My 2000 xj developed this noise a couple years ago. It seems like a lower end knock that is sometimes hard to hear, but other times is very noticeable. It only does it when up to operating temp, and it sounds louder during the summer.
Mileage is 114,000
Oil is Rotella 15w-40(I used to use 10w-30 but am trying Rotella for the summer months)
Filter is Wix 51515
Oil pressure is great, 25 at idle, 50 at speed. No coolant loss, oil is clean and stays clean.
That is normal for the 4.0L. Almost every 4.0 I have heard has that slight idle knock. Some do it more than others, especially high mileage motors. Mine has done it since I bought it, my previous '90 made that noise, my dad's '87 made that noise, and my buddy's 2000 also makes the noise. I seem to only notice it in mine when I run a 10w40 oil. 10w30 seems to quiet it down a lot.
Try Pennzoil Yellow Bottle 10w30 and see how that does. It has lots of moly in it, and seems to quiet my 4.0 down.
So I'm the second owner of mine. PO was anal about keeping records ( bless his heart ). One of the items in the extensive file he had kept on the xj, was a copy of a letter to Chrysler he sent soon after he had purchased it regarding the " knocking at idle ". According to them it is normal. I had heard that it had something to do with a coating that the factory put on the pistons. Mine sounded like a diesel at idle until I rebuilt it. Now, with new pistons and re-bore, it sounds more like a sewing machine. Quite ! Bottom line - nothing to worry about. Typo "quiet"
I'm right 97% of the time. The other 4% doesn't matter.
Last edited by 1CVPaul; 06-09-2012 at 02:56 PM..
My 2000xj just started making the same noise at 126k. It only happens at start up. The noise goes away after driving a short distance. I was starting to get worried about it, maybe this is just normal?