Piston Slap

[AZJeff]

My MJ has a HO engine from a 2000XJ in it.   About 5K miles ago, I discovered I had a crack in the famous "0331" cylinder head from this vintage of engine that was causing coolant to get into the oil.   I discovered this when changing the oil, and I noticed it was not the pure black that old oil exhibits.  It was more gray, and its viscosity was "funny".   At the time of this discovery, the engine was making no weird noises, nor showing oil pressure or temperature issues.    I was fortunate in that I found it before it destroyed the entire engine, so I did the following:

 

--New cylinder head from Clearwater Cylinder Head in FLA.   Also included new valves and springs.

--New lifters, rocker arms, and pushrods  (old ones looked fine, but I was going full bore)

--New crankshaft bearings for both rods and mains  (Clearances with old bearings were within spec., but I wasn't taking any chances.)

--New timing chain and sprockets (I was in far enough that this was cheap)

--New high flow Melling oil pump

 

With all of this done, the engine runs well, and generates acceptable oil pressures per factory specifications.  In addition,  compression numbers are pretty close to fantastic, running (literally) between 135-140 on all six cylinders)

 

HOWEVER....I have a noise that has been around pretty much ever since all this work was done.  It has not gotten worse during this time, and professional laboratory oil analysis does not show any unusual metallic levels indicating high wear.    I cannot seem to capture a good clean audio of the sound, so this is a good video of a very similar sound to mine (but mine is not a loud as this.)

 

Piston Slap Sound

 

Here are the symptoms:

1.  The sound is non-existent upon startup, and reaches its most noticeable level when the engine has been operating for anywhere between 5 and 20 minutes.   After that time span, the noise diminishes to almost imperceptible.  

2.  It is worse in cold weather.  In ambient temperatures below 60 degrees, the amount of noise is a bit more, as is the duration of the “noisy time period”.    

3.  It also will make the noise on a restart of a warm engine if it has sat for more than 5-10 minutes, but the noise will disappear within 5 minutes after warming up on the restart.

 

I have checked the flywheel for cracks and/or loose bolts, with no issues seen.  This leads me to believe it's piston slap, in spite of the fact that when i pulled the head, the cylinders looked just fine.

 

So here are my choices to fix the issue:

1. ignore it, as the engine can last 10’s of thousands of miles with the sound, assuming it is piston slap.  This is what Hagerty suggests, based on my internet searches.  
2. pull the engine and do a total rebuild of the short block.  I have an almost brand new head on it, so I don't need that part redone, but the machine shop work would be some serious coin.  
3. do a  cylinder rebuild in the chassis.  Pull existing pistons, hone cylinders, install new pistons and rings.  This, of course assumes that the cylinders don't look terrible when I pull the head, but I think they are OK, because when I installed the new head 4k miles ago, they looked good.   This approach is cheaper the #2 above, but has it's own risks, of course.

 

So for this guys who are deep into engine sounds, causes, and cures, what say you?

[gogmorgo]

Might not be the worst idea to pop the valve cover off and check nothing’s loose in there. I don’t know if I’d go to the effort of dropping the oil pan to double check everything in there, but usually if there’s a concern with something I’ve just had hands on, my first step is to double check my work. Stethoscope, at least an improvised one, might help you narrow down the source of the noise a bit before going digging.  
 

I’d also be curious if fresh or different weight oil might make a difference. But also nearly every 4.0 I’ve owned has made some level of untoward noise and none of them have exploded. I’d be inclined to ignore it if there’s no glitter in the oil. 

[AZJeff]

1 hour ago, gogmorgo said:

Might not be the worst idea to pop the valve cover off and check nothing’s loose in there. I don’t know if I’d go to the effort of dropping the oil pan to double check everything in there, but usually if there’s a concern with something I’ve just had hands on, my first step is to double check my work. Stethoscope, at least an improvised one, might help you narrow down the source of the noise a bit before going digging.  
 

I’d also be curious if fresh or different weight oil might make a difference. But also nearly every 4.0 I’ve owned has made some level of untoward noise and none of them have exploded. I’d be inclined to ignore it if there’s no glitter in the oil. 

The first thing I did was to whip out my stethoscope, and I found the noise is coming from the rear of the engine.  It's not up high, and it's not really down at the bottom either.  (Using the scope on the side with the manifolds is pretty much impossible, so I did my listening on the camshaft side.)  It's most noticeable a couple of inches above the joint between the block and the oil pan.

 

I pulled the valve cover recently, and actually ran the engine with the cover off.  The noise is definitely not in the valve train.  I run Rotella with some ZDDP additive, and my last oil analysis by Blackstone Labs said there was no unusual levels of metal in the oil.

[gogmorgo]

Is everything between engine and trans bolted together properly?

[75sv1]

My thought is a cracked piston skirt. Do you notice the noise on the driver's side? I have a 2000 XJ with 280K on it. It ran about 2-3 years with the 'piston slap'. I could only hear it from the LH side. Close to what you describe. My AC compressor is bad. Rebuilt unit, I think is locked up. Anyways, this last summer it was hot.  So, I turned on the AC. Wasn't blowing cold, so I turned it off. I was picking up my LS Block from the machine shop. I noticed a clacking. I made it home. I took off the oil pan, hoping for rod bearings. All were tight. I noticed some aluminum pieces in the oil pan. It was piston #6. As posted above, I did check the trans bolt first. I too use Rotella T6. I do not add Zinc. I has about 1200 ppm ZDP.  I thought about putting in a used piston or a new one in #6. I finished up my mini-stroker and installed it. 

        My head was actually OK. Don't know if I was replaced before or original. Its not a TUPY. The bores had a lip at the bottom of the stroke. Maybe a block for a full stroker. 

    

[cruiser54]

Have you put a wrench on the flexplate bolts? 

Did you use LocTite on them when you put the engine in? 

 

And, as gogmorgo suggests, make sure all the engine to trans bolts are there and tight. 

 

What oil and filter are you using? Your oil analysis seems fine, right? 

[AZJeff]

The flex plate bolts were tight.  That was one of the first things I checked.

 

There are no metal chunks in the pan.  I have had it off twice and never saw anything in the pan.  The oil analysis from Blackstone Labs shows no abnormal amounts of aluminum, iron, or copper.  I run Rotella, with ZDDP, and a Wix filter.

 

If anything is broken/cracked, it has not made it into the pan, nor has it showed up as particles in the oil.

 

I will check the bell housing to engine bolts as suggested.

[Chad R]

My 2001 wrangler started making a tic noise 2 years ago but would go away when up to operating temp.  This spring when I got it out the noise was louder and did not go away when up to temp.  Then it got super bad knocking noise.  Took head off found nothing dropped oil pan found two chunks of number 5 piston.    It only had 93k on it

Edited January 12 by Chad R
Add picture

[jpnjake]

hi Jeff,

 did you break in the cam again after you put new lifters in?

(1800 to 3000 RPM's for 20 minutes+ when first starting up/don't let it idle for at least 20 minutes)

 

Piston slap is usually the worst when the engine is cold, and the piston clearances are the greatest.

 

Cam wear/cam damage doesn't usually show up as a noise as much as it does lack of lift,

 so I'm not sure when a cam noise would be the loudest.

 

But it wouldn't surprise me if a noisy cam was quiet when the oil was cold, and it's thickest/highest pressure.

 

I'd pull the valve cover again and see how much lift your getting out of each lobe.

Especially the rear cylinder that you said was the noisiest.

 

Good luck.

 

[AZJeff]

8 hours ago, jpnjake said:

hi Jeff,

 did you break in the cam again after you put new lifters in?

(1800 to 3000 RPM's for 20 minutes+ when first starting up/don't let it idle for at least 20 minutes)

 

Piston slap is usually the worst when the engine is cold, and the piston clearances are the greatest.

 

Cam wear/cam damage doesn't usually show up as a noise as much as it does lack of lift,

 so I'm not sure when a cam noise would be the loudest.

 

But it wouldn't surprise me if a noisy cam was quiet when the oil was cold, and it's thickest/highest pressure.

 

I'd pull the valve cover again and see how much lift your getting out of each lobe.

Especially the rear cylinder that you said was the noisiest.

 

Good luck.

 

I measured the lift on the cam with a dial indicator when I was determining what engine damage may have happened with the cracked head.  All lobes measured within spec. (I had the dial indicator on the pushrod side of the rocker arm.)

 

I ran the engine at 2000RPM for 20 mins after I put in all the new lifters.

 

I am going to try something else.   I have a borescope, and I am going to try to poke it down into the spark plug holes and look at the cylinder walls on the back couple of cylinders when the pistons are at BDC, and see if there is major scoring from the piston skirts rubbing.

[AZJeff]

So I did two things after the previous post:

 

1.   I did another compression test.  Below is a comparison of numbers after the installation of the new cylinder head (old psi), and now after 5000 miles (new psi) on the new cylinder head.

 

Cyl.      Old PSI      New PSI

1              144             140

2              150             140

3              152             149

4              155             146

5              151              144

6              157             138

 

Then I also used my super-high-quality Chinese borescope to look inside cylinders #1 and #6, to see what it looks like.   The attached PDF gives you an idea of what I saw.

 

So I am getting frustrated with the cold start "ticking" I am hearing.  If I KNEW that it was not harmful, I could/would ignore it, but I am fearful of some calamity down the road.

 

Keep in mind that this MJ is not a beater vehicle.  It's pretty primo, and could/would command a premium price on the market if it was for sale (which it is NOT.) A noise the inhibits its durability is something I want to address.

 

270384662_BorescopeImages.pdf

[89 MJ]

A 19 psi drop in cylinder 6 seems odd 

[cruiser54]

Jeff, have you tried unplugging the injectors one by one to see if the noise is load related?

[AZJeff]

On 1/21/2024 at 7:16 PM, cruiser54 said:

Jeff, have you tried unplugging the injectors one by one to see if the noise is load related?

That was a brilliant thought, Cruiser.   I figured it would be easy to spot the cylinder(s) with bad slap by turning off the added pressure of combustion of fuel.  With combustion not going on in that cylinder, the slap sound would be diminished.

 

So I started the engine, and let it warm up into the "most noise" phase (about 5-10 minutes after startup.)  I then went down the line, pulling one injector connector at a time. (Am I ever glad i replaced all the injector connectors 2 years ago, so they were easy to snap on and off.)

 

Guess what?  Other than running rough when each individual cylinder was deactivated, the sound did NOT CHANGE when any given cylinder was shut down.  

 

So now I am mystified.   The sound never changed when any cylinder was diactivated.    So I wonder what is the source of the noise.   (I have not checked bell housing to block bolts yet, BTW.)

[cruiser54]

Yeah. Check those bolts.

 

You have checked the harmonic balancer and it's not come apart and smacking the timing cover?