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How much knock is too much knock?

alexgalexg

NAXJA Forum User
Location
Connecticut
1998 XJ
4.0I6
Head swap in December. About 6k put on it since.
Recent oil change less than 500 miles ago

Started up the XJ and it seems just a bit too knocky. They always make some noise when they are cold but this sounds like a light diesel. It did go away once the engine was at running temp and no fluids are disappearing. Filmed the knock
https://youtu.be/JwqPox1euWQ

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Sounds like a lifter. It can go a long, long time that way before failure.

I'd ignore it until it blows. Which may be another 100K miles.
 
It would get a lot louder and the engine would start missing.

Or a valve would stick open, a piston would hit it and possibly blow a hole in the side of the block.
 
It would get a lot louder and the engine would start missing.

Or a valve would stick open, a piston would hit it and possibly blow a hole in the side of the block.
Okay I was curious because this engine pings and I've had to use premium only since i cant figure out what the problem is. I can see they aren't related now

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Pinging is generally a problem with it being lean. Have you checked the fuel pressure?
 
Pinging is generally a problem with it being lean. Have you checked the fuel pressure?
I did however I used a rental tool so not sure if it was accurate or not. I had a thread on my fuel trims a while back but I didn't get much out of it aside from go research fuel trims. Haven't had the time

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I feel the need to update this thread. I can definitely confirm this is low end knock. Removed my throttle body today for a cleaning and started it to see how it idled after the cleaning and got a better recording after realizing it was louder from below.
This seems too loud even for an xj and I cant help but think it could be related to me having to use premium only since it pings with regular.
https://youtu.be/JZBaFGHtnIQ

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I don't know what your expecting! Sounded like a normal 4.0 to me!
 
I don't know what your expecting! Sounded like a normal 4.0 to me!
The black xj next to it has 300k on it and year old gas in the tank but doesn't knock. Maybe I'm just not used to owning a 4.0 that an average wreckless human owned. Late oil changes, no coolant or air filter changes ect. I dont know the history of the xj I pulled the engine from. I just know all the work i put in it since i dropped it in mine

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They always make some noise when they are cold but this sounds like a light diesel. It did go away once the engine was at running temp and no fluids are disappearing.

Rod knock gets worse as the engine warms up.

I really like how you go in-depth while trying to diagnose your vehicle problems. The next step is to buy a mechanics stethoscope and start probing; it's fun!
 
Rod knock gets worse as the engine warms up.

I really like how you go in-depth while trying to diagnose your vehicle problems. The next step is to buy a mechanics stethoscope and start probing; it's fun!
Will have to add that to my list along with a new fuel pressure gauge. I dont think harbor freight sells a non leaking one!
I will tell you this. When I put the new head on in December I, of course used assembly lube on the rods and rockers. When I started it I couldn't believe how smooth and quiet it ran. Not a knock. Ran like my other xj. But if course it only lasted that one start

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Does it disappear when you rev the engine?
Cant say it does. I haven't tried that exact method but leaving for work in the morning it sounds just as much like a diesel after backing out of my spot in the driveway as it did 2 secs ago before I hit the gas.
Once the engine is on the warmer side it clears up.
No issues with performance or stalling

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Unless you mean during the actual rev itself. Which I am not sure. If you need this information let me know and I will give it a small tap tomorrow

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Before jumping to conclusions, you mentioned an oil change. What type and weight of oil did you use?
 
Cant remember if it was 10w or 5w but it was Castrol edge and a k&n filter. Every 4k I change it

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5w leads to all sorts of noise in many older engines. Make sure it is at least 10w, especially if you oil pressure is on the low side. Short term you can run some STP oil treatment and see if it changes the noise.

Piston slap is very common in 4.0L's. For most, it is hard to tell the difference.

If the STP doesn't make much difference, I would drop the pan and plastigage the rods and mains. That is the definitive test for rod and main knock. It is a bit messy but not hard on even a mildly lifted vehicle.
 
5w leads to all sorts of noise in many older engines. Make sure it is at least 10w, especially if you oil pressure is on the low side. Short term you can run some STP oil treatment and see if it changes the noise.

Piston slap is very common in 4.0L's. For most, it is hard to tell the difference.

If the STP doesn't make much difference, I would drop the pan and plastigage the rods and mains. That is the definitive test for rod and main knock. It is a bit messy but not hard on even a mildly lifted vehicle.
I dont understand why my 300k xj has no noise and this thing does. But I will make a mental note to get 10w next time. Typically I use that in the summer anyways. Seems like a bad idea in the winter when its -14 but I wont keep this engine for too many more years if it's going to have so many issues regardless

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I dont understand why my 300k xj has no noise and this thing does. But I will make a mental note to get 10w next time. Typically I use that in the summer anyways. Seems like a bad idea in the winter when its -14 but I wont keep this engine for too many more years if it's going to have so many issues regardless

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The recommended weight oil when it was built was probably 10w30
I run 15w40 Rotella in Colorado and we get to -30F.
 
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