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Replace Rod Bearings, or Rebuild

Xj_idget

NAXJA Forum User
Location
Cincinnati
I have 2000 XJ Sport/4.0/AW4 w/ 195K that has slowly developed what is a pretty clear, light knocking sound(s), more pronounced when hot and at idle. Easy to hear when standing in front of the vehicle, not really noticeable inside the vehicle at idle or when driving.

Oil pressure is very low at idle (10psi'ish on factory gauge), BUT it has been that way since I've owned it at around 130K. Oil pressure generally trends with rpm, roughly 20psi at 2000rpm, 30psi at 3000rpm.

I'm assuming rod bearings are the most likely culprit. Does that seem like a good guess to you?

If it turns out to be rod bearings, is it worth having the bearings and oil pump replaced and continue running the motor for another 50K or so, or should I look into a replacement motor or rebuild?

Thanks for any insights. ~ Steve
 
With a 2000 model and low oil pressure I'd be taking a look at your cylinder head which is an 0331 head. They are known for cracking in between cylinders 3 and 4 because of a casting flaw.

How does your oil look? Normal, or like a milkshake? Any gunk on the fill cap?
Are you losing coolant from the reservoir with no external leaks?
If I were you I'd take off the valve cover and check for a crack on the top of the head before even thinking about doing the rod bearings.
Might also be worth getting a used oil analysis done to see if there's any water present in the oil.

I'm no engine expert this is stuff I've learned from having an 01 XJ with the same head. Keep an eye on that damn thing if you haven't been already! I'd hate to see you put a bunch of money into the motor only to have it die because you missed the real problem.

Hope this helps and keep us posted, most of the people here are great help!

Oh and one more thing, if you do find that the head is cracked, you should probably stop driving the vehicle until you get it fixed and full of fresh, water-free oil.
 
4.0 engines are known for knocking and ticking. I have found that taking the engine up to 3700 rpm for 45 seconds while hot in park, fixes many of the noises, usually a stuck lifter unsticks.
 
Rod bearings aren't usually heard at idle. A blip to 2500 will get a quick rattle just as the rpm's start to drop, or you may hear it on cold start if the rpm flares.

Piston slap is probably the most commonly heard noise at idle followed by lifters. Neither one usually means impending doom.. just one of those 4.0 things.
 
Likely what is being heard is piston slap, common on older vertical cylinder engines. The low oil pressure is not of great concern, as is normal for an engine with almost 200k on the clock.

If you decide to rebuild, pull the block, have it reconditioned and outfitted with oversized pistons to match the new bore. A crank kit may be the most cost effective way to deal with both the crank and bearings.
 
Thanks for all the input. I'll check the oil pronto. I have not been losing coolant, that I know for sure, and I have been using synthetic oil at ~5K intervals since I bought it at around 130K miles.
 
What exactly is piston slap, and what causes it?
 
Adding some MMO, Marvel Mystery Oil ,and running the RPMs up near 4000 RPM has worked for me so far, on two of them.
 
replacing the conrod bearings will bring the oil pressure up to normal. Did this on my 90 which is back to about 15 lb by the gauge. 12 is spec. at idle.

Piston slap is when the skirts of the piston slap against the side of the cylinder. It is a result of wear and overall piston design. In the case of the 4.0, it is more unusual to not hear it!
 
replacing the conrod bearings will bring the oil pressure up to normal. Did this on my 90 which is back to about 15 lb by the gauge. 12 is spec. at idle.

Piston slap is when the skirts of the piston slap against the side of the cylinder. It is a result of wear and overall piston design. In the case of the 4.0, it is more unusual to not hear it!

The noise many of us have had, goes away for a long time, if we run the engines at above 3700 RPM for a few minutes, seasonal returns in some cases (cold-hot-cold, cold winter starts seem to trigger it, which is why I think it ours is sticky lifter, paraffin wax...., from using Dyno oils), so I doubt that is piston slap, as I would expect from your description of piston slap, that it would never just go away from running the engine hard?
 
What exactly is piston slap, and what causes it?
It's the pistons slapping around in the bore because the piston skirts have too much tolerance between them and the cylinder wall. As the pistons warm up and expand the slap is reduced.What caused it on the 4.0s is worn out tooling and poor piston design along with poor quality control.
 
What exactly is piston slap, and what causes it?

1st, ALL older engines get piston slap, not just vertical orientated engines. And it is NOT just a 4.0L problem.

Imagine a piston. Modern pistons are not the old slugs of old, but are usually only a complete cylinder shape until about 1/2 way down, then the sides that are perpendicular to the motion of the rod are shaved off to save weight. The sides that remain below the wrist pin are called "Skirts". The reason the skirts remain is to counter the tendancy of the piston to rotate in the bore. The center of rotation is the wrist pin, which connects the piston to the rod.

As the engine gets older, the clearances between the cylinder wall and the piston increase. This clearance allows the piston to wobble in the bore on the top and bottom of the stroke. This is piston slap. It is literally the piston skirts slapping against the cylinder walls.

Piston slap is usually noticeable when cold, and decreases as the engine warms to operating temperature. The reason is simple: As the engine warms, the aluminum in the piston expands faster than the steel the cylinder wall is made of, and the clearances lessen.

Piston slap is less noticeable in a V or flat configuration engine, but it is still there.

HTH
 
It's the pistons slapping around in the bore because the piston skirts have too much tolerance between them and the cylinder wall. As the pistons warm up and expand the slap is reduced.What caused it on the 4.0s is worn out tooling and poor piston design along with poor quality control.

That confirms that mine are not piston slap. Mine would come and go hot or cold, in a second, there one second gone the next, which has to be something sticking and unsticking, IMHO. Thanks, now I know what to look for, call it, etc.
 
I got rid of it on both of mine.

I did too, but every once it while it tries to come back, especially on a cold, cold morning start up, and especially on my oldest one, which is nearing 300,000 miles.
 
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