Found a 99 Cherokee Sport with 71K on it sitting in a garage. It's been there for six years since the owners husband passed. Paid five grand for it.
It's for my 16 year old grandson. So far we have put 200 miles on it.. It has fresh fuel and fresh oil in it.
The lil lady was TOWED behind the mans motorhome. So I have no idea really how many miles it has been driven.
NOTHING has a bit of rust..

Now to a question.
After a 15 mile drive when all is at normal temp I hear a single knock in it. The knock is the same that I would subscribe to one piston pin.
So I'm thinking of changing the oil to 40W just to see if the knock lessons or goes away.

The only other engine of the same kind in my experience is my 02 TJ, which has never had problems.
So I'm asking does anyone have experience with a similiar knock?
It is a light sound not the deep hard sound of a rod/crank knock. Also not anywhere near the sound of a lifter. But there is a chance it is an injector... if they make such a sound. As the sound occurs with the same regularity as an injector or a piston..
Anyone?
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

The 4.0l is a very durable and forgiving engine.

Perhaps you are hearing loose timing chain??

or, exhaust manifold???

or, motor mounts??

Those are the only problems I have experienced with that engine in 2 different Jeeps over the past 10 years. One had over 195k miles and my current YJ has 121K miles.

I have never had a problem with injectors. I do run injector cleaner through the engine on a regular basis.

Do a cylinder balance test by shorting a plug or disconnecting an injector one at a time to see if the knock noise changes. If it does, it may be piston/rod related. If it doesn't, it may be a lifter/valve train, or there may be an excess of carbon on one piston.

I hate to say this but flog the crap out of it,.. its likely just deposits,.. if it pops grab another motor , if it clears up ,...sweet no use on prolonging the inevitable,..