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Unread 10-08-2008, 09:27 PM   #1
snowweb
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What could cause the crankshaft to break?

Hi, We have a Magnum V8 5.2L engine in a Grand Cherokee Limited 1996 model.

The engine stopped the other day whilst accelerating gently as around 20km/h or less.

Inspection revealed a broken crankshaft, but I'm worried about replacing it with a new one, which in the Philippines costs P48,000 (around $1000) because we have not ascertained for certain, the cause of the break.

Please can someone list the likely or possible causes and suggest whether I'm being paranoid or not?

I checked the guages, shortly before the breakdown and the engine temperature was low to cool and the oil pressure was normal and it seemed to be running very smoothly. However, the mechanic who disemmbled the engine says that the oil pump caused it, but I'm not sure that it is really faulty (since the engine management light, which works, never lit), also, I'm not sure how he can tell that it isn't working? What do you guys think?

Also, what are your views on using a 'second hand' crankshaft?

Thanks in advance.

pete

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Unread 10-10-2008, 09:25 PM   #2
snowweb
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Please guys, I'm desperate for advice here! Any help would be dearly appreciated!

Cheers. pete
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Unread 10-10-2008, 09:48 PM   #3
adam728
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The only time I've heard of someone breaking a crankshaft it's either been

A) A manufacturer defect that shows up right away

or

B) Someone running an extremely modified high hp engine

Sorry it doesn't help you. Definietly not a common failure. Where exactly did the crank break?
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Unread 10-10-2008, 10:03 PM   #4
snowweb
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thanks for your reply adam, that's interesting info!

Engine not modified at all or rebuilt recently. Could it be caused by failure of the crankshaft position sensor?

Alternatively, cam chain SLIP? (the cam chain did not break).

thanks again
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Unread 10-11-2008, 12:19 PM   #5
Indy
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The crank is pretty much the strongest piece of metal in the engine. Having a timing failure will kill valves/damage pistons, but the crank should pretty much shrug it off. When I live in laramie I had a Wyo-tech instructor do an intake swap for me once and dump a ton of coolant/water on top of the pistons before he cranked it over. That bent the crank enough to throw it out of balance, but didn't come anywhere close to breaking it.

Post some pics of he breakage. It would pretty much have to be a major manufacturer defect for that kind of catastrophic failure.
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Unread 10-11-2008, 07:31 PM   #6
never monday
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please post pics.
I've only seen two broken cranks in 20+ years of messing with engines. Usually they stem from a defect in the casting of the crank blank. I wouldn't worry about installing a new or good used one.

What made the mechanic determine the oil pump caused the failure?
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Last edited by never monday; 10-11-2008 at 08:38 PM..
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Unread 10-11-2008, 07:46 PM   #7
Unlimited04
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a few things to consider that would cause the crankshaft to break...
- not enough oil, either a clog or just enough enough oil in the engine, or a bad oil pump
- unbalanced (either not machined correctly or bad harmonic balancer) - vibrations will destroy any rotating assembly
- non-concentric front to back in the block (warpage).

Sounds like the best thing to do is rebuild the bottom end, pull the rods, pistons, everything. Make sure everything was getting enough oil, and then go from there....either reusing the good components w/ new rings & bearings, or just use brand new components.

Also keep in mind you can't put a lot of faith in those factory gauges...my temp and oil pressure gauges lie on a regular basis.
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Unread 10-12-2008, 08:54 AM   #8
snowweb
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Thanks to both of you

I will post the pics when I get the old parts back from the workshop.

I quizzed the mechanic about the oil pump failure that he blamed for the broken crankshaft. I asked him how come the hydraulic valve lifters did not stop working before the crankshaft broke? After some squirming around in his seat, he agreed that the pump was still working but said that crankshaft was starved of oil by clocked oil ways to the area of the break. I can nolonger examine those oil ways, as the block has since been cleaned at the machine shop.

I do however, still wonder if this could be the cause, since running an engine dry of oil or an oil pump failure are fairly common things to happen to engines, but in my experience, it's the top end which suffers first.

It looks like we will be obtaining a good second hand crankshaft, 2 replacement second hand conrods, new piston rings, all bearings, bushes, camchain sprockets and cam chain. Also a new camchain cover, since although the chain did not break, the cover did. Also fitting new oil pump, plugs, and valve guides.

I drive like an old man due to the gas consumption, so I'm still amazed that this could happen at just light acceleration at 20km/hr!

Anyway, really appreciated your encouragement. I'm now sufficiently confident to have it re-assembled without thinking that it's going to break again the moment we turn the key!

Cheers guys, pete
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Unread 10-12-2008, 05:31 PM   #9
Ryanpowell167
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Please be sure the mechanic is checking for warpage along the mains. Have him check main bearing clearances with the crank installed aswell.
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Unread 10-12-2008, 11:11 PM   #10
snowweb
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ok, thanks Ryan, I'll pass that on.
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