4.0 cracked block???? - Jeep Wrangler Forum

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Old 03-17-2012, 01:11 AM   #1
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Well for the last year our so I would have to add a little coolant. Maybe like a gallon total after 3-4 months. I finally got fed up with it and started checking where these little drops came from. I thought it was the freeze plugs on the initial inspection, so just to be sure I did a pressure test. I could see it start coming out from above the freeze plugs so I assumed the head gasket. So, today I ripped off the intake and header just to find that it wasn't the head gasket. The coolant is actually seeping out of these cracks. It is 4 "v" shaped cracks in the block. My main question is had anyone else ever seen this? Also, if you have, what causes it?

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This is above the second freeze plug

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This is the third freeze plug

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And this is the fourth.

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Old 03-17-2012, 01:42 AM   #2
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overheating, lack of maintenance (Coolant).
Crud in the water jacket prevents cooling causing hot spots that can turn into cracks.


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Old 03-17-2012, 10:55 AM   #3
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I know this motor hasn't been overheated. I bought the rebuilt 4.0 from a close friend that has a shop he rebuilds motors for a living. I called him and he said to bring the motor by so he could tear it down. Btw it's supposed to be bored .040, a small cam, hvop, and the head is shaved .040. I got it out and I'm taking it over there in a minute. He said in about an hour he could tear it down and he could tell me what happened.
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Old 03-17-2012, 11:02 AM   #4
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The cracks that i have seen around frost plugs have been from the coolant not strong enough and it froze.
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Old 03-17-2012, 01:02 PM   #5
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This reminds me, I need to do a coolant flush and refill!
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Old 03-18-2012, 11:40 PM   #6
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Well, he called me around lunch yesterday and said that the reason it cracked was because the extra compression was too much stress on the block. He also said that eventually it would have broke enough to blow the head off the top. This is his reason for the v-shaped cracks. So I drove about two hours away to pick up another motor. He's going to trade me the rebuilding of the new motor (bore the cylinders, shave the head, swap in my cam, and grind the crank with new bearings and seals) for me to do some trans r&r labor. I guess we'll see how long this block lasts.
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Old 03-20-2012, 05:22 PM   #7
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Man I still dont think that is right...I have seen several 10.0 compression 4.7 strokers live a long time. I'm thinking it was a freak deal, honestly
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Old 03-20-2012, 08:59 PM   #8
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Me too. He swapped all my stuff and I installed it today. This motor doesn't seem to have as much power to me. Before I could atleast chirp in 2nd, maybe I can't because it's warmer. Oh well, it's all done now, I'll just drive it until it breaks again :-) which will probably be soon.
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Old 03-20-2012, 09:29 PM   #9
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How much did you pay for the new motor? Im putting a V8 into my 4.0 and am going to sell the engine/tranny but had no clue how much it's worth. That sucks about the cracked Block!
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Old 03-20-2012, 09:44 PM   #10
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Yep..seen it last week on my cousins Jeep. Previous owner had a leaky radiator, kept adding plain ol' water..winter came and froze the block..all freeze plugs were buldged (looks like yours to0) and cracked the block in the same place as yours. To bad the freeze plugs didn't do their job

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