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7.3 "Thrashy" sound over 2200 RPM

Discussion in '6.9L IH & 7.3L IDI Diesels' started by cason234, Sep 12, 2019 at 1:26 PM.

  1. cason234

    cason234 Registered User

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    Hi folks, not sure how best to describe this, but here goes: I've got an old 7.3 IDI NA with ZF5 and 4.10 gears (4x4). Don't know how many miles, but I pretty sure it's more than 200,000.

    It starts instantly, uses no oil between changes, and runs great. If you shift it below 1800 RPM, it is like a spectacular old tractor. However, if you rev it much higher than that (let's say around 2200 RPM), which you must do if you wish to go 70MPH in 5th gear, it loses its dignity, and starts to sound kind of "thrashy", for lack of a better word. Not a knock, but it just sounds unhappy. I've slipped it into neutral at 70MPH to see if it might be the transmission or transfer case, but the sound goes away as the RPM's drop below 2000. It has done this for the 18 months I've owned it, and it hasn't changed. I've driven it for hours at 70MPH, and it doesn't change.

    If this was your truck, where would you start looking? Lifters? Injectors? Injection Pump? (also, how can I be sure this isn't gear rollover from the ZF5?) Or, is this just the nature of a 7.3 IDI? I love the truck, but I'd like to 'smooth things out' if I'm revving it at highway speeds.

    If anyone has experienced something similar and found a resolution, please share!

    Thank you
     
  2. SLC97SR5

    SLC97SR5 Full Access Member

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    Is the clutch fan seized or partially locked up?
     
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  3. cason234

    cason234 Registered User

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    Good question..... I don't think so. But that's an idea I hadn't thought of. I'll temporarily remove it and see what happens. Thanks

    (I remember the old 4.3 GM V6 engines had a horrendous 'roar' as heard from outside that was associated with an overly aggressive clutch fan)
     
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  4. Thewespaul

    Thewespaul Supporting Vendor Supporting Member

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    Your dual mass flywheel is coming apart
     
  5. cason234

    cason234 Registered User

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    Thanks Wes! I'm going to check my records, but I believe the previous owner indicated he'd replaced the DMF with a SMF..... If I'm wrong (or optimistically remembering something that really isn't true), I may just live with it for a while.... the sound is such that I can live with it, but if there was a relatively simple repair, I'd do it. A clutch..... I'll wait until I NEED to replace it. Many thanks for replying.
     
  6. IDIBRONCO

    IDIBRONCO IDIBRONCO

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    If it's the DMF, you can usually hear a rattling sound when you shut the engine off. Sometimes it will also have a shake when the engine starts. You can get rid of this by revving the engine slightly. When it returns to idle, it runs smooth again.
     
  7. cason234

    cason234 Registered User

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    I just checked my anecdotal history of the truck from the previous owner, and I was wrong about the DMF.... it had not been replaced. He'd simply put a new clutch in, and noted that the DMF was possibly headed for trouble. This lends credence to the idea that it is the DMF.......

    I can't wait 'til I get a ride in another IDI...... see what they're supposed to sound like!

    IDI Bronco - thanks for those notes..... it doesn't rattle on starting or shutdown. It doesn't rattle or make any untoward noises when idling. The thing I'm noticing is the sound at more than 2200 RPM...... and it goes away at anything less than that speed.
     
    Last edited: Sep 13, 2019 at 2:12 PM
  8. IDIBRONCO

    IDIBRONCO IDIBRONCO

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    A quick way to check your DMF is to remove the inspection cover. See if you can grab the part of the flywheel with your hand. It should not move back and forth very easily by hand. Another thing that I though of is you pump timing. Mine was WAY too advanced. It would be very noisy when turning higher RPMs. I don't have a tach so I have no idea what the RPMs were. Wes Set it back and it was SOO much quieter. It also revved up quicker, and pulled strongly (for a N/A engine) clear to the governor. I have a Zf5 with 3.55s. I would purposely shift at low RPMs because it was so loud before. I just didn't like hearing the noise.
     
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  9. Thewespaul

    Thewespaul Supporting Vendor Supporting Member

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    I think every IDI should sound like this :Thumbs Up
     
  10. The_Josh_Bear

    The_Josh_Bear Full Access Member

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    Do you have the same sound in neutral in the driveway as when driving?

    Also gear rollover only happens at near idle speed.
     
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  11. IDIBRONCO

    IDIBRONCO IDIBRONCO

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    I have to agree with that!
     
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  12. BrianX128

    BrianX128 Supporting Member Supporting Member

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    DMF's are great fun when they start to do this. Easy enough to check.

    Mine chattered so much at idle with the clutch out I got sick of hearing it and just held it in. Figured something was coming apart in my trans and was prepared for total failure one day. Then I got the worst wobble ever accelerating up a hill and it turned out my dmf was going, but so was my carrier bearing and all of my U joints. Input shaft of trans was wobbled loose (probably from dmf shaking it to death), and my back trans moutning tabs were long gone. I'm sure the dmf shaking around helped ruin all of that. This is in my 2wd truck so it wasn't that hard to smf swap it with some help from a friend, changing my totally ruined clutch / brake pedal assembly from holding the clutch down at intersections was considerably worse.
     
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  13. cason234

    cason234 Registered User

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    Thanks folks - some great feedback. I'm going to check a bunch of things tomorrow (Saturday to see if I can diagnose what's going on). I really appreciate your help..... I've got some specific possibilities, and now it's just up to me to get out there and do something about it! I'll report back with the results!
     

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