Cracked exhaust manifold on 3.8... - JKowners.com : Jeep Wrangler JK Forum
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post #1 of 7 Old 03-21-2017, 07:55 PM Thread Starter
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Cracked exhaust manifold on 3.8...

Have a ticking from what I believe is another cracked driver side exhaust manifold. Warranty is up on this one so it's on me.

Doing some research online I came across this:

"The exhaust systems on the JK Wranglers allow very little flex. This causes the forces that the exhaust sees from the engine torqueing to twist the exhaust system instead of allowing it to flex. Over time the weakest part of the exhaust system (the manifolds) fails, causing cracks to develop. It is highly recommend that a flex-pipe is installed into the down-pipe of exhaust system with the installation of replacement manifolds. This will allow the forces that the exhaust sees to be absorbed by the flex-pipe instead of passed onto the manifolds."

Seeing how stiff and long the y pipe is this makes sense to me. I don't want to keep installing exhaust manifolds on my dime now that warranty is up. I'm looking at a $90 doorman manifold and then having a couple of these flex pieces installed :



Was thinking in the 2 places I circled on blue, seems to be the only possibilities:



Was wondering if anyone else had done this? Or any other thoughts on it...
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post #2 of 7 Old 03-21-2017, 08:54 PM
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It's a good ideal but replacing a flex tube may cost as much as a manifold later on down the road.

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post #3 of 7 Old 03-22-2017, 03:15 AM Thread Starter
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Quote:
Originally Posted by Rb6700 View Post
It's a good ideal but replacing a flex tube may cost as much as a manifold later on down the road.

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The flex tubes I'm seeing are between $13 and $20 depending on where I look.
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post #4 of 7 Old 03-22-2017, 08:49 AM
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The cracks develop over time due to the manifolds getting crystallized from excessive heat. These little motors work pretty hard to pull our Jeeps and the result is a lot of heat in these manifolds. The flex pipe might do something, but will not fix the cracking issue. I have changed a bunch of these and it is very common problem.
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post #5 of 7 Old 03-22-2017, 10:52 AM
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I have a hard time believing that the cast iron manifolds are the weakest part of the exhaust system. The rest of the system is thin walled tube.
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post #6 of 7 Old 03-22-2017, 11:54 AM
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Believe it or not I am an engineer that used to design these flex couplings for the big three, also Toyota, Navistar, etc.

They are indeed meant to decouple the engine movement from the chassis. Every system is different but typical design parameters focused around XYZ movements, vibration dampening, thermal loads, and noise attenuation. The braid is there​ for abrasion resistance, strength, and dialing out any resonant frequencies.

Off the shelf can work for your application, but the lifespan will depend on all these characteristics of your exhaust system.

Definitely worth a try if your cracks are indeed related to the engine movement.



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post #7 of 7 Old 03-24-2017, 06:21 AM
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Quote:
Originally Posted by gt1guy View Post
I have a hard time believing that the cast iron manifolds are the weakest part of the exhaust system. The rest of the system is thin walled tube.

You won't be a non-believer if you examine one of these manifolds closely. The casting between the front two cylinders is very thin and eventually fails. Mine was broken there in two spots at about 48k miles. It's important not to over torque the two downpipe bolts at the manifolds. The joint there is not gasketed and designed to swivel. Bolt torque is only 20ft lbs.


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