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Who can guess what's wrong with this engine?

 
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The Jeep has been sounding a bit sad lately. It has gotten louder in the past couple days, so today I finally started tearing it down. Who can guess what the problem is from this video?



 
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Bad bearing or valve?
 
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I want to say a loose spark plug, or a missing spark plug
 
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Gonna vote with a failed exhaust manifold (header, in US terminology?) gasket.  Had a very similar sound on an engine years ago, and thought it was about to self-destruct :D
 
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Exhaust leak.     The motor is not shaking.
A bad valve or missing plug / wire and that motor would be shaking.
 
Jordan Holland
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Good guesses so far. It looks like exhaust leak at the manifold is in the lead. I will rule out spark plug wires since this is an AMC 242 straight six from '02, and thus does not use spark plug wires.

I have been scratching my head over it for a few weeks. I considered all mentioned so far, and some more, trying to imagine the mechanical mechanism making the sound. I would think one for a while, then another, then another... It sounded a bit different going down the road. The pitch seemed to be higher and sharper. It seemed to be loudest approaching 1500rpm, and higher it seemed to be quieter, but that may just be due to how much wind noise there is in a Jeep making it difficult to hear. If I floored the throttle under heavy load at high gear, it got quieter. If I eased off the throttle it virtually disappeared. It definitely made more noise under load in normal driving conditions.

Let's get a few more guesses before revealing more. I expect Pearl might like to chime in, and maybe Joylynn Hardesty has been taught something by the infamous Saab that may yield an enlightened guess, lol!
 
thomas rubino
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Hey Jordan;
No plug wires mean's a separate coil on each plug...
The thing is the motor is not shaking.
 
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Howdy,

Valves, stuck hydraulic lifter.
 
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Exhaust leak or valve train would be my guess. Hopefully just needs a new manifold gasket.

A cheap and easy way to test for leaks is to hook up a shop vac exhaust port into the tailpipe. This will pressurize the system, then use a spray bottle with some soapy water and look for bubbles.
This guy has a great video demonstrating this method and has some tips on how to fix leaks:




If this is coming from the valve train, maybe try some Sea foam or Berryman B-12 and see if that frees it up.  




 
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Q: Did the sound build up slowly or sudden onset?
 
Jordan Holland
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Pearl Sutton wrote:Q: Did the sound build up slowly or sudden onset?



Slowly. These flat tappet engines are known for their clattering. Lots of people can't stand it, but I find it kinda endearing. It's like it's a gas engine that thinks it's a diesel, lol. I thought the issue could have been from the same source just getting worse, but was wrong. Over the past few weeks it has gotten louder. A few weeks ago, in fifth gear it could not be heard at highway speeds, last week it it was quite noticable at highway speeds.
 
thomas rubino
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So Jordan;
Hows that engine run at speed?
Loss of power maybe?
Any hiccups or stuttering?
As a 2002 it has the OBD2 check engine system.
Is the check engine light on?
Do you know what codes it is sending?
 
Jordan Holland
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thomas rubino wrote:So Jordan;
Hows that engine run at speed?
Loss of power maybe?
Any hiccups or stuttering?
As a 2002 it has the OBD2 check engine system.
Is the check engine light on?
Do you know what codes it is sending?



Excellent thinking! You are doing well to pay attention to the engine and year. Some more weirdness thrown my way: the engine runs as smoothly as ever with no noticable loss in power. The only difference is that fuel mileage has gone up considerably. It usually has gone about 200 miles before the fuel warning comes on. Now it has gone about 230 miles on the past two tanks since the sound has started.

The Check Engine light has been on intermittently. The only code given is P0158, which indicates low oxygen in O2 sensor 2, in bank 2. This code and a few others were thrown some months ago as well when the engine started idling quite horribly. Installing a new O2 sensor made things right.


At this point, I will reveal that since the exhaust leak was the easiest to check, it was the first I checked, and it was not the issue. While a straight six, they learned their lesson over the years and instead of a big, long cast manifold, they gave this two short exhaust manifolds. I don't think they are a common problem.
 
thomas rubino
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OK;
How about an EGR valve or pipe?
 
Jordan Holland
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thomas rubino wrote:OK;
How about an EGR valve or pipe?



That's one that never occurred to me. Then I tried to think where the EGR system is on the engine...it doesn't have one, lol! Interestingly, I just learned that the cam profile is such that it leaves a little exhaust gas in the cylinders, performing basically the same function.
 
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...maybe Joylynn Hardesty has been taught something by the infamous Saab that may yield an enlightened guess, lol!



Ugh. Just picture me peering under the hood, nodding wisely with each suggestion.

It does sound to me like it's spitting air somewhere. Are all the exhaust hoses, connections in good shape?
 
Jordan Holland
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Joylynn Hardesty wrote:
It does sound to me like it's spitting air somewhere. Are all the exhaust hoses, connections in good shape?



I believe they were all good, but can't be 100% positive. If there was some leak, they would likely not have anything to do with the knocking heard in the video. That's part of the joy of figuring out mechanical issues. There can often be multiple problems and red herrings that convolute an already convoluted diagnostic.
 
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Geez, I have to go with the Valve as suggested earlier.
Low on oil or stuck / slow lifter.
Have you tried with a stethoscope, IE long bit of narrow timber placed between ear and different spots on the engine?
You will locate the noise and have it separated from most other noises.
 
Jordan Holland
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John C Daley wrote:Geez, I have to go with the Valve as suggested earlier.
Low on oil or stuck / slow lifter.
Have you tried with a stethoscope, IE long bit of narrow timber placed between ear and different spots on the engine?
You will locate the noise and have it separated from most other noises.



I tried an actual stethoscope, but no matter where I placed the probe it sounded the same to me. Maybe I should have tried a piece of wood or long screwdriver in retrospect. It could have given a different sound that may have been easier to diagnose. But I do have pretty good hearing. I can hear the fuel injectors running. The knocking sounded louder from the passenger side (from where I took the video) and sounded like it was coming from the rear half of the engine. I guessed around cylinder five.
 
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My guess is a cracked exhaust manifold. I keep one on the shelf, it's a common component that is prone to failure on that vintage jeep.
 
Jordan Holland
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Robert Ray wrote:My guess is a cracked exhaust manifold. I keep one on the shelf, it's a common component that is prone to failure on that vintage jeep.



Nope, it wasn't an exhaust leak.
 
thomas rubino
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I would suggest removing the valve cover next and taking a look.
Something may be obvious or you may need to start the motor to see/hear the problem.
Is this an overhead cam motor or an overhead valve?
Performance should be affected if the valve train is wonky.
I have seen overhead cams be broken and still run...
Better yet just run it over and we will put it in the shop, it's not a Subaru but for a fellow Permies I can fit it in, just don't tell anyone...

 
Jordan Holland
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thomas rubino wrote:I would suggest removing the valve cover next and taking a look.
Something may be obvious or you may need to start the motor to see/hear the problem.
Is this an overhead cam motor or an overhead valve?
Performance should be affected if the valve train is wonky.
I have seen overhead cams be broken and still run...
Better yet just run it over and we will put it in the shop, it's not a Subaru but for a fellow Permies I can fit it in, just don't tell anyone...



Thanks for the offer, but we're beyond that point now, lol! That was the first invasive procedure I tried (it needed a new gasket there anyway). A couple weeks ago when the tapping was still pretty minor I removed the valve cover, suspecting the most likely culprit to be a stuck or collapsed tappet like Randal suggested. No bolts were loose, all pusher rods were tight but could be rotated with the fingers when they were not actively opening a valve. I rotated the engine by hand and everything appeared to be normal. I tried putting toothpicks in the oil holes, but after only a few seconds of running the engine it blew a couple of them out, so I had to stop. Nothing seemed amiss, anyway.

That reminds me of the next clue: The oil pressure is excellent and has not changed. 55psi at startup and about 45 going down the road when warmed up. Just a hair under 40 at idle when warm.
 
thomas rubino
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Well, I give up Jordan;
Bring it over if you want help or let us know after you figure it out.
 
Jordan Holland
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I don't blame you, Thomas, lol. This has certainly not been fun. I'm not a motorhead or gearhead or whatever they call them. I just fix stuff mostly because I have to. If I had absolutely nothing to do, it might be different. But especially this time of year, there's a lot of stuff to be done. Things like this rarely come at the most opportune time. But it's not all bad. At least it's not the hottest or coldest part of the year. And I'm pretty sure I set a new personal record for getting down and up off the floor Saturday, lol.
 
Jordan Holland
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So, at this point we have ruled out:

Exhaust leak
Plugs\wires
Stuck\collapsed tappet
Valve
EGR
Loose rocker bolts



My line of thought seemed to have followed most people's here to this point. There were three other main possibilities I had considered that no one has mentioned as well.
 
thomas rubino
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Let's see;
Broken wrist pin...
Broken piston...
Timing chain ...
Bearings...
Sorry, but it's not looking good for an easy fix.
Let us hope you can get this figured out!
As you say at least it's not below zero or over 100 outside!
 
Jordan Holland
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thomas rubino wrote:Let's see;
Broken wrist pin...
Broken piston...
Timing chain ...
Bearings...
Sorry, but it's not looking good for an easy fix.
Let us hope you can get this figured out!
As you say at least it's not below zero or over 100 outside!



Now you're getting there...

The sound coming through the stethoscope pretty much the same no matter where I put it lead me to believe the problem was deep inside the block somewhere. But I second-guessed that because the sound was higher pitched than I would have expected for that, especially going down the road. In any case, it's easier to remove the pan than the head, so that's what I did next.

Next clue:



IMG_20230527_165756192_HDR.jpg
[Thumbnail for IMG_20230527_165756192_HDR.jpg]
IMG_20230527_165743394_HDR.jpg
[Thumbnail for IMG_20230527_165743394_HDR.jpg]
 
thomas rubino
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Hmm, my friend;
That look's like pieces of broken piston skirt...
Never a good thing.
 
Jordan Holland
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That's it! Piston number 6. It is not an uncommon issue around that year. There seems to be a lot of theories as to why, but I'm not sure which are right. There were also several heads that cracked in the same range, so probably just attempts at cutting costs until they pushed it too far.

So now the head's coming off anyway. From what I can see, the cylinder in question looks fine. After a rudimentary cleaning, I think I can even still see the crosshatch honing pattern in the area where the missing pieces used to be. I'll be curious to see if any other pistons are cracked. I've seen some people find that they have all six with cracks. I don't plan to do a full, proper overhaul. If there are no other problems uncovered, I think I will just hone the cylinders to break the glaze and replace the pistons and rod bearings, and maybe a few other things that come in the overhaul kit that are easy to get to, like the timing chain\sprockets. The good oil pressure makes me think the bearings should not be too worn anywhere. Back when it was designed, they made sure to use bearings big enough to last. The camshaft looks good. These should ideally be 300,000 mile engines before needing an overhaul, unless of course a piston comes apart.
 
Jordan Holland
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I couldn't get the head off last night because apparently it needs a 12 point 1\2" deep well socket, and I only have a 6 point. I got the rod caps off and the bearings could honestly be used again. It's hard to believe they have 186,000 miles and millions of rotations on them. I mic'd them and can't discern any wear, though you can see a tiny bit on the upper one that does most of the work. You can still see a lot of the machining marks on them.
IMG_20230529_214337737.jpg
[Thumbnail for IMG_20230529_214337737.jpg]
 
randal cranor
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Howdy,

I like this photo, very "artsy"  coloring etc., especially the reflection in bottom of oil pan.


https://permies.com/t/217820/a/212240/IMG_20230527_165743394_HDR.jpg
 
Jordan Holland
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randal cranor wrote:Howdy,

I like this photo, very "artsy"  coloring etc., especially the reflection in bottom of oil pan.


https://permies.com/t/217820/a/212240/IMG_20230527_165743394_HDR.jpg



Lol, thanks! The concrete just above the pan was in full sun and it blinded me enough to not even see the reflection when I took it. It reminded me of the stereotypical selfie in front of the bathroom mirror you see teenage girls do. I thought this could be the version guys do, lol. It would be a good profile pic for someone on a mechanic's forum, and I would bet a woman using such a pic for a dating profile would do quite well, lol!
 
John C Daley
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I like your thinking about the dating site, very clever concept.
Have to been able to talk with anybody that knows pistons to see if a change of manufacture is required?
What year and model donk are we talking about?
 
John C Daley
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I had a look around the Jeep forums and it seems there are mixed reports about broken piston skirts.
This comment was posted
"Jeep lightened the rotating assembly in 1996-1997ish so all jeeps after that have the lighter weight crack prone pistons.
I had a 97 XJ that cracked a #1 piston skirt and i had a old Renix sitting around so i swapped the 97 head on it and stuck it back in the XJ."
Otherwise everybody was guessing.
Maybe find a good quality piston from somewhere, Wiseco?
 
Jordan Holland
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John, I saw theories all over the place as well. I haven't seen any reports really of any pistons failing other than the original factory ones from that era. I ordered a master rebuild kit I saw recommended by a YouTuber who does Jeeps, but the seller said it was the wrong one for my model and the right one would cost quite a bit more. Now I'm thinking I will just buy the parts I need. I'm about to try to pull the head right now and if something else happens to be seriously wrong, I will rethink things.
 
Jordan Holland
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I got the head off without any problems. All cylinders look excellent. No damage in cylinder 6. You can't really even tell by looking that it had a piston falling apart in it. Pistons 1,2, and 5 also had cracks starting to form, all in the same place, all on the passenger's side. I don't know what's going on with that. That's the side it pushes more against during compression. It's the side opposite the intake and exhaust.
IMG_20230530_212103547.jpg
Broken piston, number 6
Broken piston, number 6
IMG_20230530_212401169_HDR.jpg
Crack in piston 5
Crack in piston 5
IMG_20230530_212803988.jpg
Cylinder wall where broken piston rubbed
Cylinder wall where broken piston rubbed
IMG_20230530_213009847.jpg
Cylinders almost look as if I already honed them.
Cylinders almost look as if I already honed them.
 
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I'm a little out of my element here but..

Same cracks forming, same side, across several cylinders.

Crank issue?

wobble would place stress on the same side of the rotation, possibly across the entire crank.... bearings
 
John C Daley
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From a forum; https://www.jeepforum.com/threads/4-0l-piston-cracks-what-is-the-cause.4043129/
"You will see a lot of number 5 and 6 have piston problems. In the racing world we often tap an additional cooling line from the back of a head
and run it to the water outlet to help keep those rear cylinders cooler..."
Form : https://www.naxja.org/forum/showthread.php?t=1076344
"I assume by your term,"stock rebuild", you mean retaining the same CR, bore sizes, stock heads, etc., but using a better quality OEM piston?
If that's the case, then in my view and experience(since the 1950s) I have a very low opinion on OEM pistons.
After many years of cracked OEM pistons, i.e. gross cracks, cracks between the ring grooves, skirt cracks, thermal expansion fatigue cracks from steel inserts cast into the piston, etc., I now always used forged pistons.
Once you have to pull an engine, strip it down, replace pistons/and or re-hone scratched cylinder walls/and or sleeve cylinders and/or replace the block, re-assemble everything, re-install the engine, break it in again- stronger quality forged pistons are very inexpensive and a cost-effective choice.
Forged pistons with dished heads can be purchased to retain your stock CR. Likewise, quality ductile or stainless steel rings virtually eliminates future ring breakage.
Also forged pistons run at significantly cooler temperatures."
also
" I've used Total Seal rings a lot and they offer stainless steel rings which are also good. Also, I've used a lot of Venolia forged pistons in the past and assume they're still in business.
If you go with Venolias, make sure they put enough oil drain-back holes in the oil ring groove.
If there are not enough oil drain-back holes, the oil will sit in the piston groove, cook, and set-up to lock the oil control ring closed.
I've found that problem in a lot of newer engines, i.e. Mitsubishi V-6, where at 100,000 miles five of the six pistons had oil control rings frozen closed. Not a good scenario."
and
"Venolia supplied the forged pistons for my last 4.0L rebuild. In the old Venolia catalog I have, they listed two pistons for AMC 232-253 6 cyl.; #6610 (Flat-top) and the #6620 (Dome or dish). They machined a set of #6610 for my 4.0L. My POC @ Venolia was "Dale"; give them a call and tell them what you want to do. They're very helpful."
 
Jordan Holland
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Will Wit wrote:I'm a little out of my element here but..

Same cracks forming, same side, across several cylinders.

Crank issue?

wobble would place stress on the same side of the rotation, possibly across the entire crank.... bearings



The crankshaft seems ok. I don't plan to remove it's bearings. If they were worn, I think it would have definitely shown up in the oil pressure. The engine ran smoothly, and the straight six layout is inherently well balanced. I'm not sure if it were out of balance it would affect one side of the piston more than the other. At least not enough to mess with the crankshaft.

Another thing I noticed is the cracks are on the side of the spark plugs, so maybe the ignition wave starting on that side makes a difference.
 
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