2008 Mack Mp7 fuel delivery problem.

Discussion in 'Mack Forum' started by Chrisjohn97, Aug 12, 2021.

  1. Chrisjohn97

    Chrisjohn97 Bobtail Member

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    Hi I have a 2008 Mack mp7 GU. I’m having a fuel delivery problem. We’ve replace the fuel filters. The fuel filter housing the fuel lines to and from the tank. The lines to and from the ecm cooler. The lines to and from the fuel pump. The lines to and from the 7th injector and after treatment fuel shut off valve. The overflow valve. The deaeration valve. The fuel pump. The 7th injector. The aftertreatment shut off valve. And the #1 injector. With all this replaced the truck hardly starts. We have the hand prime it atleast 30 pumps to get a good prime and then the truck cranks for a while. Starts hard and when the engine picks up it idles at 1000rpms for a second with no throttle response. Then after it runs for a few seconds it goes back to idle and there’s throttle response. Then the engine starts stumbling and eventually dies. We have blown out the ecm cooler. If we run a fuel line from a bucket to the suction side of the fuel pump the engine starts and runs perfectly fine. So we’ve determined the problem is on the suction side. And that it’s not the injectors or cups. We’re also not making oil or coolant. The only codes we’re getting are a mechanical failure for the after treatment shut off valve mechanical failure. The shop manager at our local Mack said this codes is coming up because the ecm sees low fuel pressure at the aftertreatment system and that’s what throws the code. We’re also getting a code for the idle validation switch. I’m not too sure what this does as a lot of our trucks throw the same code from time to time and it never seems to effect anything. Any ideas?
     
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  3. QUALITYTRUCK

    QUALITYTRUCK Road Train Member

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    Cracked pick up tube inside fuel tank.
     
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  4. Chrisjohn97

    Chrisjohn97 Bobtail Member

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    Not the pickup tube. We took the supply line off the tank and put it in a bucket with fuel it didn’t make a difference.
     
  5. Cattleman84

    Cattleman84 Road Train Member

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    Soumds like you are sucking a small amount of air somewhere... What style fuel filter is on it? If its a spin on filter is the filter base (part filter screws onto) cracked?
     
  6. Chrisjohn97

    Chrisjohn97 Bobtail Member

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    I agree with the small air suction part. But the rest I doubt. They’re napa gold spin on filters. We’ve changed filters twice. And we’ve put on 3 filter heads so far. The one that was originally on the truck, and another we had laying around, and we bought a new one. I doubt all 3 were cracked
     
    Last edited: Aug 13, 2021
    Cattleman84 Thanks this.
  7. Chrisjohn97

    Chrisjohn97 Bobtail Member

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  8. Chrisjohn97

    Chrisjohn97 Bobtail Member

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    Update. I’ve been sitting here thinking about it. Is it possible that it’s the fuel water separator bowl? I’ve heard of them leaking before but never seen it. It’s on the suction line so it’s possible that it would suck air. Also it’s the only think we haven’t replaced and it’s also left out of the system when we run the fuel pump out of a bucket
     
  9. Cattleman84

    Cattleman84 Road Train Member

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    Are you talking old school water separator bowl... Or a Davco filter? If you are speaking of a Davco then that could very well be the issue... I have had mine suck air on 3 different occasions. Each time it just required opening it up, lubricating the O-rings and reassembling it... Did it in a truck stop parking lot each time. But it would probably be a good idea to replace the O-rings just to be safe.
     
  10. Chrisjohn97

    Chrisjohn97 Bobtail Member

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    Old school water separator bowl
     
  11. Cattleman84

    Cattleman84 Road Train Member

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    I suppose it could suck air the same way... But aren't they usually a glass to metal seal???
     
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