Jeep knocks on startup, then goes away. - Page 2 -

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post #16 of 19 Old 08-06-2011, 12:56 PM
Chicken in a Turkey Suit
2003 WJ 
Join Date: Oct 2008
Location: Redmond
Posts: 8,532
I dunno, maybe since it is posted in here 2-3 times a week. Compared to 2-3 times a year someone posts with a blown-up 4.7.

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post #17 of 19 Old 08-06-2011, 02:27 PM
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1999 XJ Cherokee 
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Location: Argos
Posts: 181
I would bet that its a cracked piston skirt. Mine did it for 80K was more noticeable with a fresh oil change. It would come and go, meaning it didn't do it every time on start up. When it started knocking the last time, it did it for 20 miles, then stopped, then did it all the way home from from a field trip.
Next morning I went for coffee and while heading to work, it tapped hard for 10 seconds and the vacuum went. I drove it 8 miles to my shop, parked it and found a 2006 wrangler 4.0 to replace it. My 4.0 had a hole you could place yur fist in....sat for 2 months for me to get ready to swap engines. It started up and drove it into my shop...needless to say, my dad and I were stunned!

We have replaced 3 of the 4.0 engines, all from piston skirts letting go. If its knocking, pull it and slap some really good pistons in it.
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post #18 of 19 Old 08-06-2011, 02:29 PM
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1999 XJ Cherokee 
Join Date: Aug 2009
Location: Argos
Posts: 181
Originally Posted by billzcat1 View Post
I'm guessing cracked piston. It's far more common in 4.0s than the 4.0 fans would have you believe.
I my post! the 99 and up WJ 4.0's have poor skirt design. I bet the factory made them thinner for several reasons...cost, faster revs, MPG and so that the dealer will replace it for a ton of money down the road. lol
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post #19 of 19 Old 08-06-2011, 02:41 PM
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2001 WJ 
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Location: Northern
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Drop your oil pan and have a look, not to pricey to do and you can inspect everything else.

2001 WJ 60th anniversary edition 4.7 quadra-drive
5" hybrid lift, 265/70/17 Guard Dogs load E, bilsteins 5100's, addco sway bars, JKS front quick disco's, JKS adjustable track bar, KOR durometer track bar bushings, IRO crucial path front L.A.s, 1.25" Spidertrax spacers

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