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Dreaded tapping at idle into rev

Nolaws

NAXJA Forum User
Location
Santa Clarita
Well I am guessing I have a bad rod bearing in my XJ. I just noticed it this morning after I of coarse filled the tank! It is a small tapping at idle and it increases in RPM as the motor does. Could it be something else? I am now wondering how hard it is to do the fix on this. I am guessing I will need to pull the motor but was wondering if there is any advice any of the more seasoned veterans out there can give about doing this fix. I have changed several motors and done a few hotrod engines but they were all V8 platform. I have very little experience with the in-line 6.

Any help would be appreciated,

Robert
 
Let the engine warm up fully, then in Park run the engine speed up to 3800 and hold it there for about 60 seconds, then let it slowly back down to idle. If the noise goes away, it is just a typical 4.0 sticking lifter. Many of us have had that issue for a gazzilion miles with these jeeps. Seems that at about 3800 rpm they unstick quickly. May need to do it every once in a while, especially very cold day start ups, but let it warm up first. Also some oil additives like MMO or a different oil will help some. What oil do you use? Is this jeep new to you?
 
Drive it until it blows up.

x2

you can drive a 4.0 with no oil pressure for a month 30 miles back and forth to work.

seriously.

if it makes you feel better, be on the lookout for a used 4.0. They last nearly forever, the only "tricky" bit about the install is the e-torx bolts on top of the bellhousing. Mostly because no one expects them. They're the Spanish Inquisition of bolts.
 
Before you condemn the engine. If you have an automatic, then check the torque converter bolts, and check for cracks on the flex plate. I guess they can also make a knocking noise at idle when bad.
 
Thank you for all the advice! I have had the XJ for around 7 years and it has around 251,000 miles on it ATM. I am currently looking into a rebuilt long block with a warranty. So far most of the ones I have seen are around $1600. Anyone know of any good places for rebuilds? I will definately look into the flex plate issue but it sounds like the noise is coming from the oil pan area near the number 2 cylinder.
 
Drop the pan, roll a new bearing into place, reinstall the pan and drive it.

FTW--the 4.0 crank bearing caps and rod bearing caps like to work themselves loose. Anytime you have the pan off re-torque them.
 
Mines had a knock for about a year and a half. I have a DD so I really don't care what happens, when it goes it goes. Ill pull it and throw in another high mileage 4.0 afterwards. Like others have said, they make funny noises but run like a top. Id be more worried about overheating or no oil pressure than a tick.
 
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