So I will start with a little background. My wife wanted an XJ of her own to join me wheeling. I found a clean 2000 with a supposed blown motor with an included new used motor. They guy said the motor had a knock and never got around to changing out the motor. It's hard finding not rotted xj around me so I jumped on it. I was curious as to what was causing the original motor to knock so I tore it down. I have torn down 3 previous 4.0s and never seen one in such good condition with such little signs of wear. For 190k on the motor the rod bearings showed no copper on the upper caps no scoring. The crank journals look and felt like they were fresh from the machine shop. The cylinders are clean with no scuffs, scores or even any amount of ridge on the top. All piston skirts are were intact and look fine. Cam lobes, lifters and rockers all clean and no signs of wear. First thing I though thought was **** maybe the flex plate bolts were loose. Problem is the p.o. started raring into it already has them removed. Since the replacent motor looked a bit sketchy and had some sludge buildup under the valve cover I took the risk and tossed the old motor back in after installing new gaskets and what not. She started right up. I thought I was in the clear. I heard a little noise in the beginning but know that 4.0s are noisy when cold. After taking it for a test drive I realized that there definitely was a knock or heavy rock. When it's warmed up it quiets down a lot but under load while driving it's loud enough to hear over the heater blower on high. So I started to try and figure this noise out. Below is what I have done.

Double checked flex plate bolts
Added bottle of Marvels mystery oil
Unplugged injectors one at a time with no chang​​​​​​e
Used stethoscope and could not pinpoint the sound

Took the valve cover off and felt each rocker while running. They all felt the same!

Rechecked rocker bolt torque spec.
I haven't taken off the serpintine belt but I am pretty positive it's not the power steering pump or any other accesories.

I know what piston slap sounds like and this is not it. I'm at a loss. Any ideas?

 

Hmm... did you reuse the same lifters? Could be a lifter. If not, I'd be looking at other common parts that were shared between the engines

 

How about this?

 

I used the same motor never installed the replacent the p.o. purchased. The lifters all looked good they were pumped up with oil and looked good to me.

 

This...lol....

 

I could not pinpoint the sound with a stethoscope. I have a video so everyone can here the noise but I don't know how to post it up. Do I have to put it youtube and submit a link?

 

Try removing serp belt and starting yet see if its still there ?

 

That how I do it.

 

I have had a bad power steering pump make a knocki
knocking before but I'm almost positive this in the motor. When I get home today I will take the belt off just to check that off the list.

 

Here is a video of the noise im hearing. https://youtu.be/fWvebZqpBYI

 

Sound like it needs a rebuild.

 

Telling you this is the cleanest 4.0 i have ever opened up. I see no signs of wear. No ridge on the tops of the cylinders. Cylinders have no marks or grooves with lots of factory cross hatching. Crank and can journals are smooth. The crank bearings have no visble wear and none of the nickle plating is worn off. Oil pressure is about 40psi once warm. The only think I can think is maybe a wrist pin or maybe a skirt is bent in a bit causing slap. How to I test for a wrist pin withowithout removing the head, oil pan and removing Piston?

 

I have had two water pumps that sounded like a lower end knock. Both 4.0's. One with 212k and one at 163k. The first scared me and the second I knew what it was.

 

I haven't had a chance yet to take off the serpintine belt to check. I had a power steering pump do the same thing!

 

It's unfortunate that it looked so nice that you didn't plastagauge the rods when it was easy. Anyway if you DID go there, me wonders if one would feel a worn wrist pin by fiddling with the unbolted rod?

Like mentioned I might remove the belt just to be double sure....sound can be very tricky. Btw besides the stethoscope, don't forger a length of hose to your ear might tell you something.

I know you unplugged injectors. I've always unplugged plugs to isolate a bad rod. (loosening the wires on the cap first, cuts down on getting zapped!)