I looking at buying a 98 zj 4.0 as a daily. It's was described as having a lifter knock, so I went to look at it. It is pretty noticeable sound, seems louder then your average 4.0 lifter noise. The noise stayed pretty consistent the few minutes it was idling, but it did get little louder and quieter then went back to original noise. It defiantly sound like it was from the top end, and much less noticeable from underneath the jeep. Oil pressure and oil level are good. I only got to see and hear it idling and parked. He said it started all the sudden last week and he has only out 30 miles on it since then and it never knocked before now. He also has a lot of recent service, including valve cover gasket and oil pan gasket within the last 2k miles.
Any ideas? Maybe just a more severe lifter/valve issue? Or maybe a bad piston skirt?

 

i know you mentioned it sounded like it was coming from the top end.. but sounds can travel... could actually be the torque converter bolts loose... i have the same noise and im going to be checking there first..

 

Put it in gear. If the torque convertor is loose, it should shut up.

 

Well I heard it knocking when he pulled up in the parking spot

 

In my experience, a collapsed lifter sounds like a sharp clicking sound whereas a piston is a deeper bell-like sound.

From what you say about the new gaskets, I'd be willing to bet he knows what it is and is trying to hide it. In which case, it's a piston skirt or wrist pin. Just remember, in order to change a lifter on this engine, the head has to come off. It doesn't have a "trap door" in the side like older 6's.

 

Well he has receipts for the work. But certainly sounds like a lifter, just louder then usual

 

It is possible one of the rocker mounting bolts is loose.

 

This happened to my 89. I'd just take the valve cover off and check the lash on each valve along with checking the rocker mounts.