It's Knocking, But Why? - Tearing Into Our 4.0 Jeep Daily Driver And Picking Through The Carnage 

My '98 Wrangler busted a piston skirt a few years ago. I pulled the engine and my local engine guy rebuilt it for me, boring it 0.030" over. She's been flawless ever since. Jeep made a piston change in the latter half of the 1990s. 4.0s are known for it, but the 2.5s like mine can do it, too. With better pistons in my block now, it won't happen again. My engine was never abused in overrevved, but I have no regrets about rebuilding it. I've had it for 21 years and just keep on truckin'! 😎👍🇺🇲

Learned a long time ago that both the 4.0 jeep and 300 ford dont reeeally need skirts on the pistons. Just make sure you dig all the chunks out every oil change and keep going

I hate working on daily drivers

that jeep will out last any new jeep they make today

4.0s are the best. My first one knocked like crazy. Drove great! And the detonation was almost surely overheating. It is an odd XJ if it has never struggled with temp.

Hey, Tony! Make sure you check the EGR and it's passages. Lack of EGR can lead to detonation as well. .

A man after my own heart referring to a 20+ year old Jeep as “late model” 😂

Change all six pistons. You don't want to have to do this job over again six months down the road.

My first car had a pretty beat 3800 Series in it with 150000mi. The compression was likely crap and the rings were so cooked I had to run 20w50 to keep oil in it. But it ran well, and always started. It saved my bacon because here in Canada it hit -40C (also -40F) and being a southern car out of Arizona it didn’t have a block heater. My dads car was plugged in all night, and his didn’t start. Mine was so worn out it cranked right up and he had to take my car to work and drop me off on the way.

Retired tech from Chrysler for 30 years, and I have seen this a million times. 4.0 pistons have a lot of pin offset to quiet them, and as a result they are prone to cracking the skirts when cheap gas and lack of maintenance is present. It doesn't take much detonation at all, to start the ball rolling on these engines.

This is nostalgia. I remember my father-in-law doing rings and bearings on a six cylinder Falcon in about four hours. Gotta' love those inline sixes.

You're falling in love with the 4.0L

Similar issue in my mid 80's f150 .. Ford used non forged pistons, in there trucks engine's..

I just finished building one of these motors in my 92 cherokee. 5 out of 6 pistons were cracked and 1 of thr 5 had completely dropped the skirt all the way up into the ring lands.

It's videos like this that remind me why I subscribed to UTG . So interesting and reminds me to be sharp and diagnose fine details that are screaming information at you .....thanks Tony nice work son 👍

Talked to some guys that love that engine. They seemed to be in agreement that the same engine with carburetors didnt crack pistons.

We had more hard part failures from detonation than any other source.

Had a 97, 98, 99, and a 2000 Jeep all with the 4.0. Loved that engine.

70k miles in 2 years? Impressive

I had a 92 wrangler with the 4.0, best driveline I’ve ever owned. I beat the piss out of it for a dozen years and in all that time I had to replace the water pump once and I had to eventually put a clutch in it, that’s it. It’s too bad they rusted so bad, you couldn’t kill them otherwise.

70k is way longer then I could stand. I imagine you have a good radio. Open engine bay and easy to service stuff makes for much more enjoyable maintenance.

Uncle Tony I would love to see you cover rebuilding this 4.0! This was a fantastic video & is great info to know. I have two 2000 XJ 4.0 one my 16 year old son now drives that I have owned for six years & my daily which seems to have a cracked skirt & I use on some lengthy road trips for work. Love the XJ & 4.0 especially despite some of these known issues.

Analyzing Piston Failures To Determine Root Cause: Regardless of whether it runs on diesel, rocks a crazy rotary motor configuration, or is just a pump fuel-powered project car, every time an engine explodes, a thorough investigation is sure to follow. It doesn't matter which portion of the motor took a dirt nap. What matters is why it happened in the first place. That, and how much damage did said component's failure cause in other areas of the engine? All too often, people see a destroyed piston, and they immediately presume that the piston was at fault. When in actuality it could be any number of culprits. However, it's not always a big-ass explosion that causes a piston to fail. Oftentimes it's a continuous, gradual grinding scenario that is the harbinger of doom. For instance, contamination or combustion buildup can cause the skirt to start scuffing the side of the piston bore wall, thus leading to overheating. This results in an uneven ring groove, and a pinched ring gets stuck in a set position and will never float like it should. The piston can be a window into the engine's failure.

Its just amazing when things just work and dont stop working because of a hiccup

Saw that happen on an old 1928 fire truck engine. The detonation past the rings was so bad that it deformed the piston, lost compression and sent the detonation into the oil pan. Found spider cracks in the pan. Great job Tony. Learned a lot from this video 👍🏻

I did this same job last year. I fixed it the same way. It ran great. I did a lift put a set 35" mud tires I got off market place. I had a lot of fun with it and sold it to a young guy a view months ago. I'm now in the middle of LS swap into a JEEP Wangler JK I'm just doing a 5.3 with ax15 5 speed. never done a swap like this but I wanted to learn.

Is there actually a bad straight 6 cyl? Straight across the board from ford,GM and Chrysler, there all pretty much indestructible...

I used use a 99 jeep cheerokee there my favorite jeep.they are small like a car but also have good handling and good 4 wheel drive and okay gas mileage. I put 180k miles of side street driving on that jeep it only broke down 1 time.but it was always enjoyable to drive . I hope I can find one again for a daily driver.there my favorite year and trim.mine was green.

The advantage of working on OEM stuff is that when you get done, it will likely work right on the first try. The negative is that room on newish Mopars is usually an issue. Much more room and simplicity on older stuff. Aftermarket junk is hit or miss, however. My guess for root cause is junk gas and a partially clogged injector.

Love the 4.0 manifold assembly there fun

Very good information. I like working on my daily driver because it's a 1964 Valiant.

I love working on my 1996 Buick Roadmaster sedan. When I can afford parts. But when you're barely getting by and you have a few hundred dollars worth of repairs that need done immediately. It kinda takes the fun out of things. Especially when you only have one car and you have to drive it to work. But other than that. I love working on cars

I hate heavy engine work on newer stuff

I would definitely check all the rings and ring lands, detonation doesn't discriminate, it's all inclusive.

Thanks Tony! When are you going to hit on how you eliminated the weight on Bottle Rocket. Thanks.

Weird thing I discovered when I had a 4.0. Dodge Neon injectors fit and atomize the fuel much better than a 4 hole. I think they are 8 hole. My Jeep ran much better.

I polished the crank on mine ,lost oil pressure but didn't spin the bearing , just the #6 rod bearing was showing brass and crank was ugly , I polished the crank with some emery cloth and small rope , got a .001 undersized bearing on #6 and 5 fresh standard bearings in the rods, a high volume oil pump and timing chain, put 50k on it till I wrecked it. Question, injectors being dirty and out of balance, cause a lean cylinder to cause this ?

At one time the 4.0's were so well known for piston slap they were nick-named the Jeep diesel.

I suggest a new distributor drive gear if not an entire distributor. Well done !

Thanks Tony...

This is a great video UT. My friend drove his like that so long the piston broke and seized the engine. I pulled the engine expecting a broken rod of something. We had to sleeve the cylinder, the rest were perfect still. Not knowing the blocks changed from 97 I got a 80k mile motor from pic a part for $90 and found out we couldn't put it in. So we stole the pistons and rods out of it, along with a few other things put fresh bearings and timing chain in it and he drove it until the body fell apart. There's 3 things that suck working on your daily, having to get the work done on a time line, and working on any car newer than the mid 80s....

🥝✔️🕊️🕳️ Usually, the injectors are only able to meter to 5% of ideal fuel load. The O2 sensor then allows pulse width to be varied to get target air fuel. When one injector goes off its ideal fuel flow rate, the rest of the injectors muck into to ensure the engine hits target air fuel. Just a simple check of the injectors flow rates can avoid knocking. In Australia, GM's US sourced GM3800 and Hi End V6s and Ford's six cylinder engines ran better quality pistons to ensure a bad injector or crook fuel didn't take out a position skirt. Piston skirt loss is a typical US in line six cylinder problem that GM, Chrysler and Ford hit with much better than stock US pistons. The HO Jeep 4.0 liter really needed better pistons with more ring gap and better.(forged) rods from day one, but it only had to meet the 300 hour test and EPA emissions, so they never went the next step up in piston quality like the overseas engines did. Always check your injectors first.

if you dont think another piston has a crack in it, you are dreaming

the thin side skirts is the common denominator in this piston issue. instead of changing one piston why not swap out all 6? you are already deep in this thing and you might have another one let go sooner than later? cheap insurance.

I would be concerned the other 5 pistons could be stressed enough to crack. Why not pop 6 new pistons in. You will already have the rods and pistons out. The plug on that cylinder looked noticeably whiter, it was going lean. Properly flush and test those injectors.

Anyone that has been in this automotive game for decades hits the wall,we all do I call it the biological stretched throttle cable lol Father time has his way

I've been waiting for this teardown!

Hey U. T. did you see the update on Steve Mags that posted on his channel today? If not, his condition is improving! He still has a long road ahead though.

distributor bushings ...........detonation , intermittent hard starting , broken starter housing .........etc prolly

Working on dailys sucks the soul out of me. But I gotta get to work.

Very lucky wrist pin didn't tighten up and pull piston apart. Might you consider having atleast that 1 rod checked for bore size, possible resize?

1) Are you inspecting the other piston skirts? 2) Whether you do or don't, seriously consider getting the injectors flow-tested. Subtle differences in flow can cause intermittent detonation. There are places that will clean and test the injectors for about $150 and swap in good ones so you have a matched set. They need to be consistent and with a 1990s multiport system, it's not to be counted on that they even were consistent from the factory, plus whatever gunk they accumulated. It's a daily driver and you're about saving money, but is this Project Darwin Award? We just got done hearing about fuel starvation on Slaghammer.

Working on my XJ right now. This video was good for my soul.

Engine ping is a shock wave that is moving at the speed of sound. That wave is bouncing back and forth in the cylinder, that's why you hear it rattle.

I quit doing it when it became work and it wasn't fun anymore. oh yeah and the massive stroke I had, where it took me 4 days to replace my starter on my Chevy 350 normally a 20-minute job and O'Reilly's parking lot with only a pair of vice grips cuz that's the only tool I had in the front seat, 4 days I guess you need two hands to do a starter.

Love your stuff bro. Very cool and informative

Are higher quality pistons available aftermarket?

Really interesting, I like how you trouble shooted to see why did the piston break. Cool video Brotha! Boom Baby!!!

I miss my XJ's!

Beauty! I always learn from your videos. I think your an awesome teacher, Thanks for sharing with us.

Check knock sensor and advance, or use premium fuel

Are you saying you are going to do this work in the car? Or remove the engine? I like this kind of repair where you save money by just replacing the bad stuff, not rebuilding the engine. I suppose a timing chain would be needed for this engine.

It's not worth replacing the other pistons while you're at it?

Same here. But with me it's modern 4.6 fords, not jeeps. I love crown vics/grand marquis/town cars. It's been a whole new learning curve that takes time and will to understand.

I recently picked up a Cherokee ( my 4th!) and i know it'll need a rebuild at some point (it has 287k on it now) looking forward to seeing how you build this ;)

So what does that mean, that Chrystler ran these motors lean to save .001% fuel economy? If this is a common problem it has to be something common to these years of Jeeps because earlier ones don't have this problem that I am aware of. My parents 92 has almost 300K on it and doesn't have a knock.

“ from two blocks away… doc doc doc doc” 😂 UT coming around the corner with a half smoked cigarette he has no intention on finishing

Rebuilt a 2000 Cherokee 4.0 for a customer this year, #2 piston was broken ,bearings were perfect 201k miles found harmonic balancer had separated at bonding causing imbalance, some advice, don't use DNJ engine kit, chinese junk , use sealed power.

Nice! Those engines have a long lineage clear back to the Nash days, even before the venerable 258. A history dive may be in order! Cheers U.T.

Reminds me of Pintos and Pontiac Grand Ams from way back. Notorious for piston skirt damage.

Thanks for finally do a video on a mid 2000s daily driver 🙂

Way to go, hope it runs great when your done.

Looks like carbon induced detonation. Notice the flat portion that comes close to a flat portion on the head. They didn't hit each other but they were close enough to share heat the carbon build up exasperated the heat causing the detonation to take place.

uncle tony is sherlock holmes....amazing...i love it...

Always fix the problem, not the symptom. Have seen many people pay to have things "fixed" over and over because the real problem was never corrected. I'm too cheap for that, I want to fix things once and be done with it.

I thought you quit smoking? I am glad you made this video I'll know what to watch out for on my 94 XJ

I had the same jeep. Loved it. Had a 4½ lift on it and a drunk driver decided to park underneath my rear end while I was sitting at a light. That was the end of that

This is my favorite part of my job, automotive autopsy.

On my 2001 4.0 Cherokee I had cam bearings wiped out from a cracked head that allowed coolant to get in the oil. Oil pressure was ok cold but too low at operating temp. I had replaced crank and rod bearings to correct the knock thinking that would correct the oil pressure problem too. Engine sounded great but still had low oil pressure hot. Pulled the engine again and found the cam bearing problem. Check them cam bearings UT so you know they are good 👍

Include me in your list of 4.0 broken piston skirts. Happened to my ‘01 Cherokee 2 years ago

Why not do a three angle valve job when you have the head already off??

Thanks Uncle Tony. Great content. Maybe put another injector on that cylinder? Maybe running leaner than the others?

4:00 piston skirt is overrated. Besides, no skirt , no friction. Mpg gains,bigtime. Bust off all the skirts and get 63mpg highway,ask me how i know.

That was a great engine and pretty fun vechile dont see them much any more up here

Great Video!

UT, it's not you it's the car companies. They've over engineered these things to the point that only a factory trained tech can get out from under the hood in less than six hours. This is why we stick to the old iron because they make sense.

Great info Tony. Love your Detctive Work. Thanks

Just about all 4.0 have knock issues.

the skirt is over rev. running it with the piston tilted and broken probably pinched the lands

Late model has a knock sensor, no? Was the chicanery causing the det beyond the scope of what sensor, engine computer, available timing swing not enough? I mean just how FUBAR was the air/fuel/timing mix that it defeats modern efi? Inquiring minds want to know!😀

A man who does know his away around the garage! Respect 👍🏻

Good stuff uncle tony, very informative.

Hey Uncle Tony, we would like to see the detonation in relation to the combustion chamber. Where is the spark plug in relation to the detonation area. Maybe some insight into the actual cause of many 4.0 that have suffered the same thing.

The 3.9 v6 that they put in dakotas had problems with skirts breaking too

Owned a 96 Xj for eight years. Now I have a 95 Grand Cjerokee (ZJ). Both have the piston slap noise. Never a problem. I once read that Chrysler lightened the AMC pistons in part to go with the HO pkg, and the result was pistons sort of wiggling up and down the cylinder as they travelled. More than few broke or chipped the skirts with or without deton. If anyone can confirm this, I'd appreciate it. Rebuild suppliers seem to only stock one number for a standrat build, or I woud be able to compare the weight. I' can't find original AMC pistons tocheck.

I love this type of content

My friend has a 1955 t-bird and it wants to start but drags as if the timing is off. I was thinking could the dis-cap be advanced to give it the proper function to turn over? Newly subbed to you.

Hey, Clouseau, would the engine rotate freely because the rings on the other pistons are seized? And, are you going to ring the other pistons? Great channel.

Excellent video.

Could it also Been carbon build up on top of piston??