Knock = Flywheel Cracked?

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  #1  
Old 09-01-07, 04:47 AM
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Knock = Flywheel Cracked?

Hi everyone, I have a knock that is coming from my engine. The car is a 2000 Dodge Caravan with the 3.0 v6. The knock is most prominent on cold starts and lasts about 5 minutes or so before it goes away (in idle). If i put it in gear or turn the wheel it starts to knock, but will go away as I give it gas. Its definitely a knock and not a ticking sound.

I took it to one mechanic who removed all the belts to determine if it was something outside the engine. Turns out its in the engine, but cant tell me what it could be exactly.

So, I take it to another mechanic. I tell him what the other guy did so as to not do unnecessary work. He tells me he thinks he sees a crack on the flywheel. He investigates further and cant determine for sure, but tells me it is probably the fly wheel and will cost about $550+tax.

My question is, could this be the flywheel? I dont understand why the knock would go away upon warm up if it was the flywheel. I've had a bad flywheel before and I thought it was constant knocking sound. Believe me, I'm not opposed to spending the money at all. I just want to make sure that when I get the bill the knock is gone. Also, do any mechanics use a stethoscope to isolate/determine engine noses?
 
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Old 09-01-07, 01:17 PM
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Are the mechanics checking the engine while it's knocking or after it has warmed up and disappeared?

In terms of a flywheel being defective - it will cause a knock. And it can change as the engine warms. An open crack when cold can close and change the flywheel shape when warmed.

However, you've described two different loadings to get the knock. One is loading the engine through the flywheel by putting the van in gear. That could cause the knock to come back due to a cracked flywheel.

But turning the steering wheel isn't loading the engine through the flywheel so if you're getting the same knock by doing that you could have a problem somewhere else.

Something that brings about what you've described can be as simple as a loose part such as the exhaust which bumps when an engine is running rough as in a cold engine. When the engine warms up it smooths out and the knocking disappears only to return when you load the engine by putting it in gear or with the power steering.

I wouldn't rule out the flywheel as being at fault, but when the engine is cold and you have the knocking, raise the hood and see if the engine appears to shake as with a rough running engine. If it is check mounting points of the engine as well as exhaust components.

In terms of stethoscopes - there are electronic devices which can be very effective in isolating noises from moving parts. They can be calibrated to eliminate certain frequencies leaving only the suspect noise. They are somewhat expensive and I can't say I know of a lot of shops that have them.

Hope this helps,

Bob
 
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Old 09-01-07, 03:20 PM
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Did any mechanic pull individual spark plugs to see if the knock goes away. In cases of a bad rod bearing, that is a good test. The reason I see that even your power steering affects it, as that too is putting the engine under load (when turning the wheel). It sounds like 2 common denominators are any sort of engine load and cold engine temp; sign of bad bearings. Possibly.

Any oil pressure or finding engine without oil, EVER? Have you owned the van since new?
 
  #4  
Old 09-02-07, 08:26 AM
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I would suggest that you measure the oil pressure with a direct pressure guage. If the pressure is on the low side, I would suspect main bearing. The conditions discribed are consistant with worn mains.
 
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Old 09-02-07, 01:55 PM
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One more question...

Unless I missed it, I didn't see how many miles this engine has on it. From my meager experience, the 3.0 is notorious for knocking and oil leaks when it has high miles.

I have family members who've had caravans with the 3.0 where the engine sounds quite troubled because of knocking, but the knock didn't cause any problems.

The 3.0 is very tough. I haven't heard of any major trouble out of this engine from the many that've been in my family.... besides when the water pump goes out. One thing I've noticed from ALL of them is that they knock when the miles get high. Almost all of the Chryslers with the 3.0 in them (that I've had experience with) last well over 200 thousand miles with this knock.

You have a fairly well designed engine (IMO). It has its quirks such as knocking and eventually oil leakage, and have fun when the water pump goes. If the knock doesn't sound too dastardly, it might just be getting old. Use thicker oil and be on your way.
 
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Old 09-02-07, 03:35 PM
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Originally Posted by Jason R View Post
Use thicker oil and be on your way.
Now THAT ain't a bad idea. I and this other mechanically inclined guy I work for both swear by some of the good luck we've had with the various 'motor honey' products out there. They can actually stop a car that has been getting hotter than it should and losing oil pressure at idle, to then sustain closer to normal pressure at idle under same condition with the addition of the product. Also you MAY notice that your oil consumption goes down It is okay to substitute 20% of your oil capacity with this type additive. It will say that on the bottle or can of the stuff.
 
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Old 09-04-07, 02:00 PM
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Thanks for all the replies!
Sorry i forgot to list the mileage, I have 64K on the van.
I've use the "motor honey" in my previous 3.0 that was burning oil, and the stuff worked great. That one had 100K+ on it and no engine noise.

"jeff194307" that's what I'm afraid of, but why would that happen at so low of miles? Especially when the van was and still is running great (minus the knock). How would I go about measuring the oil pressure with a direct pressure gage?

"ecman51" No, no one has pulled the spark plugs, nor has anyone offered that as a diagnostic option. I have never had this type of issue before, not too sure how to go about solving. That's why I came here , I know this site is rich with experience.
 
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