My son has a 2002 Jeep Cherokee with a 4.0 6cly. About a too weeks ago it started making clicking noise. He had new lifters in stalled and new rockers kits in installed. still making the noise. What could it be? It has about 120,0000 miles on it.

This is hard to diagnose without more details.  Typical valve train noise comes from worn lifters.  However, a loose worn timing chain can cause a similar noise.  If the Jeep has an automatic trans, loose bolts on the torque converter can make a “rod knock” sound as well.  I would suggest checking online with some of the “ask the mechanic” sites specifically for Jeep.  There may even be Jeep repair blogs.  Most vehicle models share the same problems and you may find some documented history on this particular issue.

Many times replacing lifters w/o replacing a cam will produce noise.  The lifters and cam wore together, now new parts are mated to an old part and can cause problems.  That is one of the reasons why when you take lifters out of an engine to do a valve job you return the lifters to the original location if you are not replacing them along with the cam.

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My 1996 Jeep Cherokee loves MMO!  I don’t know what it is, but acceleration, idle, and general smooth performance are so noticable when I run MMO v. when I do not (which is not very often, just when I need to fill up and forget to put a bottle under the seat).