I was driving home today and a silver dodge with stacks came up behind me, got the jump on me and I let him pass, but just barely. He was already on boost by the time I stepped on it. Put my bumper on his and didn't let him go. I tried to get him to pull over so we could drag from a stop--to no avail. At that time, no problem.
Anyway, this was a few miles from my home, as I was taking the slow drive down my street and noticed the truck shaking. Felt like something out of balance-or even rough pavement. My thoughts changed when I slowed to 3mph pulling up my drive. The truck was shaking violently.
Again, no smoke and the engine revs, although I have not pushed it. My question is:
Harmonic balancer? or spun a main?
How can you tell the difference on inspection?
I had an old 6.5 that the harmonic balancer came out of wack and that's what it feels like.
No engine light or codes. Just extreme engine shaking with no strange engine noises or knocks.
Ideas?

Misfire from a dead injector?

I've seen that before, this isn't just a hickup, I mean it's shaking something serious.

Sounds like you have injector wiring problem-- also check the wiring to the PCM next to the alternator!:)

Mine did the same thing and it was the harmonic balancer, it had come loose and spun partway around.

I spun a fan blade off once. Sounded like a rare "diesel backfire" and put a hole in my hood. So it was pretty easy to diagnose that one. But top engine RPM was 1800 - 2000. Serious shaking though.

You didn't happen to go offroad or anywhere you could cake your rims with mud did you? Sounds stupid, but enough mud in the rims to unbalance a tire or two makes the truck feel like you have a bent axle.

Sounds almost identical to what happened on my LB7. It ended up being the pin on thcrank was sheared, allowing the harmonic balancer to stop spinning and/or be out of position. Scary shit huh?

LOL Good luck!

http://dieselplace.com/forum/showthread.php?t=131675

ouch, sounds like you are on the right track harmonic balancer or broken crank. Remove the serpentine belt and look at the balancer while its idling. If it is wobbling and the bolt is tight you have a broken crankshaft.

Turns out it was a harmonic balancer. The fellow at the dealership had a few come back like this, he thinks that the newbie mechanic didn't torque the main pulley to 200 ft lbs.

This is exactly the reason I designed our Super Damper as a press fit to the crank snout. Not only does it dampen better with a press fit it also relieves the stress on the alignment pin.