I don't think that's piston slap. (shiny oil)

[brucecooner]

The Heartbreaker's just been a big paperweight for a while, waiting for me to sort out my health, and what seemed like a misfire, and the increasingly loud lifter chatter.

 

After a few months I told my son he could start driving it if he helped with upkeep, so at least it wouldn't just be sitting there.  He's a maintenance tech at a freight yard and has a couple of old project trucks he's spent some time on, though not enough to reliably keep them on the street (he works nights).

He was excited to be rowing the gears in a genuine truck again, and he started his Jeep journey by giving it some new plugs and wires, which fixed absolutely none of the issues we had set out to address.

 

He's always, always suspicious of bearings (some past trauma I think), so at his behest we pulled the oil filter and drained it into a plastic lid, and we had to shield our eyes from the light reflecting off all that brass.  

Maybe I'm exaggerating, but it still seemed like a lot of brass for oil that only had a few hundred miles on it.  (pics at the end)

 

Apparently I have managed to acquire the only 4.0 ever that didn't even make it to 190K.   Maybe I'm exaggerating again, but it's still disappointing.   I'm pretty sure this particular Comanche, though well maintained and running fantastically when I acquired it a few years ago, was a tow rig in its former life and worked very hard for that first 180K miles.

 

So the Heartbreaker really earned its moniker this time. 

And I'm trying to figure out which is worse, the pain of wanting an old Jeep, or the pain of owning an old Jeep.

 

There are no tall cliffs conveniently nearby to push it off of so I can listen to the satisfying thud as it finds its final resting place, so now I have to figure out what I'm actually going to do with it.

 

I could take the L and go with the default choice, a Tacoma.  They're worth their weight in gold, as prices will attest to, but I've just never wanted one. 

 

We've got a newish Maverick that's getting the truck stuff done around here now, so I could take the L and dump the Comanche for a Wrangler of some sort, which would be great for adventuring.  Some of the JK's seem okay, and I could get a manual to boot.  Or I could roll the dice again and get an old TJ, which is what I'd rather have, and I'm sure the one I chose would seem very solid the day I buy it and then turn out to be useless as a box of broken bolts once I've driven it a few miles.

 

I could get a modern truck for adventuring.  But I don't want something new with all the bells and whistles and things. 

I want something old and simple, and unfortunately the Comanche ticks that box.

 

Usually I ignore the giant gray paperweight shading a patch of my driveway, but sometimes I look at it, and I remember the time I saw one years ago and realizing how much I wanted one, and then I'm sad.

 

A great big weed has come up underneath it.  Finding it a little funny I snapped a picture the other day and then I noticed how good it still looks, and how much I wish I was driving it around, and I got sad again.

 

I used to have dreams, stupid stupid dreams, about driving this thing to out of the way places in Arizona.

And oh god the time and effort I put into this thing, getting it to stop, and sometimes getting it to go, all those hours wasted thinking I was getting closer to bouncing up the road to the top of Mount Ord again.

 

The last thing I did when I thought I had it going good was to put those nice LED taillights on it, which really dressed up the rear end.  And here it sits, day after day.

 

 

Life-wise, things are looking up.  My health is at least better than it used to be, so it's time to make some sort of definitive decision.

 

I guess the first thing I should do is at least investigate replacing the front half of the powertrain (I've already swapped the old Peugot for an AX-15, yet another gigantic thing I did on the way to driving it around).

 

I don't have the space, time, or skills to tackle a job this big, but I have a decent job now, and I didn't spend quite all of my inheritance on the Jeep, so I can at least, theoretically, engage the skills of someone who has skills and space.

 

Rebuilding this 4.0, or acquiring another, is the obvious option.  Is it still possible to get engines rebuilt reliably?  My experience of watching my son attempt to have a Ford smallblock rebuilt would seem to indicate nobody actually does that anymore.  At best, "rebuild" shops seem to just power wash a seized engine from a wrecked vehicle, pack it up, and ship it to your door.  (no offense if you are a rebuilder, I just have not had good luck with it)

 

Sacrilege though it would be, I wouldn't be opposed to a 2.5 downgrade.  It's towing days are done, and I pushed around a Ranger with the 3.0 Vulcan and 5MT (best truck ever) for twenty years so a slow truck would just be like going home to me.  I'm not sure if the 4 banger is a big penalty offroad though.

 

At the other end of the spectrum is the Chevy smallblock, which seems like a really popular option for XJ's and MJ's.  Though seeing how it barely contains the 4.0, I have to wonder where a V8 even fits under the hood.  I guess the 8, being shorter than the inline 6, might fit a little better?

 

Any information out there on amenable options for the owner of a dying 4.0 is welcome.

 

 

 

 

 

 

 

 

 

 

[pizzaman09]

I'm going to start with an unknown question, does the engine actually run and does it sound bad?  I'm looking at those oil pictures and all I see is relatively fresh oil that does not have many miles on it.  You could send an oil sample to Blackstone labs and they'll let you know what's in it.  

 

The reason I ask is, if it's running fine, drive it!  I replaced the rear main seal on my 270k mile 4.0L, and was surprised to find the big end main bearing looked basically new, and it was quite evident that my oil pan had never been off.  

 

If your engine actually needs bottom end bearings, that really isn't that terrible of a job, and can be done with the engine in the truck.  

[Gojira94]

I'm going to concur that something's hurt that's not easily remedied. If you were my neighbor I'd be over in a minute...

 

The part that sounds to you like a misfire and lifter chatter:  You might want to pull the valve cover off and look for a loose rocker arm (nut is tight but there's slack at the rocker tip/ pushrod). That would be an immediate giveaway that you might have a wiped cam lobe/ lifter. Any debris from that would quickly find its way to the bearings.

[89 MJ]

20 hours ago, brucecooner said:

Apparently I have managed to acquire the only 4.0 ever that didn't even make it to 190K. 

Not quite, my MJ needed an engine at around 150k miles. It was knocking and I still drove it another 10k miles, roughly. It never blew up, I just found another to swap in. I would definitely do more diagnosing though, these engines are pretty easy to work on. That said, if this engine ever starts having problems, my truck is getting an LS. It's considerably more expensive, but its over 100 hp more, will result in the same or better fuel economy, and I've never seen one that needed more than a starter or exhaust manifolds, and I've been around a lot of them.

 

The 4.0 has an exaggerated reputation in my opinion. They're not powerful, they're not good on gas, and they can be a pain to keep from leaking. They're a great engine if you want to beat it and not have to take care of it, but don't care about leaks or power or fuel economy. 

 

In short, my opinion is diagnose the problem first. If you can get away with fixing the engine, go that route. If you'll need to pull the engine apart and fully rebuild or replace it, drop an LS in it and never have to work on it again.

[75sv1]

Mine was that the harmonic balancer went bad. I suggest on replacing it. The copper flakes is probably one of the bearings going bad. I'd think the main thrust bearing. That's what went bad on mine, when the harmonic balancer went bad. I snagged another crankshaft from a neighbor, and the bearings. The bearings looked new. The original crankshaft had damage to the thrust bearing surface. I have a few machine shops left in my area that do engines. I'd have the cam bearing replaced if you take the engine out. 

     Another option is Car-Pats Inc. You can search for parts in your region or across the US,  Mexcio and Canada. 

[AZJeff]

My 4.0 was a transplant from a 2001XJ with the infamous “0331” head.  Unfortunately, sometime in its past, one of the owners allowed it to overheat badly, popping the head gasket under the manifold side if the engine.  And not knowing about the vulnerability of that generation of head casting, they just slapped a head gasket on it, fixed the overheating cause, and called it good.  (They apparently didn’t know those heads can crack in a subtle way during an overheating episode.🙁)

 

Not knowing it’s history, when I got the MJ, the engine still sounded good, and had clean oil….until I drove it about 1500 miles with the (unknown) crack in the head that allowed the oil to convert itself into a brown milkshake. 😳

 

After putting on a new head from Clearwater Cylinder Head, and replacing all the bearing shells and polishing the crank, I thought all was well.  (The bearings really did not look too bad, but I wasn’t taking any chances.)

 

5000 miles later, I was getting this weird ticking sound that was VERY noticeable on a hot restart and stayed present to some degree whenever the engine was hot.  Compression testing showed #6 cylinder was getting progressively lower as time went on over a period of 6 months.

 

the problem wound up being a cracked piston skirt, probably caused during the severe overheating episode.

 

I pulled the engine and sent it to a machine shop in Prescott, AZ, and had him put in a new cam, new oversized pistons, and polished the crank and put in new standard sized bearings.    Then I had a small 6 month interruption while they replaced to valves in my heart due to a mysterious blood infection I contracted.🥴

 

After I recovered, I put the engine back in, and it’s as quiet and smooth as ever.  I have about 2000 miles on it now, and am ready to switch to synthetic oil to deal with the high temperatures here in the PHX area.

[brucecooner]

You guys are right, I should at least look inside it a bit before writing it off.

 

 

On 2/15/2026 at 6:27 AM, pizzaman09 said:

I'm going to start with an unknown question, does the engine actually run and does it sound bad?  I'm looking at those oil pictures and all I see is relatively fresh oil that does not have many miles on it.  You could send an oil sample to Blackstone labs and they'll let you know what's in it.  

 

The reason I ask is, if it's running fine, drive it!  I replaced the rear main seal on my 270k mile 4.0L, and was surprised to find the big end main bearing looked basically new, and it was quite evident that my oil pan had never been off.  

 

If your engine actually needs bottom end bearings, that really isn't that terrible of a job, and can be done with the engine in the truck.  

 

It still starts and runs well, though it sounds like a tin can full of ball bearings in an earthquake.  Doesn't hesitate or stall though.

It's been unnerving that the various noises seem to be getting louder.

The top end was just lifter #1 at the beginning, but more seemed to be joining in last time I listened to it.

 

It's crazy to me that you can replace the bearings with the engine in the truck.

 

On 2/15/2026 at 8:58 AM, Gojira94 said:

I'm going to concur that something's hurt that's not easily remedied. If you were my neighbor I'd be over in a minute...

 

The part that sounds to you like a misfire and lifter chatter:  You might want to pull the valve cover off and look for a loose rocker arm (nut is tight but there's slack at the rocker tip/ pushrod). That would be an immediate giveaway that you might have a wiped cam lobe/ lifter. Any debris from that would quickly find its way to the bearings.

 

Good to know, and that's an easy one to check.

The compression has checked out fine the couple of times it's been done, even after the noises started, which led me to think it was a bottom end problem.

 

18 hours ago, 89 MJ said:

Not quite, my MJ needed an engine at around 150k miles. It was knocking and I still drove it another 10k miles, roughly. It never blew up, I just found another to swap in. I would definitely do more diagnosing though, these engines are pretty easy to work on. That said, if this engine ever starts having problems, my truck is getting an LS. It's considerably more expensive, but its over 100 hp more, will result in the same or better fuel economy, and I've never seen one that needed more than a starter or exhaust manifolds, and I've been around a lot of them.

 

The 4.0 has an exaggerated reputation in my opinion. They're not powerful, they're not good on gas, and they can be a pain to keep from leaking. They're a great engine if you want to beat it and not have to take care of it, but don't care about leaks or power or fuel economy. 

 

In short, my opinion is diagnose the problem first. If you can get away with fixing the engine, go that route. If you'll need to pull the engine apart and fully rebuild or replace it, drop an LS in it and never have to work on it again.

 

I'm perversely glad to know I'm not the only person whose 4.0 didn't make it to 300K.

The 4.0 overall sounds a bit like the 3.0 Vulcan(?) in my old Ranger.  Not a lot of power, not at all efficient, but it ran without any drama for 200K.  Only work it ever needed was the camshaft synchronizer, and that was well past 100K.

 

It runs well enough to suit me, if i could figure out it's okay, or replace a part that calmed it down I'd be happy.

I just don't want to be ignoring something that will go boom one day.

 

10 hours ago, 75sv1 said:

Mine was that the harmonic balancer went bad. I suggest on replacing it. The copper flakes is probably one of the bearings going bad. I'd think the main thrust bearing. That's what went bad on mine, when the harmonic balancer went bad. I snagged another crankshaft from a neighbor, and the bearings. The bearings looked new. The original crankshaft had damage to the thrust bearing surface. I have a few machine shops left in my area that do engines. I'd have the cam bearing replaced if you take the engine out. 

     Another option is Car-Pats Inc. You can search for parts in your region or across the US,  Mexcio and Canada. 

 

Interesting. How did you figure it out? Would it shake in the mounts if the balancer was bad?  

 

3 hours ago, AZJeff said:

My 4.0 was a transplant from a 2001XJ with the infamous “0331” head.  Unfortunately, sometime in its past, one of the owners allowed it to overheat badly, popping the head gasket under the manifold side if the engine.  And not knowing about the vulnerability of that generation of head casting, they just slapped a head gasket on it, fixed the overheating cause, and called it good.  (They apparently didn’t know those heads can crack in a subtle way during an overheating episode.🙁)

 

Not knowing it’s history, when I got the MJ, the engine still sounded good, and had clean oil….until I drove it about 1500 miles with the (unknown) crack in the head that allowed the oil to convert itself into a brown milkshake. 😳

 

After putting on a new head from Clearwater Cylinder Head, and replacing all the bearing shells and polishing the crank, I thought all was well.  (The bearings really did not look too bad, but I wasn’t taking any chances.)

 

5000 miles later, I was getting this weird ticking sound that was VERY noticeable on a hot restart and stayed present to some degree whenever the engine was hot.  Compression testing showed #6 cylinder was getting progressively lower as time went on over a period of 6 months.

 

the problem wound up being a cracked piston skirt, probably caused during the severe overheating episode.

 

I pulled the engine and sent it to a machine shop in Prescott, AZ, and had him put in a new cam, new oversized pistons, and polished the crank and put in new standard sized bearings.    Then I had a small 6 month interruption while they replaced to valves in my heart due to a mysterious blood infection I contracted.🥴

 

After I recovered, I put the engine back in, and it’s as quiet and smooth as ever.  I have about 2000 miles on it now, and am ready to switch to synthetic oil to deal with the high temperatures here in the PHX area.

 

Glad to hear you got yourself and the truck sorted out. 

 

This motor is, as far as I know, the original '88 Renix, but it could have mismatched stuff that I am not aware of.

It's got an aftermarket radiator, I guess the original owner using it as a tow rig decided on that.  Other than that I haven't noticed any signs of work, not that I'd be able to though.

 

And we're neighbors, in a way.  I'm at the exact opposite end of the valley in Goodyear.

 

I'd be curious to know that machine shop in Prescott?   Someone here (Scout I think?) led me to AZ AMC Restoration, seems to do mostly older carb'd V-8 work, but I've talked to Dan(?) there a few times, knows his stuff.  I should give him a ring and see what he says.

 

 

I've thought about selling it, but when I think about after that, I feel like I'd be regretful of my decision.  I've never really actually wanted another vehicle like I wanted this damned thing.  If I won the lottery, sure, I'd acquire some rare iron. 

But for something to scratch my crawling the backroads itch that I actually want to drive, it's this Comanche.

 

 

[pizzaman09]

If it sounds like ball bearings bouncing around you might want to check your valve train.  Stuck / bent valve, collapsed lifter or loose rocker can all make noises like that.  Bad bearings are more likely a deep knocking sound.  

[Gojira94]

Not to throw more to consider into the mix, but the timing chain guide might be worn out and adding some of the noise. Like not really the problem, of course, but adding the mix to confuse and possibly misdirect.

[AZJeff]

1 hour ago, Gojira94 said:

Not to throw more to consider into the mix, but the timing chain guide might be worn out and adding some of the noise. Like not really the problem, of course, but adding the mix to confuse and possibly misdirect.

A timing chain is easy to pinpoint with a stethoscope, as that noise is very localized.  A valve train noise rocker arm or lifter can also be narrowed down by stethoscope.

 

A 4.0 can be run without a valve cover for short intervals at low engine speeds to also narrow down valve train noises.

[89 MJ]

5 hours ago, brucecooner said:

And we're neighbors, in a way.  I'm at the exact opposite end of the valley in Goodyear.

 

I'd be curious to know that machine shop in Prescott?   Someone here (Scout I think?) led me to AZ AMC Restoration, seems to do mostly older carb'd V-8 work, but I've talked to Dan(?) there a few times, knows his stuff.  I should give him a ring and see what he says.

Bring it to the cactus cruisers car show at the end of March. @eaglescout526 and I will be there and we could listen to it, as could Dan from AZ AMC Restorations. 

[eaglescout526]

36 minutes ago, 89 MJ said:

Bring it to the cactus cruisers car show at the end of March. @eaglescout526 and I will be there and we could listen to it, as could Dan from AZ AMC Restorations. 

Oh yeah that’s very true. And Dan said he won’t be there this year. Which is wild cause he always has like 3-5 AMCs there even though he doesn’t have his American collection anymore. 

[75sv1]

       On the harmonic balancer. There was some squealing with the accessory belt. I did not catch the harmonic balancer for some time. You can put a white line mark crossing the hub and outer ring. If the out line of that moves, then its bad. Mine was past bad. I'd look at the elastomer. If any cracks in it, replace it. They are $40 to $50. 

      I do replace the timing chains on mine. If you get the cover off, the chain should not deflect more than 1 inch. That is from pushed in and pushed out. Get a set and not just the chain. The 'silent' or stock chain might be stronger than the roller chains. At least according to an article I read. The issue is at 5K RPM. A silent chain starts to stick.

      I do have a 2000 XJ with about 280K. I don't know if the head has been changed. It was OK. I have been around two of them, that the head cracked at 80K. They were well maintained. 

     Seems the #6 cylinder has more issues with cracked pistons. I'd pull the valve cover and check the rocker arms. If OK, then the oil pan. A bit of a pain if not lift. Then the head. Or just pull the whole engine and inspect. See if it needs work and what work. 

     I mentioned Car-Parts Inc in my previous post. Still, Junkyard motors can be iffy. I got burned on mine. Head was warped and someone put a boatload of Barrs Seal in it. 

[pizzaman09]

22 hours ago, 75sv1 said:

       On the harmonic balancer. There was some squealing with the accessory belt. I did not catch the harmonic balancer for some time. You can put a white line mark crossing the hub and outer ring. If the out line of that moves, then its bad. Mine was past bad. I'd look at the elastomer. If any cracks in it, replace it. They are $40 to $50. 

      I do replace the timing chains on mine. If you get the cover off, the chain should not deflect more than 1 inch. That is from pushed in and pushed out. Get a set and not just the chain. The 'silent' or stock chain might be stronger than the roller chains. At least according to an article I read. The issue is at 5K RPM. A silent chain starts to stick.

      I do have a 2000 XJ with about 280K. I don't know if the head has been changed. It was OK. I have been around two of them, that the head cracked at 80K. They were well maintained. 

     Seems the #6 cylinder has more issues with cracked pistons. I'd pull the valve cover and check the rocker arms. If OK, then the oil pan. A bit of a pain if not lift. Then the head. Or just pull the whole engine and inspect. See if it needs work and what work. 

     I mentioned Car-Parts Inc in my previous post. Still, Junkyard motors can be iffy. I got burned on mine. Head was warped and someone put a boatload of Barrs Seal in it. 

If you replace the harmonic balancer, it's technically an easy job, but do make sure you take the giant thick washer off when you remove the bolt.  I fought trying to pull the balancer through the washer for about an hour before I realized that I had a washer stuck to the front of the engine due to years of grease and gunk.

[75sv1]

Also, there is an installation tool, to install the harmonic balancer. The stock bolt is a bit short. It is not advised to smack or hammer on the harmonic balancer. Cast Iron can be brittle. A bit loner bolt from the hardware store might do. 

   Also, if you pull the head off, then check for cylinder wear. Look at the top of the cylinder. See if there is a ridge or not.