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LB7 broke a crank

Discussion in '01-04 LB7' started by 3500 dually, Jun 23, 2016.

  1. 3500 dually

    3500 dually Member

    77
    3
    Mar 7, 2011
    Peculiar, Missouri
    Well guys, my sons 2003 gmc 2500HD crank broke about 2 1/2 weeks ago. Just got running again. My million dollar question is what causing these??? Ive heard a lot of different answers, so I thought I would ask my friends here.
     
  2. NVW

    NVW Well-Known Member

    8,416
    745
    May 5, 2008
    Castor, AB.
    Bad HB can and will do it, I hope he put a new one on.
     
  3. Will L.

    Will L. Well-Known Member

    5,667
    3,011
    Jan 25, 2013
    Boulder City Nv
    Almost every diesel with a broken crank is from a bad hb. All that compression and torque comes at a price, and that is beating the crud out of the crank. Every bit that vibration doesn't go out the rear- goes out the front.

    How many miles on it? Over 150...
     
  4. THEFERMANATOR

    THEFERMANATOR FRANKENBURBAN Staff Member Lead Moderator

    8,413
    1,501
    May 9, 2008
    ZEPHYRHILLS FL
    Harmo,ic balancers rarely fail on the duramax, I've yet to hear of one failing even on the million mile trucks. I know many have a hard time grasping it, but unfortunately it's the luck of the draw. It's a known problem, it does occur, and there is ZERO explanation for it. Theres been trucks go half million miles turn around and reuse those cranks in a race engine and put down a thousand hp with no crank problems. And then theres guys who have had them break with 6k miles on a NEW engine.

    Theres many theories on it ranging from the firing order. GM has 1&2 firing back to back, and the theory is that it overstresses that throw as most break near the snout or around the rod journals for 1&2. So they now have alternate firing order camshafts. A few have still had crank failures though, but they were all used cranks that had been run with the stock firing order. But there has yet to have been a reported failure with a new crank and an alternate firing order cam that swaps 2&7 iirc. Another theory is the width of the crank across the rod throws, and the radius of it there.

    Several have had the broken cranks examined, and the general consensus has been poor casting due to how the fractures appear to have porosity in many cases.

    We DO know several things that have been proven to lead to crank failure. Poor tuning is the #1 killer. To much fuel down low with WAY to aggressive of a timing table. Particuliarly stacks of boxes like how we used to do it to get power. Another thing that was linked to ALOT of failures was running a fluidamper and turning the RPM's higher than stock. The fluidamper weighs I believe 11 lbs MORE than a stocker and all that weight is hanging out front of the geartrain. Another killer has been bad injectors. The ECM will balance all the cylinders at idle as best it can, but as soon as you touch the throttle, they all zero out, and fueling is even to all cylinders. If you have bad injectors, you can have dramatically different fueling from one cylinder to the next which invokes a nadty harmonic in the crank from the uneven firing pulses.

    Ultimately it is a harmonic issue as the DURAMAX DOES have harmonic issues. It really isnt the smoothest running engines across the board as they have NUMEROUS RPM points that if you hold them at, they appear to run smooth, but will rattle and hum the trucks to pieces overtime. 650 RPM's is one known point that overtime the harmonics of the engine will wreak havoc with rattles and harmonic hums in the truck. Coincidentally, I believe this is where LML's idle at, and they seem to have the highest rate of broken cranks stock of any of them.

    It's mostly all theories, but ultimately, it's just bad luck with the DURAMAX to have a crank break.
     
  5. Will L.

    Will L. Well-Known Member

    5,667
    3,011
    Jan 25, 2013
    Boulder City Nv
    From 2 fleets - 1 I was a mechanic, the other cheif mechanic. The first had 43 duramax, the second had 210 when I left. The larger fleet was the one that had trucks putting on 100,000 a year on.

    I saw somewhere near 30 broken cranks. Keep in mind we went through somewhere near 600 trucks for that between the 2 fleets. All but 1 had very obvoius bad balancers. Engines were free crate replacemements under GM field testing program, so we ran them till they popped. When you get to where you can geel the harmonic spots in it pretty bad- swap it out. If it were a tire shake or a driveline vibration you'd think nothing of it, but somehow part of the engine is untouchable.

    For years I've been trying to get people to hear me on balancers for diesels. I'm telling ya, swap a balancer in duramax every 150,000 and watch the crank live.

    Sure some people get lucky. Ive seen 1,000,000 miles on untouched engines that we only added oil to, never changed oil, and just did the hallow oil filter housing for no filtering. Doesnt mean thats the best answer.

    Think about the 6.5- most people finally agree to swap hb around 100,000. But i bet if we did a pole here, there is 25 people with over 200,000 miles on a balancer.
     
  6. NVW

    NVW Well-Known Member

    8,416
    745
    May 5, 2008
    Castor, AB.
    I'm guilty, but most of mine had those miles on purchase.
     
    Will L. likes this.
  7. ak diesel driver

    ak diesel driver 6.5 driver

    11,589
    1,062
    Feb 7, 2009
    alaska
  8. THEFERMANATOR

    THEFERMANATOR FRANKENBURBAN Staff Member Lead Moderator

    8,413
    1,501
    May 9, 2008
    ZEPHYRHILLS FL
    It's still uck of the draw. Theres HUNDREDS of broken DURAMAX cranks from guys running aftermarket balancers on engine with cirtually no run time. LML's seem to pop around the 150-200K mile mark, others are a gamble. As to harmonics, these same harmonics are present in engines running ATI balancers, it's just a natural harmonic in the engine. Wagler and I believe Callies are both working on forged cranks for the DURAMAX, and theres been some interesting stuff coming from that as well, but still not much.
     
    Will L. likes this.
  9. C20ELEPHANT

    C20ELEPHANT Member

    96
    25
    Apr 4, 2013
    Phoenix, Az
    With only 60,000 miles on my LBZ when the water pump fails I will most likely replace the balancer and drill a Empire Performance Engineering pin in it. Why replace such a low mileage balancer, you should have seen the rubber isolator on the driveshaft carrier bearing at 40,000 miles when I had to replace it, it was dry rotted real bad.

    I'm usually pretty sensitive to seat of the pants vibrations throughout the truck but I never felt one, found it during an inspection about a week before driving to California. What vibration I did have was the mirrors vibrating, after I replaced the carrier bearing the mirror vibration disappeared (no u-joints were replaced).

    On top of that I'm a little concerned about the balancer at the differential pinion shaft, start buying parts now or sell it but damn it is a nice driving and riding truck...
     

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