Hello fellow jeepers

I took a deep breath and a couple of weeks off working on the xj after seriously considering asking someone to bring a trailer.

so far;
Oil pressure great
pulling the fuel connector one cylinder at a time makes no difference in sound
flexplate and bolts seems fine
knock gets louder when warm


HOWEVER! after diagnosing further I started to hear a screech at certain rpms. Also I felt the knock might come from further forward- ENTER BALANCE CRANK PULLEY THINGY!

after removing the belt the knock got even worse. I tried to look closely to see if it was wobbly, but when I slowed my video down I can definitely see it moving up and down at the centre hub. What do you think?

 

Roundy roundy good
Upy downy not good

 

yeah moves up and down right?
does this mean that the front crank seal is shot, and I guess this could make the knocking?

 

have you done a compression test?
have you drained the oil looking for metallics?
have you removed the valve cover and inspected the push rods and rocker arms?
have you started it to watch for any abnormalities with the rocker arms and or lifters and push rods or a stuck valve at an idle.?
if you removed the serpentine belt and it still knocks, then the harmonic balance pulley should be out of play for knocking unless it is slid off the inner part and rubbing the block. Can you see any areas around the harmonic balance pulley showing any friction wear or shiny areas?
did you get a cheap mechanics stethoscope to isolate where the knock is originating from?

 

I have drained the oil and checked the pan and oil for metal with a magnet as well. Couldn’t find any metal shavings or chunks.

I have yet to do a compression test and take the valve cover off to inspect the lifters and push rods.. First time for me so a bit hesitant.

as mentioned I have tested to cancel one cylinder at a time but no change really

I’ll put the belt back and do these as next steps, thanks for the input!

 

here is some additional information provided by Co-pilot:
A Jeep 4.0L engine knock that appears only when warm is often linked to worn internal components, oil-related issues, or heat-sensitive clearances. Common culprits include piston slap, rod bearing wear, lifter noise, or even oil viscosity and filter problems.

Here’s a breakdown of the most likely causes:

🔧 Common Causes of Warm-Only Knock in Jeep 4.0L Engines

• Worn Rod Bearings or Main Bearings
  
  • Heat thins oil, reducing its cushioning effect. If bearings are worn, this can lead to knocking only when warm.
  • Check for low oil pressure when hot—this is a strong indicator.
• Piston Slap
  
  • More common in high-mileage engines. As pistons expand with heat, excessive clearance can cause a knock.
  • Usually harmless but can worsen over time.
• Lifter or Valve Tappet Noise
  
  • Hydraulic lifters may stick or collapse when oil thins out. This can sound like a knock or tick.
  • Some owners report improvement with heavier oil (e.g., 15W-40) or additives like Rislone.
• Oil Viscosity or Filter Issues
  
  • Switching to a different oil weight (e.g., from 10W-30 to 5W-30) or using a poor-quality filter can affect oil flow and pressure.
  • Filters with faulty drain-back valves can cause delayed oil delivery to top-end components.
• Cracked Exhaust Manifold
  
  • Though more of a tick than a knock, it can mimic engine noise and worsen with heat expansion.
• Timing or Knock Sensor Issues
  
  • Less common, but incorrect ignition timing or a faulty knock sensor can cause detonation-like sounds when warm.

🛠️ Diagnostic Tips

• Oil Pressure Test: Use a mechanical gauge to check hot idle pressure. Low readings suggest bearing wear.
• Compression Test: Helps rule out valve or piston issues.
• Stethoscope or Chassis Ear: Pinpoint the knock source—bottom end vs. top end.
• Scan for Codes: Even if the check engine light isn’t on, stored codes may help.

 

I had a knock on my 350 and it sounded like it was coming everywhere. It ended up being carbon buildup on the pistons.

 

[QUOTE=blazer 2008;3733764]I had a knock on my 350 and it sounded like it was coming everywhere. It ended up being carbon buildup on the pistons.[/QUOTE

ok!
did you manage to remove it somehow and get rid of the noise?

 

[quote=Skuggan;3733766]
but I don't think this will fix your problem. And I would advise to research how to do this a lot more before attempting.

 

[QUOTE=BruceB;3733768]Yes won’t be doing that until I know for sure what’s going on, and probably not with a garden hose
I have crazy vibrations going on as well, especially around 1000-1200 rpms. And the HB rubber looks kind of crusty. Anyways, tomorrow valve cover comes off, let’s see what to learn from that.

 

Your crank pulley / harmonic dampener appears to be coming apart, although I'm sure that's the source of your knock sound and vibrations.

 

I just rebuilt the motor. I had it so far apart trying to find the noise was easier to rebuild it

 

Good luck with this! I would definitely recommend getting that harmonic balancer pulley replaced before going further with the diagnosis.

I've had several occasions where I though the 4.0 in my XJ or my wife's ZJ was knocking, but every time so far (knock on wood) it has always been a failing belt-driven accessory. You mentioned that you had a crazy vibration too, which is likely from that pulley.

My wife's ZJ once had really bad vibrations and was knocking - it turned out to be a bad water pump bearing.

 

thanks! Yes I fired the parts cannon and I am waiting for a new HB and front seal to arrive soon. I removed the belt and the knocking got even worse, I guess that means that the other accessories and pulleys are outruled for this specific problem. They feel solid. And that the HB got even more freeplay without the belt on, causing even more knocking. Looking at it -it won’t hurt replacing anyways..

 

you will need a hb puller to get that thing off....