Hello, I'm a new member who recently purchased a 1998 xj. It was extremely inexpensive, but that cost came with some caveats. The engine makes a loud consistent knocking sound that doesn't change with operating temp, increasing in frequency and intensity with rpm. It isn't a double knock or whine like rod knock or a spun bearing, but more like loud click. The vehicle has 260k miles, so I'm assuming it is piston slap. I'll post a video link when I get the chance. Also the battery doesn't charge, causing the vehicle to require a jump to start. Voltage at idle is around 9volts, but surges to 19 when driving. I'm certain the alternator is the culprit here. Also AC doesn't work, the previous owner said they think it was the AC clutch but I didn't investigate that issue yet as the previously mentioned issues take priority. Anyway, I'm new to the jeep world and am not experienced with the 4.0, my area of experience is gm v8s. Is the piston slapping a common issue with this model year? Assuming the bore isn't scored, I'd like to take the piston from a junkyard 4.0 (Assuming it is piston slap). Can I take the piston from any 4.0? Xj, zj, wj, etc? Does year matter? I appreciate any and all input, thank you for your help!

 

I'd check the flexplate bolts first.......https://www.cherokeeforum.com/f51/ho...ocking-125547/

 

My vehicle has the ax15 5 speed so it shouldn't be prone to this issue

 

Piston slap goes pretty much away when warmed up. Sounds more like something else. Besides, you wouldn't just do one piston, you'd do them all. No, cant use a junkyard piston, its got the same issue. No, not all 6 cyls have the same piston. They are very different.
You said your alternator and your AC aren't working, try fixing them first.

 

Thank you. What about a broken piston skirt? The noise is only affected by engine rpm, increasing as I accelerate. Also when I decelerate it sounds horrible, like a tumbling sound before it stabilizes again.

 

First off, stop driving it.

How is the oil pressure and engine temperature?

 

it's parked right now, I'm not touching it until I figure this issue out. Oil pressure is normal and the engine sits around 210, doesn't overheat.

 

I'll try and get a video up when I have the chance, I know it would help a lot

 

I wouldnt think it is the timing chain. But dont rule it out.

You could remove the valve cover and see if anything is broke. Easy to do and would eliminate that possibility.

 

That will be my first course of action, I'll post pics when I get the cover off, but it probably won't be until Wednesday. Are there any other common issues on these 4.0 engines? People always call them bullet proof so I'm assuming there can't be too many common failures? Again I'm a jeep newbie, been a GM guy forever and that's what I know lol

 

It's possible there is a bad lifter causing a tick and my guess is that is probably the issue. Timing chain noise isn't typically a tick but rather a deep almost grinding noise that you can distinguish in the front of the engine. Doesn't sound to me like piston slap - unless the bore is terribly scored which by that point your bearings are suffering and your oil pressure won't be that wonderful. Also try to isolate the noise and get an idea of where it is coming from.

 

https://www.cherokeeforum.com/f2/eng...shrods-213335/
I looked up some related threads involving the valvetrain like 1/1200 cherokee suggested and stumped across the thread linked above. In his video with the valve cover off, the sound of mine is slightly different (probably because my sound is resonating in the cover) but the rate at which the tick occurs at idle exactly the same, 8 ticks in 1.25 seconds. Maybe my problem is a bent pushrod like in this thread? Also here is a picture of my oil pressure.

 

Easy to pull the VC and look. You want to see anything out of the ordinary. Could be as you suggest a broken skirt. Could be rod bearing. Could be lots of stuff time to start disassembling. And no you cant take a piston from a junk yard and put it in.

 

Yes you can use used pistons, but you're going to need to tool up and look for specific pistons. That could entail going through a few used blocks to get the right letters or else the new old ones will continue to slap.

Find out what the issue is, but if it's sloppy pistons it's probably going to be easier and cheaper to just get a good used engine.

 

If you're going to do ANYTHING with pistons, right off the bat you're going to need these things:




so there's your new used engine already.