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Thread: Anybody ever see one suck air into the fuel *here* ?

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    Former SSG 98G's Avatar
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    Default Anybody ever see one suck air into the fuel *here* ?

    1984 M923 with NH250.

    I've seen bunches of them leak fuel here, but I've never had one suck air from this point. Anybody else?

    History - I drove this truck from IL to NM a couple years ago without incident. The new owner drove it a little bit, ran it out of fuel, dumped old nasty biodiesel into it from some cans, and let it sit and grow algae.

    He then decided that the problem was the nasty fuel, replaced the fuel filter, may or may not have got all the seals on the filter housing, and then drained and removed the fuel tank.

    Fuel tank has been cleaned and reinstalled. I checked the lines for obstructions by blowing compressed air through them. We primed the system up to the hand priming pump by pressurizing the tank and opening the line at that point.

    The truck now runs, but poorly with clear symptoms of air in the fuel.

    Today's activities will include changing the filter housing and priming again.

    I'm tempted to replace the rubber fuel line even though there's no signs of it being the culprit.

    It leaks fuel from this point. I know any point fuel can get out air can get in, but I've seen bunches of them leak fuel here without getting air into the fuel.

    20190815_175514.jpg

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    Former SSG 98G's Avatar
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    As of now everything is new or cleaned from the tank to the IP. All rubber is new.

    20190823_114127.jpg
    Last edited by 98G; 08-23-2019 at 16:29.

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    Former SSG 98G's Avatar
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    Ok, has to be sucking air where its leaking fuel. Any solution to this short of rebuilding the IP?

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    4 Star General frank8003's Avatar
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    How many gaskets are there between supply point and the goeintahs?
    You have what I named a "venturi leak"
    Means that the more the liquid flows then the more intrusion of air.

    Common extremely hard to find problem in power plants that move every kind of fluid every which way.
    But it got figured out, even with 3500 hp machines, just a pump.
    2400 psi on the one side and 28.7" abs vacuum on the other side.
    Any faulty seal or gasket will cause it.
    Thimk about seals and gaskets.
    Last edited by frank8003; 08-23-2019 at 16:59.
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    Former SSG 98G's Avatar
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    Rerouted the fuel line to a clean jug of fuel. Routed the return line to a clean clear jug.

    Steady stream of air bubbles in the returned fuel, varying in intensity with rpm.

    @&$&$^@!!!!!

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    The Venturi effect is a valid theory. In aviation, it explains what makes an airplane fly. The difference in airspeed above and below the wing causes a difference in pressure.

    It holds true with fluid as well.

    Look at the Venturi effect as it pertains to bottlenecks in the fuel flow. This might tip you off as to where to look for cracks or leaks.

    Pressurize your fuel line from the tank. You might see bubbles or leaks. When fuel is sucked through the system, it pulls in air.

    It just explains the physics behind it. Its an academic way of telling you what you already know. You are sucking air in. You are welcome!
    Last edited by Buffalobwana; 08-25-2019 at 10:41.
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    With the engine drinking clean fuel out of a jug

    20190823_175427.jpg

    20190823_114132.jpg

    through a newly made 4300psi hose, having bypassed the rest of the fuel system up to that point, still seeing overt and blatant air bubbles in the returned fuel.

    With the entire fuel system intact, pressurizing the fuel tank doesn't show any leaks between the tank and the IP.

    At this point I'm reluctantly of the opinion that the throttle shaft on the IP is the culprit. We're pulling the IP and sending it off for rebuild.

    I'll update this post either way when we get it back and get it reinstalled. It may be a few weeks.

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    The faster the fluid goes thru the pipe the faster the air gets sucked in...................
    Found a lot of then right at the NPT threads on things (3/4" per foot taper) as that actually only seal on one thread at about 500 of the minor diameter.
    I was here, had a good time.

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    That is a pressurized point in the pump, keep in mind, at idle it is just flow, but, the symptom you get when the throttle shaft leaks is "air in fuel". It will be kinda hard to start, slow to build RPM and the RPM will hang when the throttle is replaced. It is a .20c o-ring leaking, but the front of the pump needs removing to get at it. Too much throttle leakage can have that symptom too, so, IMHO, it is best to have a shop do it. Yes there have been others here that have done it themselves.
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    Have you tried spraying something like ether on where the assumed air is being sucked in? If the leak is there and you spray you should be able to hear the idle change.
    1989 M923

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