Jeep 4.0 engine and piston skirts

[Donald]

A guy I know traded some vehicle he owned for two Jeeps with 4.0 engines. When I first alked to him I told him about his Jeeps I mentioned the piston skirts cracking and falling into the oil pan. A few months latter the engine in one went and he found chunks of piston skirt in the oil pan. Went to look at the other engine and while the engine had not (yet) failed it too had chunks of piston skirt in the oil pan. He is looking at a junk yard engine. So maybe there are a lot more Jeep 4.0 engines with cracked piston skirts than we think, just not all cause engine failure. Who takes off the oil pan to look for chunks of aluminum?

My Jeep with 4.0 engine had an engine failure with cracked piston skirts. I had a reman engine put in.

 

[KCJeep]

The problem was by far the most common with the 99s to 01s, while not unheard of in other years it was exceedingly rare in 04s for example. Are they XJs or WJs? You'll need to match those up too...

 

[Chris142]

Twas very common on the Jeep boards about 10 years ago. Page after page of threads about it. Happened to the 4cyl too.

 

[zrxkawboy]

Happened to the 4.0 in my ‘98. Talking with the owner of the local salvage yard and told him about my engine, he asked if it was cyl 5 or 6. In my case it was #6. He said all of them he’d seen had been either cyl 5 or 6.

 

[AutoMechanic]

Happened to my dads Jeep it’s a 2001 Jeep Cherokee

 

[SwampSurvivor]

Protip: The "lifter tick" that these Jeep 4.0 engines have is piston slap. I have pulled apart a few that were somewhat loud slappers but nothing alarming to find small pieces of piston in the oil pan. As long as it's not a complete failure, they'll still run quite a while with broken pistons.

It's a result of a few things. The tooling was really wearing out towards the end of the run. Also the High Output 4.0 uses a different piston design that is more prone to cracking than the Renix / AMC 4.0 engines

 

[supton]

Wouldn't losing a chunk of piston cause an imbalance? maybe it's too small to really unbalance things.

 

[92saturnsl2]

Wouldn't losing a chunk of piston cause an imbalance? maybe it's too small to really unbalance things.

I'm no engineer but I wouldn't think it'd be a major problem given the pistons are only moving in one direction (up/down.) They're not rotating as in the case of the rod journals or crankshaft counterweights, which would cause a serious vibration or worse if it were to become unbalanced.

 

[Nick1994]

What year are his and what years are affected?

 

[Donald]

I am unsure exactly what year his were but in the 99 to 01 range if I remember. Mine was a 2001 at 96K miles when it failed.

 

[Donald]

Protip: The "lifter tick" that these Jeep 4.0 engines have is piston slap. I have pulled apart a few that were somewhat loud slappers but nothing alarming to find small pieces of piston in the oil pan. As long as it's not a complete failure, they'll still run quite a while with broken pistons.

It's a result of a few things. The tooling was really wearing out towards the end of the run. Also the High Output 4.0 uses a different piston design that is more prone to cracking than the Renix / AMC 4.0 engines

So you are saying the tooling in the plant making engines was wearing out and they had no plan/process to check their machinery (boring machine) every xx engines to see if in spec and/or adjust? I thought that would be part of Manufacturing 101.

 

[jeepman3071]

I wouldn't say it is "common" as the percentage that fail is still pretty low. From the ones I've seen it has been on mostly the 99-02 WJ 4.0Ls which is actually a different block than the 1999-2001 XJ and 97-99 TJ 4.0Ls. 2000-2006 TJ and 99-04 WJ 4.0 share the same block.

 

[supton]

I'm no engineer but I wouldn't think it'd be a major problem given the pistons are only moving in one direction (up/down.) They're not rotating as in the case of the rod journals or crankshaft counterweights, which would cause a serious vibration or worse if it were to become unbalanced.

It's been a long time since I read through any Hot Rod magazines but I recall them mocking up piston and rod assemblies (somehow) for balancing purposes--they certainly were matching piston weights to each other. Maybe you're right, they go up and down, although the big end of the rod does rotate with the through... Dunno.

I like Miller88's contention "...nothing alarming to find small pieces of the engine in the oil pan..." (not a direct quote). Chrysler engineering at its finest, lol! [Or was it an AMC engine? either way...]

I wonder if most of them have simply done it by now. None of them are new nor low mile.

 

[jeepman3071]

It's been a long time since I read through any Hot Rod magazines but I recall them mocking up piston and rod assemblies (somehow) for balancing purposes--they certainly were matching piston weights to each other. Maybe you're right, they go up and down, although the big end of the rod does rotate with the through... Dunno.

I like Miller88's contention "...nothing alarming to find small pieces of the engine in the oil pan..." (not a direct quote). Chrysler engineering at its finest, lol! [Or was it an AMC engine? either way...]

I wonder if most of them have simply done it by now. None of them are new nor low mile.

The issue has nothing to do with the actual engineering or design of the engine. It's a casting flaw in the pistons, combined with bad machining tolerances in the cylinder bores allowing more piston slap. Same with the cracking cylinder head issue, it has to do with piss poor manufacturing and casting processes. It's the same reason why some engines from the same year with the same potential problem can go hundreds of thousands of miles with no issues, and some fail like Donald's at 96k miles.

 

[Eddie]

The 4.0 and 4.2 version were great depedable engines until AMC decided to change piston supplier. Before the "new" pistons, the engine was usually good for 250,000 + miles with any reasonable care and many got 400,000 miles. Ed

 

[SwampSurvivor]

So you are saying the tooling in the plant making engines was wearing out and they had no plan/process to check their machinery (boring machine) every xx engines to see if in spec and/or adjust? I thought that would be part of Manufacturing 101.

Yes. It's been posted on different jeep forums over the years that the bores are slightly ovaled and not the same size. A lot of these engines left with multiple sized pistons too ---- one thread here https://www.cherokeeforum.com/f2/replacement-pistons-size-question-178595/

 

[SwampSurvivor]

The 4.0 and 4.2 version were great depedable engines until AMC decided to change piston supplier. Before the "new" pistons, the engine was usually good for 250,000 + miles with any reasonable care and many got 400,000 miles. Ed

*Chrysler changed the pistons. AMC made a heck of an engine.

The Renix 4.0, all the way up to 90 has better pistons than Chrysler's High Output 4.0.

If I ever find myself a clean 4.0 TJ or YJ jeep, I am making it my duty to swap in Renix 4.0 block with the appropriate High Output head. They're just an all around better engine when it comes to rotating assembly.

 

[Donald]

I wouldn't say it is "common" as the percentage that fail is still pretty low. From the ones I've seen it has been on mostly the 99-02 WJ 4.0Ls which is actually a different block than the 1999-2001 XJ and 97-99 TJ 4.0Ls. 2000-2006 TJ and 99-04 WJ 4.0 share the same block.

My point of this thread is the percentage might be much higher if all the oil pans were checked for chunks of piston skirts.

 

[SwampSurvivor]

My point of this thread is the percentage might be much higher if all the oil pans were checked for chunks of piston skirts.

Definitely! They may only break off a small bit of the piston. I've heard legitimite lifter ticks in these engines. With the exception of one or two, all of them with "lifter tick" were definitely piston slap from having piston pieces in the oil pan. In most cases, they will run forever with a broken piston.

 

[benjy]

interesting read as my aunts rust free but poor paint 99 cherokee sport with almost a 100 thou is great except the oil leaks!! sisters 2000 was "rapping" when she traded it + a pro garage friend just threw a remanned engine in an early rust free cherokee he found. everyone wants to buy my aunts cherokee as their popularity for 4 wheeling is amazing!! a local junk yard next to the AOAA outside shamokin pa where i livebuys any he can + fixes as needed for resale, i guess its the low prices + REAL 4WD!!!