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Truck has 330k miles, exhaust, intake, boost fooler, 1.0 ar housing, stock otherwise. Runs loud as **** at idle, but as soon as I get past 1/4 throttle, runs perfect. Diesel mechanic said it's not a flywheel, sounds like crankshaft or a spun bearing.
 

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A truck with a broken crankshaft isn’t going to run perfect at any speed. Shouldn’t run at all given the front wouldn’t be spinning and the cam is driven from the front. I’d guess a bad injector would be more likely.


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I've never personally experienced it but I have heard of crankshaft fractures where the shape of the break keeps both "pieces" more or less aligned so the engine will still run. Obviously its going to make a hell of a racket.

The only broken cranks I have experienced were on 6.2 Chevs - 2 of them on the same truck - and they were catastrophic show stoppers. After the second one I put a 350 in the truck and traded it on a Ford. That was 2 Chevs in one for me - 1st and last.
 

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I still can't see any scenario where the crankshaft would break in a way where it would be noisy at idle and quiet at speed. I have seen out of round rods that were quiet at idle and got progressively louder at speed. Can't imagine a break where the bearing play would be increased. You could spin a bearing or destroy a bearing, but that isn't going to get quiet at speed.

An injector that is overfueling can produce a real racket at idle and might clear up when the engine speed got to the point where it could handle that fuel. Are you getting any smoke at idle?
 

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camshaft end play will cause crap like I think you are describing. Rough idle that will make cyl deviation numbers all over the place. The sensors used as the recall fix sensors have sensitivity to the poles on cam being too close. I have two trucks running around with thinner gasket paper cut out shims I made by hammering sockets of two different sizes into the paper to cut the OD and ID that would let it slide over sensor. The o ring on sensor seals just fine with the sensor out .015-.020 inch.

There are grey and dark blue sensors and some say the blue fix the issue as they are less cranky about the pole to sensor tip clearance. Blue one did not fix the trucks I shimmed.
 

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Discussion Starter #7
I still can't see any scenario where the crankshaft would break in a way where it would be noisy at idle and quiet at speed. I have seen out of round rods that were quiet at idle and got progressively louder at speed. Can't imagine a break where the bearing play would be increased. You could spin a bearing or destroy a bearing, but that isn't going to get quiet at speed.

An injector that is overfueling can produce a real racket at idle and might clear up when the engine speed got to the point where it could handle that fuel. Are you getting any smoke at idle?
No smoke at idle, just a lot of racket.
 

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Take your fan belt off and start it to see if it clears up.
 

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NO. Removing belt to eliminate any of the engine driven accessories as cause is a very commonly used method. You don't go drive and tow with no belt as water pump is not being driven. Running the engine even 5 minute unloaded is not going to cause a problem with overheat as thermosyphon will still cause some circulation.
 
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