Jeep 4.0 engine and piston skirt failures

[Donald]

OK, so this has my curiosity. I had this failure on my 2001 Jeep with just under 100K miles. I have read that when the piston and cylinder wear the piston can rock back and forth and finally break the piston skirt and total engine failure is not far away. Further reading says at some point they moved up where the rings are (emissions) and that made it worse and more prone to rocking when worn.

So I looked in RockAuto and the same piston # is listed for 1996 thru 2002 and it changes for 2003. (Did not look before 1996).

What to conclude:
1) From 1996 to 2002 (inclusive) may have the piston skirt failure problem.
2) Cannot conclude anything as the piston sold by RockAuto can replace any of those years but may not be identical to the original piston and may or may not have the same problem.

 

[jeepman3071]

I have done quite a bit of reading on this. The pistons from '96 onward are all the same.

The piston skirt failures however seem to be all around 96-2000 or so.

These are the engines I've seen that had the failure:
2001 Wrangler
1996 Grand Cherokee
1998 Grand Cherokee
2001 Grand Cherokee (yours)
1999 Cherokee
2001 Cherokee

From 1999 onward, the Wrangler and Grand Cherokee 4.0L blocks were changed, and they won't swap into a Cherokee. The accessory mounts and motor mounts are completely different. These blocks were redesigned in 1999 to reduce NVH. My guess is they left the Cherokee blocks alone knowing they were going to discontinue the vehicle.

Many think that the coil on plug ignition is to blame for the piston skirt issues. From what I've seen I don't think that is accurate. That wouldn't explain the piston failures on the older Jeeps ('96, '98 Grand Cherokee, and 1999 Cherokee) that had distributor ignition.

Another theory I've seen is that it only happens to engines that see a lot of high revs. I personally asked a bunch of guys on Jeepforum involved with 4x4 clubs and many had not seen a 4.0 with the piston skirt failure. Offroad is where many of these engines see high revs and hard abuse.

My opinion is that like the 0331 head casting flaw, it is a casting flaw in the piston. Thousands and thousands of 4.0Ls from late '99 until '02 had the bad 0331 head, yet I've only seen one or two cracked. I believe this is the same kind of scenario with the pistons. Its a casting flaw that only impacts some of the jeeps.

Since you have a rebuilt motor that likely uses pistons from another source or are from a different casting, you are most likely safe from having the failure again.

Figures that Chrysler would be able to ruin an otherwise very reliable engine.

 

[440Magnum]

Originally Posted By: Donald
OK, so this has my curiosity. I had this failure on my 2001 Jeep with just under 100K miles. I have read that when the piston and cylinder wear the piston can rock back and forth and finally break the piston skirt and total engine failure is not far away. Further reading says at some point they moved up where the rings are (emissions) and that made it worse and more prone to rocking when worn.

So I looked in RockAuto and the same piston # is listed for 1996 thru 2002 and it changes for 2003. (Did not look before 1996).

What to conclude:
1) From 1996 to 2002 (inclusive) may have the piston skirt failure problem.
2) Cannot conclude anything as the piston sold by RockAuto can replace any of those years but may not be identical to the original piston and may or may not have the same problem.


It obviously can happen... my only doubt is as to how common it is. There are a gadzillion 4.0s out there, and there are multiple forums with hundreds of Jeepers talking about them. It is one of the "more common" failures of the 4.0, but less talked about than the 2000/2001 "0331" head casting crack problem.

As for the cause- as manufacturers have gone to centered wrist pins(*) and small "slipper" pistons with shorter skirts and higher ring lands, piston slap has become more and more widespread. The 4.0 (and many other engines) used a skirt coated with a dry lubricant (think 'teflon', and PTFE is part of the coating composition as I recall) to allow a tighter skirt fit and the coating is almost like a "wear to fit" sacrificial layer. It prevents piston slap from being audible, but that's about it. As far as I'm concerned, the jury's still out on whether piston slap causes accelerated wear or premature failure. There's some evidence it does, but then other engines prone to slap run forever with it.

Has your 4.0 that had the failure ever been exposed to over-revving? Or sustained high RPM (with "high" for the 4.0 being anything over 3500-4000)?

(*) Some may not know that in the past, engine makers would DELIBERATELY build pistons with the wrist pin a bit off center so that the piston would always be "cocked" in the bore, which pre-loaded it and prevented audible piston slap. But that pre-loading added friction, and with fuel economy becoming more and more important it was one of the things to be eliminated.

 

[440Magnum]

Originally Posted By: jeepman3071
.

Figures that Chrysler would be able to ruin an otherwise very reliable engine.



OK, let's be fair and square here. You really should read Bill Weertman's book on Chrysler engines. Chrysler and AMC merged shortly before he retired as head of engine design, and he in fact almost raves about how good the AMC 4.0 and 2.5 engines were. They were good enough that the AMC 2.5 replaced the Chrysler 2.5 (which was Weertman's own design!) in the Dakota. It just had more power per pound without a turbo, and had great reliability.

But they weren't perfect, and in fact had a good bit of baggage that needed to be addressed. One of the first things to go was the godawful Renix engine management system AMC was using. The result of that was the 4.0 "HO" engine that had significantly more power and a less complicated engine management system with fewer sensors. Also, the 4.0 block and crank were more prone to flexing than Chrysler engineers really liked (I guess a few were around that remembered the Slant-6). There were multiple steps taken over the years aimed at stiffening that block, and you mentioned the last ones for the Wrangler and JGC. The XJ did get the main one, though, which was a bearing cap girdle internal to the oil pan.

OK, so some changes had flaws (the 331 head, maybe some piston skirts). Chrysler is guilty as charged on that one, but they hardly "ruined an otherwise reliable engine." They took a literal diamond in the rough that was actually cantankerous and trouble-prone, had NOT gotten enough improvement since its introduction, and are the ones that MADE it great. No one praises AMC-era Renix 4.0s the way they do Chrysler-era 4.0 "HO" engines (even after that designation was dropped).

I still contend that the 331 head failures and piston skirt failures are relatively rare (compare them to GM lower intake gasket failures, or Toyota sludge failures for example). But they kinda stick out because there are so FEW 4.0s that fail for any reason before a ripe old age.

 

[Hokiefyd]

Originally Posted By: 440Magnum
One of the first things to go was the godawful Renix engine management system AMC was using. The result of that was the 4.0 "HO" engine that had significantly more power and a less complicated engine management system with fewer sensors.


Interesting fact. I didn't know the engine management system was the change responsible for the power bump. I forget when that occured (you probably know), but my dad had a 1989 Cherokee Laredo that (I think) was a pre-HO 4.0L. In fact, I think they may have come out with the HO version shortly after, in 1990 or 1991. That Cherokee was a workhorse; it dragged a 5,000 pound travel trailer from California to Virginia, during the heat of the summer. The rear axle on the Jeep burned up in Michigan, but the engine and transmission were solid.

I miss Cherokees. The only complaint I had about them was the rather narrow front seats and footwells.

 

[SwampSurvivor]

Chrysler's quality was in significant decline at that point. My thoughts are that they kept paying less and less for the pistons until it got to the point that they were of such poor quality that they seemed to fail.

From what I understand from reading about piston skirt failures on the 4.0 is that the same year as the 0331 head seem to be affected. Personally, I don't think they are related - just two coincidental failures that happened to be caused by chryslers p@#$ poor quality.

A lot of people blame pinging from running E10 fuel - while it is the equivalent of higher octane, the older ignition and fuel injection system can't compensate and start to ping pretty bad. I've never had my 4.0 ping. Ever.

I know of one person who had a head fail, replaced the engine, then on the second engine the piston skirts failed, then replaced that engine. He ended up ditching his grand cherokee and going to a 2.5 Wrangler.

The head generally will NOT fail unless you overheat it. And that's true for any 4.0 - I've seen a few failed 0331 heads on craigslist, but JUST as many on 99 and older. 4.0s don't blow head gaskets, they crack the head. Overheat your 0331 head and it's a bit more likely to crack.

 

[Kruse]

Originally Posted By: 440Magnum

I still contend that the 331 head failures and piston skirt failures are relatively rare (compare them to GM lower intake gasket failures, or Toyota sludge failures for example). But they kinda stick out because there are so FEW 4.0s that fail for any reason before a ripe old age.


I've got two '96 Cherokee Sports with the 4.0 engine. One is a manual, the other an automatic. (One of which has had a cracked head, even though it wasn't the infamous 331 head, btw)
While the 331 head might be exaggerated by the internet, my personal gauge on if there is a genuine problem is how aggressively the factory and aftermarket make a fix for the "problem". The 3.1 and 3.4 GM intakes have a definite problem and the aftermarket (as well as GM) has taken an aggressive fix on the gaskets. Some Ford 4.6 engines had a bad intake manifold and both Ford and the aftermarket came out with the fixes.
Not only was the 331 head revised by Chrysler making the replacement in Brazil, but also the aftermarket making a replacement in China. Both good replacements, IMO. I think there is a real problem with the head or there wouldn't have been such an aggressive fix. I do know that if I owned a Jeep with a cracked 331 head, I would NOT replace it with an original Jeep 331 head that is prone to cracking, even if the majority of them never do crack. It would be aftermarket all the way.

 

[Chris142]

I think the cause of the skirt failures is from the short pistons. I believe being so short allows them to rock in the bore which make some crack and then break.

 

[Donald]

My 2001 Jeep GC that failed never had high revs in the last 40K miles before the engine failure. Unsure about the first 60K of its life however.

I do hope that the reman engine I got from Marshall has different enough pistons that I will not have that problem (again).

 

[hypervish]

Originally Posted By: 440Magnum
Originally Posted By: jeepman3071
.

Figures that Chrysler would be able to ruin an otherwise very reliable engine.



OK, let's be fair and square here. You really should read Bill Weertman's book on Chrysler engines. Chrysler and AMC merged shortly before he retired as head of engine design, and he in fact almost raves about how good the AMC 4.0 and 2.5 engines were. They were good enough that the AMC 2.5 replaced the Chrysler 2.5 (which was Weertman's own design!) in the Dakota. It just had more power per pound without a turbo, and had great reliability.

But they weren't perfect, and in fact had a good bit of baggage that needed to be addressed. One of the first things to go was the godawful Renix engine management system AMC was using. The result of that was the 4.0 "HO" engine that had significantly more power and a less complicated engine management system with fewer sensors. Also, the 4.0 block and crank were more prone to flexing than Chrysler engineers really liked (I guess a few were around that remembered the Slant-6). There were multiple steps taken over the years aimed at stiffening that block, and you mentioned the last ones for the Wrangler and JGC. The XJ did get the main one, though, which was a bearing cap girdle internal to the oil pan.

OK, so some changes had flaws (the 331 head, maybe some piston skirts). Chrysler is guilty as charged on that one, but they hardly "ruined an otherwise reliable engine." They took a literal diamond in the rough that was actually cantankerous and trouble-prone, had NOT gotten enough improvement since its introduction, and are the ones that MADE it great. No one praises AMC-era Renix 4.0s the way they do Chrysler-era 4.0 "HO" engines (even after that designation was dropped).

I still contend that the 331 head failures and piston skirt failures are relatively rare (compare them to GM lower intake gasket failures, or Toyota sludge failures for example). But they kinda stick out because there are so FEW 4.0s that fail for any reason before a ripe old age.





Yeah, but the Toyota engines that failed due to sludge were because of owner neglect and lack of maintenance not cost cutting and inferior parts.

 

[BillyTheKid]

I am surprised to hear about any of this. I had two 4.0 in Jeeps in the past, and they were darn good! I would venture to think piston skirt failures were probably contributed to some abuse or neglect. Also, short skirts or even no side skirt pistons were being used in Formula 1 engines for a very long time, and those things rev to insane RPM and make power. Granted, they were not made to last forever, but if short pistons were prone to failure, I highly doubt they would be integrated into racing which is more harder on parts.

 

[Donald]

Originally Posted By: Miller88

I know of one person who had a head fail, replaced the engine, then on the second engine the piston skirts failed, then replaced that engine. He ended up ditching his grand cherokee and going to a 2.5 Wrangler.


Was his second engine a reman or just a used engine?

 

[Donald]

Originally Posted By: BillyTheKid
I am surprised to hear about any of this. I had two 4.0 in Jeeps in the past, and they were darn good! I would venture to think piston skirt failures were probably contributed to some abuse or neglect. Also, short skirts or even no side skirt pistons were being used in Formula 1 engines for a very long time, and those things rev to insane RPM and make power. Granted, they were not made to last forever, but if short pistons were prone to failure, I highly doubt they would be integrated into racing which is more harder on parts.


On the Jeep forum, about 2 people a week report this kind of failure. Percentage wise not that high, unless it happens to you and then its basically 100% from your viewpoint.

 

[BillyTheKid]

Yeah, I figured it could not be a large percentage.

 

[SwampSurvivor]

Originally Posted By: Donald
Originally Posted By: Miller88

I know of one person who had a head fail, replaced the engine, then on the second engine the piston skirts failed, then replaced that engine. He ended up ditching his grand cherokee and going to a 2.5 Wrangler.


Was his second engine a reman or just a used engine?


#2 and #3 were from a junkyard. So that could have very well been the reason why #2 vehicle was junked.

 

[440Magnum]

Originally Posted By: Miller88
Chrysler's quality was in significant decline at that point. My thoughts are that they kept paying less and less for the pistons until it got to the point that they were of such poor quality that they seemed to fail.

From what I understand from reading about piston skirt failures on the 4.0 is that the same year as the 0331 head seem to be affected


You may be on to something, but a number of changes happened for model year 2000 in the Cherokee:

1) it got the 331 head
2) distributorless ignition
3) Dual pre-catalysts right off the exhaust manifold with 4 (maybe 5? can't remember) O2 sensors instead of a single upstream/downstream as in '99
4) A cast iron dual-outlet exhaust manifold instead of the tubular steel header it had through '99

#s 3 and 4, SIGNIFICANTLY bumped up the under-hood temperatures, enough so that there's a TSB to reduce vapor-locking on the #3 injector on hot restarts. 've always thought the iron manifold and cats right next to it probably aggravated the head casting problem, never thought about it possibly affecting piston skirts.

 

[440Magnum]

Originally Posted By: Hokiefyd

I miss Cherokees. The only complaint I had about them was the rather narrow front seats and footwells.


And the ride like a &#*@! buckboard wagon, especially my 01 with the tow package and heavier leaf springs. The Grand Cherokee got trailing arms/panhard rod and coils to smooth things out a lot. I keep reading that a mild (1-2 inch) lift with Old Man Emu brand springs, and longer spring shackles really smooth out the XJ, but I haven't tried it. If I keep the 01 and convert it to 4x4, I will try that for sure. I've already stashed a set of ZJ front springs to use... ;-)

 

[440Magnum]

All opinions aside, I find it kinda odd that Donald has seen/knows of a substantial number of these failures, while I have never KNOWN anyone to whom it happened, and haven't even seen it that much on the forums. I assume we've both been around similar numbers (LOTS) of 4-liter Jeeps. By the way Donald, what forum do you see it on? I used to hang out mostly on Jeepforums.com in the XJ Tech section, but I'd poke around NAXJA a bit too.

Our different expereince is one of those weird things that statistically shouldn't happen, but still do from time to time.

 

[84zmyfavorite]

Have any of You seen the difference between a 1987 thru 1999 Jeep 4.0 oil gasket and a 2000 thru 2006 Jeep 4.0 Oil pan gasket? The area that become distorted or Burned and Shriveled is much wider on the 2000 thru 2006 4.0 because of the dual collectors going Right by the oil pan, about a 1 1/4 inch away. On 87 thru 90 Jeep 4.0s, the collector is single and about 1 1/2 inch away and 91 thru 99 which are my favorite exhaust for a 4.0, it curves away from the oil pan and is of course a single collector. Currently in possesion of 7 Jeep Cherokees, 5 of which have the 4.0, I see the differences these exhaust designs have. Sound, Engine Performance, Oil pressure(I personally Believe), MPG are all changed because of the different exhausts. My 87 with the 1st year 4.0 has 297,881 miles on Her original everything. Her 4.0 still has Excellent compression and gets Excellent mileage(18Town/22-24HighWay). Her Renix system has only needed New Injectors(These years are Prone to injector leak) and a few sensors(Map, IdleAir, TPS(7 wire/dual connector), CPS and a lone Oxygen Sensor have kept her exhaust Very clean.

 

[84zmyfavorite]

Most 1999 Grand Cherokee 4.0s got the DistributorLess Ignition with Cast Iron Exhaust manifold, dual collector, but Only Grand Cherokee that year. From 2000 thru 2006 Every 4.0 got DistributorLess Ignition, Cast ExManifold with dual collector, 0331 head, 3 or 4 O2 sensors, Pre_Cats on late 2000 thru 2006, HIGH Flow Intake,..I Love my Red 99s 4.0 combo, One of the Best I think.