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Thread: Cummins STC variable Fuel Timing

  1. #1
    Senior Member Mike Mc's Avatar
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    Cummins STC variable Fuel Timing

    My '79 KW with a Cummins 855 Big Cam Four has the variable step timing control. The STC is suppose to hold the injector down to help get better mileage and the truck has a reputation for wearing out injector cam lobes. It makes me think all the noise coming STC makes is hammering on my new cam and wearing it out. I don't like all the noise it makes and would like to disconnect it if possible? Do the injectors have to be changed too?
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    thats a good question.....a 3406 would fit nicely in that spot tho

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    The Jake brake you have on it is a lot harder on the cam than the STC.

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    Senior Member 2stickbill's Avatar
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    Quote Originally Posted by oregon96pd View Post
    thats a good question.....a 3406 would fit nicely in that spot tho
    I'll second that.
    Cat,5X4 and a good cigar now we're hauling
    And my overload permit starts after sunset.

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    We are just buttoning up the rebuild of my grandpa's engine that also has the step timing. In the past I had the timing control box plug up on me, I bypassed the box just to get me home and it ran just fine like that for the next to months. It was a little rough at idle and we did consider just retiming the rockers and leaving it that way but in the end we finally rebuilt the timing control box. The injectors were never an issue for us.

  6. #6
    Senior Member Mike Mc's Avatar
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    The engine was completely rebuild 40,000 miles ago with reman heads and new injectors. Not much noise or smoke when idling. It makes most of the light knocking noise when running 1,600-1,900 rpm. It will still makes the knocking sound when under heavy load but it's drowned out by the engine working hard. Sometimes if I fluxuate the throttle when climbing I can get the noise to soften up.

    The only way it runs quiet is to keep it under 1,600 rpm.

    Can I disconnect the STC?

    Thanks for the replies!
    Last edited by Mike Mc; 11-02-2011 at 12:57 AM.

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    Have you checked the timing? did you time all 3 cam followers when you replaced the cam ?

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    You can disconnect it but you might want to try adjusting the rpm it switches at first. If it is changing the timing right in the middle of your pull that could be what your hearing and feeling. The bolt on the timing control block with the lock nut adjusts it, the bolt will either be on the top or bottom depending on wich way your block is mounted but it's hard to miss. The bolt puts tension on a spring inside the timing block that changes the oil flow to the injectors, backing the bolt out lets the timing change at a lower rpm while tightening it up makes it change at a higher rpm. I backed the bolt out as far as I could and now the timing changes and goes to normal as soon as the engine comes off of idle. I would give that a shot before you just bypass the system.

  9. #9
    Member Sparkie's Avatar
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    I tried looking in my fuels book, but thats no help. One little section about SCT. Although it did tell me that the engine should be under 140 degrees when setting it If you cant get it figured out I can ask my fuels teacher when he gets back on Friday

  10. #10
    Senior Member Mike Mc's Avatar
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    gypo, a shop rebuild the engine. I just slipped down and took some pictures during the rebuild. It runs great with no smoke, just makes a lot of racket over 1,600 rpm.

    MT, I bought this old truck with plans to use it as a historical vehicle. The engine needed work when I bought it but once they got in there everything was worn. So I spent my life saving rebuilding this Cummins $$$$. So I'm a little scared of F***in something up by messin with it. But I hate all the noise and it seems to be getting louder.

    Sparkie, yes please, the more info the better.
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