Fueling problem CAT D9T (C18 engine)

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This topic contains 13 replies, has 10 voices, and was last updated by  Tony 4 months, 3 weeks ago.

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  • #25538 Reply

    C18 engine in D9T Dozer Fueling problem.

    At low idle I get fuel pressure of about 410 PSI
    At high idle(2000 RPM in N) it drops to about 350PSI and engine loses RPM sporadically.
    At high idle under full load RPM maxes out at 1600RPM

    -> Replaced filters : OK
    -> Checked all fuel lines : OK
    -> Checked for debris in all components a pressure regulating valves : OK
    -> Replaced back end of mechanical fuel pump : Found slightly better results but still losing RPM sporadically as before.
    -> Replaced all rubber seals while dissassembling.

    I noticed an electrically controlled valve on the return side of the filter housing, could it cause this problem if it is faulty or not receiving signal?

    I only replaced the back end of the mechanical fuel pump, could the gear side of it be damaged and if so would it cause the high idle RPM loss but not at low idle?

    Is there anything in the fuel system I’ve overlooked?


    #28304 Reply


    try changing the check valve in the primary fuel filter base.

    #28305 Reply

    Is that part of the electric primer pump housing which has a spin on filter? Or the other stand alone filter base?

    #28145 Reply


    it is the other filter, it looks like just a plain plug. whem you pull it out it will be a check valve.

    #28314 Reply


    Jason, it sounds to me like you are drawing in a lot of air into the fuel system somewhere. Also, it seems like you are reading the KPA scale on your pressure guage instead of PSI. You should get 78 psi at idle and 95 psi at high idle. Check you fuel level (level guage could be misleading). Then install a clear sight glass in the fuel suction line at the inlet of the transfer pump. If you see air coming in, keep plumbing the line in further down the system toward the fuel tank until the air stops coming in. Then you will know where it is coming from. If you don’t have that kind of tooling to use, the primary fuel filter base fittings (or filter o-rings) and the fuel tank stand pipe are common places for air to enter the system.

    #28315 Reply

    Jason is right about your questionable fuel pressure measurements. Spec is 78-95psi as he said. Your secondary fuel filter base, up on the fender in the filter compartment, has a fuel pressure differential switch and a fuel pressure sensor on it. There will be a fuel temp sensor in your fuel manifold on the side of the engine. Those are probably the electrical components you were wondering about. There is an inline check valve connector attached to your primary fuel filter base (with the electric priming pump). There is also a check valve plug in the bottom of the fuel manifold on the LH side of the engine, just forward of the ECM.

    You will have a pressure regulator orifice in the fuel return line. I think it will be in the line right where it leaves the engine head. This orifice is responsible for maintaining a constant pressure in the fuel system. If it was blocked with debris, fuel pressure would be high.

    The fuel transfer gear pump has a pressure relief valve in it, but it appears that you would have replaced that when you replaced the back end of the mechanical gear pump. The results of your fuel pressure testing and air-in-fuel testing will tell you if your fuel transfer pump gears are ok.

    Checking for air in the fuel system is definitely a good recommendation. If you get to the point where you’ve eliminated the fuel suction lines as possible causes and you know you have air entering the system, try running clean fuel from a clean bucket into your primary fuel filter. If problem disappears when the machine’s fuel tank is bypassed, then you know you may have a stand pipe problem. Pressurizing the fuel tank with no more than 5 psi is also a good way to check if your stand pipe is allowing air entry.

    Maybe double check your fuel return lines for kinks or tight bends that could cause high fuel pressure or fuel volume problems.

    #28317 Reply

    Jim Owen

    A lot of very good advice has already been presented.

    One suggestion that is missing is: a plugged or constricted line can cause vaporization of the fuel. That means the suction force is enough that a restriction causes the fuel to boil or vaporize in the lines. So, instead of air in the lines, as was suggested, you could have fuel VAPOUR in the lines, which can also result in fuel starvation.

    #26297 Reply

    Thank you, I have cleaned all the lines, I’m suspecting a cracked manifold, I’m in a pretty remote part of Africa so special tools are a problem, I have to rely on replacing components to diagnose because I cant test, I’ve arranged for a new manifold.

    Thanks all for the great advice.

    #28316 Reply

    You aren’t in Djbouti are you? I’m in the navy seabees, i definitly know what its like to be deployed to the middle of nowhere with no special tools or testers and tryin to troubleshoot. Even to get parts delivered can be a nightmare and to finally get what you think you need and it turns out to be wrong can be extremely frustrating. I feel for ya. It sounds like you have a plan with the manifold theory. If that doesn’t pan out, I wouldn’t rule out the front side of that pump as well. Since you had slightly better results when you replaced the back half of the pump, i would think you were on the right track.

    #28318 Reply


    I think your diagnosis from a defect in the fuel system may be flawed.
    Suspect to the engine goes into safe mode and limits the power of alone. Perhaps because of incorrect sensor reading.
    In several cases, the exchange atmospheric pressure sensor to help me, you can also check turbocharger outlet pressure sensor. But it is still possible that the electro-valve high pressure pump is not working properly. I had such a case, when when the engine starts- the solenoid valve gave too low, or maximum pressure without showing an error code. But this came after the analysis with ET. There are no other machines to replace it from another machine to test?

    #28319 Reply

    Guys, thanks for all the great advice, I’ve learnt a lot about the fuel system on a machine I’m not familiar with, that’s what I love about this site.

    The problem was something I had considered and discarded a few times, in hind sight it should have been one of the first things I checked: plastic bag in the fuel tank. *FACEPALM*

    @ Joshua : I’m in the Congo on a remote copper mine, Yeah I hear ya, it get’s tough out here with limited skills, tools, parts and support and then to add to that is the fact that the rainy season ensures we spend 12 hours a day knee deep in mud, no pressure washer to clean machines, rudimentry lean-to for a workshop designed for light vehicles, but that’s what we signed on for right 😀

    #28437 Reply


    I sympathize with you.. been to that one before. Very frustrating to find..

    Operators who sabotage equipment should be hung. by their nuts. with fishing line. LOL..

    #28948 Reply

    Peterson Cat must be God’s gift to mechanics!!

    #50883 Reply


    What is the max fuel tempreature for a c18 cat

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