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Piston Slap Question

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stephenspann27

NAXJA Forum User
Location
College Station, Tx
I recently installed a new crank, rod, and main bearings, timing set, oil pump on my '87 XJ. About 200 miles before doing this I replaced the head, and lifters. The motor is basically new except for the pistons, rings, cam, and cam bearings. I know it sounds stupid but I actually replaced more than I could afford as it was.. and I couldn't afford pistons.. and having them put on the rods..

Anyways.. I still have some thumping in the motor.. I can hear it echo off walls.. and other cars.. my oil pressure is stellar.. and it runs really well.. it occasionally doesn't idle great. However, I'm getting 20mpg hwy. So its running pretty well.

I can only assume that the noise I'm hearing is piston slap.. The pistons have 260k miles on them so I don't blame them. The jeep never smokes, and doesn't use oil.. that's another reason why i left well enough alone... or what I thought was well enough.

My question is.. if the noise is pretty moderate.. any guesses on how long I can go before it gets much louder.. or a skirt breaks off?
 
I don't consider piston slap in the 4.0 a terminal illness.
 
If it's piston slap, you're living on borrowed time. Expect trouble within the next two hundred thousand miles or so....

Seriously, if it's not too bad, and if you are able to determine that it really is piston slap and not something else, I would not worry. It's common on a 4.0. My 95 has been happily slapping away for the last 150 thousand miles I've had it (it's now at 263K). However, make sure that you're not hearing something else masquerading as piston slap, or hiding behind it, such as a dying water pump, cracked manifold, etc.
 
is the noise you hear at idle or under load. at idle its ok, under load is not good.

when my 89 with 243k miles gets a little noisy, its a reminder to change the oil.
 
stephenspann27 said:
Has anyone's slap gotten rapidly louder.. or actually had a piston skirt break?
Click to expand...

I have had collapsed pistons on a Ford 6, resulting in a dropped skirt after a long period of slapping, but that's a bit different. All the skirts on that one were cracked, probably from overheating before I got it.

My two XJ's each with well over 200K miles, have not gotten any worse over time. Oddly enough, my 87 did not have audible piston slap. It was quieter than the later ones.

One thing that does occur to me is to check the timing chain. Perhaps get a stethoscope on it, to make sure it's not rattling.
 
I took your advice and probed around with a stethoscope and the noise is actually the most pronounced with I probe the side of the distributer.. I wouldn't think that the gear being worn would cause this kind of tapping noise.. maybe its actually a lifter and the noise is traveling through the cam.. and to the distributer? The lifters are new.. as is the timing set.. I'll go stick the probe on the timing cover real quick.
 
No noise at the timing cover. However if I probe along the oil pan the noise is the loudest in the middle.. I could also hear the noise the loudest at the #5 injector. It must be that its the #5 piston that's slapping? That or the lifter has issues.. It does sound a bit like a rod.. but all that crap is new.. and I checked it all with plasti gauge the clearance was perfect.
 
Piston slap should change when you cancel spark to the affected cylinder. I say should, because it's not a guarantee.

This is one of those things that would have been very obvious had you checked when it was all apart.
 
ask this ,does it get lighter in sound after it warms up? if it does, and does'nt get loud under load afterward then ya know it's piston slap and as long as ya don't push her hard then ya can probably nurse it until ya can bore and replace the pistons. these old 6's can take a lot of abuse.if the sound is consistant no matter what the engine temp, then it's a rod or a valve train problem.plus rod slack kind of sounds like a bag of sea shells being shaken under load so to speak
 
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I've heard rod knock from this motor for sure.. before I put a new crank it it.. two of the journals were destroyed and I had no oil pressure at Idle. The noise does go away under a load but i have to give it quite a bit of gas. For instance if I hold the brake, and start giving it gas, the first couple seconds the noise gets louder with RPM, until the torque converter really locks up, and I bear down on the gas, then it goes away. I spent about two hours last night reading about piston slap, and out of all the stories I read, I couldn't find one that ended in catastrophic failure, I'm sure its possible, but doesn't appear to be likely.
 
well I just installed the motor.. and tightened the crap out of the flex plate bolts. It's not especially loud at start up.. I really only notice it after about a minute of running. Its not louder if take the oil cap off.. either.. I"m kinda stumped. I tried pulling some spark plug wires to listen for a change in the noise but managed to shock myself.. even with a welding glove on.. It may be something that I just have to learn to live with until it makes itself more obvious...
 
Can you identify whether the noise is a double knock, or more random and rattly?

I have read that if you squirt a couple of ounces of oil into the suspect cylinder, then turn it over a few times without starting to get the oil distributed, then start it, slap should temporarily quiet down. A rod or wrist pin will not.

this is a stumper, isn't it?
 
I appears maybe I'm being to anal about my engine noise. I had no idea what a 4.0 was supposed to sound like. Today i cranked up a co-worker's '00 XJ with 130k on it, and it has the same noise. With the hood closed they really sound almost identical. However, when you raise his hood and put your ear to the motor, his isn't as loud as mine. But it made me feel a lot better to hear that.. I need to just chill out and drive it.
 
I noticed that the brand and weight of oil made a difference in my piston slap noise.
If the noise really bothers you, and you don't mind using snake oil, the product called "restore" may reduce your noise for a while... it did on mine. A V-8 size can made my noise less offensive for about 2500 miles. But that is not long enough to justify dumping 6 qts of oil, and I would not put 2 cans in succession before a change.
It's up to you.
I have 236000 miles and it still hauls tail, and does not use oil.
But boy does it sound like a diesel when cold at idle. When it is warm, the noise is very much reduced, and under load is goes away completley (can't hear it).
Just drive and enjoy.
 
another thing you can try if you think it's a lifter is to get some marvel oil just before an oil change, put a quart ofit in there, rap it out a little and see if that doesn't change things, that marvel oil will loosen stuck lifters alot of times, but I still think it's probably a piston,also run a compression check onthe cylinders, if the suspect cylinder has the lowest reading then there ya go.but do it when it's cold so the piston is not swelled giving you a false reading.
 
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