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  #1  
Old October 26th, 2019, 22:42
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alopeks alopeks is offline
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Piston Slap?

Sorry in advance for the long post. I'll start with the critical stats:

99 XJ
4.0, AW4, NP242 - basically bone stock.
221,000+ miles, and it's not just my daily, it's my only.

Engine-wise, in the last 3 years (since I bought it) I have replaced:
Radiator
Vapor Canister Purge Solenoid
Water Pump
Head Gasket (done by a professional shop)
Heater Core
A/C system (practically all of it)
Oil Pressure Sensor
Muffler (Thrush Turbo, stock size)
and as of last week, all 12 pushrods

So, about 3 weeks ago I noticed a sudden disturbingly loud engine noise. As I started the Jeep, it made a harsh scraping-like sound, that was consistent with engine RPMs. I had been driving at that point for well over an hour, with a couple brief stops (no more than 5 min each) and most of that time at highway speeds. The engine did NOT make the noise until I started it up the 3rd time, and yes, I would have noticed.

The sound at first was VERY similar to what I remember hearing years ago when my first XJ decided it was tired of having a straight 6, and wanted to see how well a straight 5 would function. Sadly, the answer is "not well." Thankfully, this time I did not have the same end result, as my first XJ was making the sound of a piston blowing a hole in itself right through the top, which resulted in a very brief trip and a very long wait for a tow truck. Instead, I have been able to keep driving, everyday, for almost 3 weeks. The sound never goes away, but usually gets a little quieter as the engine warms up. It is MOST disturbing (and LOUD) when I first start the engine, especially in the morning.

Oil pressure, temp, voltage were and still are all normal. I listened carefully and timed the sound to determine it was happening once every 2 revolutions, and at first thought it was valve related. Doing some research lead to me to believe it might be any number of things, including flexplate damage, a cracked exhaust manifold, a bent pushrod, cracked or otherwise damaged rocker arm, collapsed lifter, or even an issue with a valve. I also discovered the concept of the dreaded piston slap, which frankly sounds like a painful and cruel way for someone's older brother to pick on them.

Anyway, in trying to eliminate all of the easiest possible problems first, I confirmed that none of the bolts in the flexplate were loose, oil was being pumped consistently to all the rocker arms, and none of the pushrods were bent (though most of them has a good bit of sludge or carbon residue, thoroughly baked onto them. I also straightened out the rocker bridges (3 were somewhat tweaked) and of course replaced the valve cover gasket while I was in there. I was disappointed (but not very surprised) when this didn't solve the problem.

So, what do I try next? If it is piston slap, is there a reason the sound is only heard half as often as the RPMs? Are there (easy-ish) ways of confirming an engine is having piston slap? Your help is VERY MUCH appreciated.

-alopeks
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Old October 27th, 2019, 09:02
lawsoncl lawsoncl is offline
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Re: Piston Slap?

To make sure it's in the engine, you can try pulling the serpentine belt to see if the noise goes away. Also have a good look at the harmonic balancer and make sure it's not rubbing.
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Old October 29th, 2019, 18:43
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alopeks alopeks is offline
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Re: Piston Slap?

Okay, pulled the belt, still there.

Pulled spark plug wires (one at a time), still there. Also, shocked myself a little.

Harmonic balancer has no visible play, and isn't rubbing anything.

The otherday when I had the valve cover off, I ran the engine for a few minutes, and the sound was timed perfectly with the #5 cylinder exhaust valve. Maybe a cracked manifold? Need to borrow an inspection camera...

Also, just to confirm, a spun bearing would likely NOT only make noise every other revolution, correct?

Thanks in advance, again!

-alopeks
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Old October 29th, 2019, 22:58
00rottweiler00 00rottweiler00 is offline
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Re: Piston Slap?

Quote:
Originally Posted by alopeks View Post
Okay, pulled the belt, still there.

Pulled spark plug wires (one at a time), still there. Also, shocked myself a little.

Harmonic balancer has no visible play, and isn't rubbing anything.

The otherday when I had the valve cover off, I ran the engine for a few minutes, and the sound was timed perfectly with the #5 cylinder exhaust valve. Maybe a cracked manifold? Need to borrow an inspection camera...

Also, just to confirm, a spun bearing would likely NOT only make noise every other revolution, correct?

Thanks in advance, again!

-alopeks
Let me preface this with the fact that I'm not an expert in any way, but instead of an inspection camera or mirror, I'd bet that you can hook up a fog machine/shopvac in "blow" mode with flour or something to the tailpipe and find any exhaust leaks, like a cracked exhaust manifold. Again, not a pro, but it's something you could try if ya wanted to try it.

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Old October 30th, 2019, 00:13
lawsoncl lawsoncl is offline
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Re: Piston Slap?

Quote:
Originally Posted by 00rottweiler00 View Post
Let me preface this with the fact that I'm not an expert in any way, but instead of an inspection camera or mirror, I'd bet that you can hook up a fog machine/shopvac in "blow" mode with flour or something to the tailpipe and find any exhaust leaks, like a cracked exhaust manifold. Again, not a pro, but it's something you could try if ya wanted to try it.

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Please don't blow flour up your tailpipe.
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Old October 30th, 2019, 04:24
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alopeks alopeks is offline
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Re: Piston Slap?

Yeah, I'm not gonna blow flour up the tailpipe...

It would gum up the muffler bearings.

-alopeks
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Old October 30th, 2019, 08:30
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tjmotter tjmotter is offline
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Re: Piston Slap?

I would perform a leak-down-test to see if I was getting any inconsistent readings between cylinders. The advantage here is that you can tell a LOT about the health of the bottom end. For example, once you identify a low cylinder:


1) if you hear air escaping through the exhaust, you have a valve problem
2) if you hear air escaping out of the oil level tube, you have a ring or piston problem
3) if you hear air escaping out of the throttle body, you have a valve problem
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Old October 30th, 2019, 22:32
00rottweiler00 00rottweiler00 is offline
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Piston Slap?

If it's not air escaping from somewhere, I've heard that a similar noise can be made from a bad lifter or extremely rarely the torque converter bolts will back out, but I dont think itd line up with the 5th cylinder exhaust valve if the latter was the case.

For my flour comment, sorry, I dont think I was sotally tober at that moment, but the fog machine I think still has merit as a cheap thing to try

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Old November 3rd, 2019, 09:56
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alopeks alopeks is offline
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Re: Piston Slap?

No worries about the flour comment; I assumed you were joking.

Afraid it was something serious, though I don't know yet what it was. Engine clattered and died suddenly last night, and I haven't had a chance to investigate yet. Hoping my mechanic doesn't have a new boat soon...

-alopeks
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