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Shaking at Idle

Discussion in '6.9L IH & 7.3L IDI Diesels' started by bilbo, Apr 16, 2021.

  1. bilbo

    bilbo Full Access Member

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    My truck has been shaking a bit at idle. With the hood open it doesn’t look like the engine moves much but I can feel it in the cab. Going up a little from the 750rpm it is set at for warm idle eliminates the shake. It’s smooth when driving regardless of engine or road speed. The little bit of reading I’ve done suggests injectors may be to blame.

    I have no idea the age or condition of the injectors currently installed, is there a way to test them to see if they’re really bad before replacing them?

    There’s a shop in town nearby that claims to be familiar with older stuff. He has been doing his thing for 40 years or so. He said he could rebuild them but can get new ones cheaper. But I’ve read some horror stories about the new ones available today.

    The truck is a 1994 7.3 NA with 285,000 miles. 280k when I got it. Just buying injectors feels like throwing parts at a problem without really diagnosing it.
     
  2. Cubey

    Cubey Full timer RVer

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    Engine mounts, perhaps.
     
  3. IDIBRONCO

    IDIBRONCO IDIBRONCO

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    If you buy injectors, get them from Mel, Russ, Wes, or Justin (Conestoga Diesel, russrepair.com, Classic Diesel Design, or R&D IDI Performance). Those are the only places to get good pumps, injectors, and more. Unless you know for a fact that your IP is fairly new (and has come from one of the four that I mentioned earlier), then don't just put injectors in it. The injectors and IP wear at the same rate and should be replaced as a set. The way to test the injectors is to pull them out of the engine and test them with what's called a pop tester. Those will let a person see what pressure they fire off at, check the spray pattern, and will also check for a leaky injector. As for the local shop with the guy who's been doing this for 40 years, I took mine to a similar shop. Within a year, I had a leaky injector and the IP wasn't working right. In fact, I don't think the IP lasted 10,000 miles before it was so bad that I had to crank the engine over for 10 seconds or so for it to start and I wasn't entirely sure that it would stay running long enough for me to get the 18 miles home. I then bought a pump and injectors from Mel and only paid about $300 more for them.
     
  4. Old Goat

    Old Goat Full Access Member

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    A little side note here:
    If you do want to DIY and buy a Pop Tester to test the spray pattern......

    NEVER>>>NEVER put a finger or any other part of your body in the spray stream from the Injector...

    The pressure is so great it will be injected through your skin and into..say your finger. There is no way to clean out the fuel from your finger......what I have read, amputation is the fix.

    Same with any high pressure hydraulics.


    Goat
     
  5. Fredrickson

    Fredrickson Full Access Member

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    Not one to propagate internet "I read about a guy..." stories, but to back up what @Goat said with some facts.
    University Of Michigan has this article
    https://www.uofmhealth.org/health-library/tm6523
    And the national institute of heath
    https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2532970/

    And if you are better with pictures, that words..
    just google "fuel injector injury" and click on the IMAGES tab (not pretty)

    The take aways are don't put anything in the path of the spray, and time is a major factor in saving the digit/limb..
     
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  6. lotzagoodstuff

    lotzagoodstuff Supporting Member Supporting Member

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    Find somebody with a proper timing meter and find out where your current timing is set.
     
  7. bilbo

    bilbo Full Access Member

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    I’m guessing the motor mounts are original. Maybe they’re bad, but I don’t see any nasty rocking when I rev the engine. I know that’s not a foolproof test though. I probably should replace them regardless. Are they a royal pain to do?

    Thanks for the tip on the injectors. I work in an industrial manufacturing place with lots of hydraulics so I’m familiar with the maiming power of fluids under pressure. Still, it’s good to see reminders once in a while to keep it near the front of my mind.

    I do have timing equipment and the truck is set to 10.5 degrees. When I originally checked it it was over 14 degrees. It doesn’t jump around or anything.

    If I need injectors/pump I will likely get it from one of those you suggested, IDIBRONCO. Just wanted to feel out the local guy a bit. I like to use local shops/resources when I can. I’ve had those relationships pay dividends for me in the past.
     
  8. BrianX128

    BrianX128 Supporting Member Supporting Member

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    If the motor mounts are good, it could be pop pressure on 1 injector. When I replaced the injectors on my 6.9 one injector ended up leaking on the thread body. I sent it back to mel to get a replacement and put an injector from advanced auto in to drive the truck. It ran worse with that injector than it did if I unhooked the fuel line to it.
     
  9. catbird7

    catbird7 Full Access Member

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    Before tearing into anything, you might try a jug of power service (grey bottle). Mix about 12 oz in 1/2 tank and see if that makes a difference. New fuel filter might be a good thing to try also before turning any wrenches. I used to run a lot of 50/50 motor oil diesel mix and when the truck started running rough, I ran a couple tanks of regular diesel with heavy mix of power service and it always cleared up.
     
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  10. franklin2

    franklin2 Full Access Member

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    A quick way to see if each injector is contributing at idle is to get it idling, and then loosen the injection line at each injector one at a time. All you have to do is crack the nut till it starts seeping. That will kill that injector. Listen and watch the engine. A good injector will make the shaking way worse that it was. The bad injector will make little to no difference in the shaking when the nut is loosened.
     
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  11. bilbo

    bilbo Full Access Member

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    I'll try the fuel system cleaner when I run my tank down a bit. I did try cracking injectors but each one seemed to create more shaking. Either they're all doing something or I am not seeing the difference. Could air ingress cause issues like this? I haven't noticed any issues with cold starting.
     
  12. franklin2

    franklin2 Full Access Member

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    Air while it's running usually causes surging when going down the road.
     
  13. bilbo

    bilbo Full Access Member

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    Maybe not then, no surging while driving or at idle.
     
  14. catbird7

    catbird7 Full Access Member

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    Just throwing out another guess, is it possibly due to low fuel pressure? Are you running a stock mechanical fuel pump?
     
  15. bilbo

    bilbo Full Access Member

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    Could be, I was randomly thinking I should put in a fuel pressure gauge today. What a coincidence. It is the stock mechanical pump, I can just check using the Schrader valve correct?
     

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