Broken Crankshaft ?

[partsandservice]

Here I am working on another teir 3 6.8 748H. I need to talk theory. I have a severe mid-range vibration. This is a direct drive machine, so the drive line to transmission was removed and vibration remained. Balancer on front of engine is runnig true. Removed what Deere refers to as a dampner( flexplate of sorts) and found the rubber torn but not seperated. So it was replaced. Still there. Removed the new dampner and found the vibration gone. Reinstall vibration returned. The next conclusion is that the new dampner is ouy of balance. Next new dampner has the same result. The engine doues not have any sort of skip or abnormal engine smoke. Here is my theory, The Crankshaft is broken, adding the dampner amplifies the vibration. The Crankshaft does not have excessive end-play. Theoretically it seems possible that the crank is broken and the engine still runs. Practically is the question. I have never seen this. Soon I will have to test this theory as I am at the point it has to be internal and will have to break it down to see.

 

[ScottAR]

Seen it on a chevy pickup once. it ran although poorly... It broke mid throw so the two halves kept each other rotating.

 

[excavator]

I've seen it several times where the engine still runs OK but there was a very noticable knock. Had it one time in a cab-over Freightliner with a 335 Cummins that it broke between #4and #5 throws, it was driveable if you could stand the noise in the cab. Does the vibration change at all under load? I would think that if the crank were broken it would change. What about a bearing problem? Oil pressure is OK? How about removing fan/alternator belt and any accesories on the front of the motor. Just some random thoughts.

 

[Scrub Puller]

Yair . . . partsandservice. What about the flywheel/clutch end, nothing amiss there, hanging up, come loose?

I don't know the machine but I try to eliminate all possibilities. A broken crank in a runner seems a bit extreme . . . as the other fellers have said it happens but most of us don't see it in our lifetime.

Just my two bobs worth and best of luck with it. Please keep us posted.

Cheers.

 

[willie59]

Just so we're on the same page, just what is this dampener that you're referring to that when you remove it vibration goes away.

 

[shopguy]

Thanks willie that is what i wanted to ask,I dont think a vibration caused by a broken crank would go away by removing anything or a change in rpm .

 

[stumpjumper83]

check an oil sample, if there is internal vibration to the tune of a broken crank, wouldnt the oil be full of metal traces?

 

[willie59]

Balancer on front of engine is runnig true. Removed what Deere refers to as a dampner( flexplate of sorts) and found the rubber torn but not seperated. So it was replaced. Still there. Removed the new dampner and found the vibration gone. Reinstall vibration returned. The next conclusion is that the new dampner is ouy of balance. Next new dampner has the same result.

That's my question, is this "damper" the one on the front of engine on crankshaft? And you've fitted two new ones?

 

[excavator]

I think what he's refering to is a rubber coupling between the flywheel and the drive line.

 

[partsandservice]

I am talking about the rubber coupling bolted to the flywheel that drive line bolts to. I did cut open the oil filter and found some metal, but it was so fine that I would loose it before I could determine if it was steel or aluminum. The engine fan is hydraulic and it not connected to the engine. Seems odd to me too. The flywheel is balanced by drills not weights and does not appear to be cracked. This may require exploratory surgery. Oil pressure is good. Removing the dampner does make the viberation much less and almost diappears but it does not completly disappear as it may have come across in the begining of this thread.The vibration that remains without the dampner would not even be noticed if that was all it had when everything was hooked up. And yes, two brand new dampners from JD. So that is a total of three dampners including the original. This engine has 5900 hours and was rebuilt by me after a valve drop with around 4800 hours. The vibration just started a coulpe of days ago. As far as putting it under a load the owner pulled out the back=up skidder and has not pulled wood with since this started.

 

[Jbullfrog]

There were alot of broken crank issues with 7.8L engines in combines and tractors 5-6 years ago.They had a batch of bad cranks, and provided the parts, but you had to pay the labor. We had a 50 series combine that had a vibration and ran fine.

 

[rare ss]

measure the end float of the crank with a dial indicator from the front end and back end, if there is a differance it'll point (get it, point.. cause it's a dial indicator lol) to a broken crank

 

[JeremiahSr]

Even though the front harmonic balancer looks like it is running true i would take it off to inspect the rubber on it. I had an older 6.8 balancer that did this. In a 644h or g? This older one i had to pull from crankshaft and may have weakened it. Not sure if that newer engine you are working on needed to be pulled with a puller or if it just bolts on and slips right off. Still...just because it dont lopok like it is bouncing around does not mean it is not the prob.

 

[John C.]

I have seen a broken crank in an old 950 Cat that still ran OK but lost a bunch of horsepower in a push and lift. Ran an oil sample which showed nothing. Swapped injectors which made no difference. Pulled and cut the filter and saw nothing. We finally pulled the motor and stripped it down seeing no problems until my helper grabbed one end of the crank and I the other end and tried to pick the it out of the block. The short end just tipped into the block.

I never took an oil sample report at face value ever again after that. That engine ran at least a couple of hundred hours that way.

 

[willie59]

my helper grabbed one end of the crank and I the other end and tried to pick the it out of the block. The short end just tipped into the block.

No doubt led to a brief moment of absolute stunned silence.

 

[partsandservice]

Well, we now know it is possible. Talking it out helped me work through this problem logically. Talking it out also made me make sure I covered all the bases before removing the engine for tear down. This is a common rail fuel system and an engine I dont have programming for. I purchased a new fuel line cut it and welded the line then used this tool to block off one cly at a time. Found number five cly almost dead. Replaced the injector and the vibration is gone. The bad injector producred no smoke and no codes so it must have been stopped up or tip stuck closed. Thanks to all.

 

[Nige]

Easier than having to rip out the motor that's for sure.

 

[excavator]

We are never to old to learn. Happy you figured it out and now we've all learned something. Thanks.

 

[Scrub Puller]

Yair . . . partsandservice. Good one mate . . . I bet that was a relief.

Thanks for posting the resolution of the problem. I get a bit frustrated when an issue is aired here and we dont get to hear how it was resolved.

Cheers.

 

[Plant Fitter]

Instead of blocking off the injector line, on a common rail system, I believe you can just unplug the electrical connector going to the injector to stop the cylinder from firing.

Or will this upset the electronics? Can anyone confirm?