weekend and see. Thanks guys.

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    Quote Originally Posted by PowerAddict View Post
    So...Its interesting you add that point. I took the fuel vents off thinking it was the supply hose (shut up...I was didn't look at it close enough, I was pissed and in a hurry.... ). and after we took them off and put them back on is when it started running right again. I thought that maybe a clogged vent was the problem, but quickly dismissed it before I thought it all the way through, Now that I think about it more though, I see that if the tanks don't have atm. pressure, there is no "suction"

    So maybe I should look there first. That would make me very happy!


    I can rule out pretty much 1,2, 3, 6 and 4 I kind of doubt.

    I'll take a look at it this weekend and see. Thanks guys.
    Before you rule out anything, start at point of origin! Are your'e tanks set up for the delivery required by the motor? Most pick-ups are a 1/4 intake and a butt-load of even smaller 90 degree fittings! Don't condenm the feul system before checking the FEUL SUPPLY. The best feul system in the world can only supply what it gets! IMLAO M

  9. #7
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    Default

    The tanks and lines were in the boat, and the guy had a brodix headed 468 in it before, putting out about 600 hp, thats why I don't think the lines are the problem.

    The pump is brand new.

    The cam used to be run with an electric pump, and probably has 20 minutes of run time on it with a mechanical pump.

    I'll check it all out though guys. I do appreciate it.

  10. #8
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    Default Pump?

    Quote Originally Posted by PowerAddict View Post
    The tanks and lines were in the boat, and the guy had a brodix headed 468 in it before, putting out about 600 hp, thats why I don't think the lines are the problem.

    The pump is brand new.

    The cam used to be run with an electric pump, and probably has 20 minutes of run time on it with a mechanical pump.

    I'll check it all out though guys. I do appreciate it.
    Was he running an electric pump? Electric pumps "push", mechanical pumps "pull".... Anytime you have dual tanks running into one pump there is a chance a "pulling" pump can do funny things depending on how the dual tanks are plumbed. Any chance one tank was low, or empty??? Is there a shut off, or selector switch??? If one pick up tube is marginal it will affect both unless the "not so good one" is shut off. AND yes, the "drop tubes" are tiny, and so are the associated fitting orifices unless they have been changed.....

    I would rig an axillary fuel tank, 5 gal can, directly to the pump and run it off that and see if the problem goes away.......

    Ray
    LOUD BOATS SAVE LIVES

  11. #9
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    Both tanks were full, and there is no selector switch.

    So are we leaning towards pick ups? Why wouldn't they suck close all the time?

    My mind is all over the place...how do you replace pick up tubes?

  12. #10
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    Default Try the aux tank first

    Quote Originally Posted by PowerAddict View Post
    Both tanks were full, and there is no selector switch.

    So are we leaning towards pick ups? Why wouldn't they suck close all the time?

    My mind is all over the place...how do you replace pick up tubes?
    Try the add on aux tank before you start tearing too much apart.... Simple to do, just be careful and use your head about securing and plumbing..... A short burst of speed should verify if the pressure stays up or not.....Then back on the trailer and address the actual verified problem.... BTW, the mechanical pump should hold pressure after the engine is shut off. If it doesn't, you have found your problem.......Ray
    LOUD BOATS SAVE LIVES

  13. #11
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    How long should it hold pressure for after the engine is shut off?

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