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Thread: New to me M936A2 engine idle / load issues

  1. #11
    4 Star General porkysplace's Avatar
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    Quote Originally Posted by dcowan View Post
    Yes the lift pump was brand new. I would hate to count how many times I've pushed the prime button on the lift pump during all of this.
    Brand new or 20 year old NOS ?

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    Brand new

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    Colonel Nomadic's Avatar
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    Another suggestion from my armchair, can you check the fuel pressure near the injectors? Maybe the fuel pressure is low.

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    4 Star General 74M35A2's Avatar
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    Clamp the return line shut out of the injection pump. Replacement overflow valve may be held open by yuck. Donít hold it too long like this, but good to test.
    Last edited by 74M35A2; 10-04-2019 at 21:52.
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    Moderator WillWagner's Avatar
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    Yiu said the fuel system was full of "tar". Cummins makes a cleaner that works wonders, I have seen it work on the dyno. The 1 gallon p/n is CC36096. You will need to make up lines to pull from and return to the can it comes in. Give this stuff a try before you go into the process of having the pump and injectors gone through. You should check the restriction from the chassis to the engine and the actual gallery fuel pressure. Someone asked if there was a way to check actual injection pressure. No, not on the engine. Another thing, it could be that he base timing is off. Use the pin on the back of the gear housing to set the engine at TDC, remove the cap on the side of the pup gov hsg and see if the timing pin will go all the way into the hole.
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    Mission Specialist jonesal's Avatar
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    Quote Originally Posted by WillWagner View Post
    Cummins makes a cleaner that works wonders, I have seen it work on the dyno. The 1 gallon p/n is CC36096.
    I looked all over for this and could not find anywhere to buy it. Is it only available to jobbers?

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    Moderator WillWagner's Avatar
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    https://cumminsfiltration.com/sites/...es/MB10106.pdf

    Any Cummins dealer, Frtlnr, Peterbilt, KW, etc. should be able to find it. It is a Cummins Filtration product, not a Cummins part.

    Another thing I thought about, the idle stop adjuster has a tendency to wear if the engine has been operated with a leak of some sort around the throttle arm area on the governor. The dirt and fluid combination acts like lapping compound and will wear the spot on the stop bolt and cause exactly what the OP is describing. It won't idle but if you gently bring the idle down to 7-800 ish it will sort of maintain that speed, but if the go pedal is touched, the engine dies. Try turning the idle set up a bit. If it works, you can take the screw out and re profile it on a lathe, drill motor, whatever turns so you can make the screw uniform all the way around.
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    WillWagner thanks and I will try your suggestions. I thought I should include a picture of the spin on fuel filter I removed to give an idea of the buildup I am seeing.

    IMG_20191005_144548.jpg

    And the sludge/tar I drained out of the fuel tank after the E85 soak.

    IMG_20190927_150520.jpg
    Last edited by dcowan; 10-06-2019 at 14:05.

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    Man, that is NASTY!! I am kinda fighting this kind of issue on a deuce, been sitting for +13 years. The shut off was stuck hard. the HH arm kept wanting to spring back to the no fuel position. I have let it run and get some heat in it and it is better. I moved it the other day and upon clutch engagement, it didn't die, so that is a good thing, seems to be working itself out. Maybe that is all your thing needs, some heat and exercise.
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    4 Star General Csm Davis's Avatar
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    Quote Originally Posted by WillWagner View Post
    Man, that is NASTY!! I am kinda fighting this kind of issue on a deuce, been sitting for +13 years. The shut off was stuck hard. the HH arm kept wanting to spring back to the no fuel position. I have let it run and get some heat in it and it is better. I moved it the other day and upon clutch engagement, it didn't die, so that is a good thing, seems to be working itself out. Maybe that is all your thing needs, some heat and exercise.
    Will try some WD-40 it worked well for my M35 that was the same as yours, it cut the varnish out pretty quickly. As for the OP man that's got to be one of the worst fuel systems in a 939 series i have seen and that's say a lot. I believe that you have a non diesel contaminant in your truck if I had to guess it is vegetable oil or sugar both will get the same result if it is not cleaned out. I like Will's way too but I think you might get better results with 100% biodiesel. If it is in the IP and injectors like I believe it is the biodiesel will clean it right out.

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