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Does my lousy sounding 4.0 need a rebuild?

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Old 03-20-2017, 07:06 PM   #1
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Default Does my lousy sounding 4.0 need a rebuild?

I am sure that someone has asked about this but I couldn't find it. Why does the Jeep 4.0 six get so noisy as it ages? The engine in my '99 Cherokee (170,000 miles) rattles and knocks but every Jeep guy I talk to says "it's normal - just keep driving it." I really depend on this Jeep and I don't want to end up stuck somewhere with a thrown rod or something. The engine has good compression, good oil pressure, doesn't burn much oil, and makes good power for what it is. How do I tell if this engine needs rebuilding?
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Old 03-20-2017, 07:20 PM   #2
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It depends on what the noise is. You can live with a valve tap. A rod knock is another matter entirely.

My '99 has about 230,000 miles and the engine is still quiet, no bad noises. So from my admittedly limited experience I would say is is not normal to have rattles and knocks develop. (I have had a lot of experience though with the AMC six-cylinder engine the 4.0 is based on. The most common noise at high mileage on those is a valve tap due to worn or stuck lifters.)
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Old 03-20-2017, 08:29 PM   #3
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I agree than a knocking engine is bad news. I bought this Cherokee used in 2004 with 96000 miles. I have no idea what kind of maintenance it received. It had some valve clatter but not bad. It was just a beater that we intended to use for a few years and unload (funny how attitudes change). Within a year it started to knock and I thought it would be dead by Christmas. 13 years and 76,000 miles later it is still knocking and clattering and still running. Sometimes the knock goes away for a short time but it always comes back. However, it never gets worse. In a year or two if it still hasn't died I plan to do a rebuild. Until then I guess I'll "just keep driving it."
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Old 03-20-2017, 11:03 PM   #4
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I had an 02 wj that was that way. Usually it was a deeper tone more like somewhere in the bottom end. Wasn't super loud just audible. Sometimes it would go away completely. Currently have an 01 xj with 207k and no noises at all. Also have a 98 with 213k that just started getting noisy after last oil change when I added 1/2 quart of mystery oil. I have a feeling it thinned the oil too much. Also have another 98 with 163k and it's quiet also.

What are you running for oil? Also are you sure it's not the water pump or something else? I have had a bad water pump scare the bejesus out of me thinkin it was a bad bearing or worse. Lastly how well do you keep up with the oil changes and what not?
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Old 03-20-2017, 11:54 PM   #5
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sure it's not loose flexplate bolts?

Put a stethoscope on it. Pretty easy to do.....
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Old 03-21-2017, 12:21 AM   #6
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Thanks for the feedback. Haven't thought of flexplate bolts but I will check into that. It has crossed my mind that at least some of the noise might be from a bad water pump. I'll check into that too. My annual mileage is kind of low so I have been changing the oil and filter every 2500 miles. I've been using Castrol GTX 10w30 until about two years ago when I switched to 10w40. I also add a little ZDDP Plus for the flat tappet cam but not enough to wreck the cat converter. My driving is about 50/50 city/highway. The Cherokee is stock and does not go off road -- I use my '04 Wrangler for that. By the way, the Wrangler's 4.0 runs nice and quiet. I run synthetic in that one.
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Old 03-21-2017, 01:54 AM   #7
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ive been told its piston slap. a gal here had hers blow a piston, trash. mine quiets when warmed up. it may survive?
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Old 03-21-2017, 08:20 AM   #8
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piston slap doesn't generally go away at any temp. it may not be as pervalent at some RPMs as others but generally wont go away. Rocker or valve tap might. May be worthwhile to pull the valve cover and check the rockers and their torque.

Also I've now owned to XJs and both of them had flex plate to torque converter bolts that had loosened up over time. this can be checked by pulling the inspection plate behind the oil pan at the transmission.
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Old 03-21-2017, 09:40 AM   #9
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Thanks for all the help. I definitely have to get under the Jeep and look at those bolts. One of the noises is a strange rotational rattle which could very well be loose flexplate bolts.
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