Jeep 4.0 engine and piston skirt failures

[Donald]

Originally Posted By: 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.



I poke around the JeepForum.com sometimes and saw where a regular stated they see about 2 posts a week about piston skirt damage. Another BITOG member posted all the Jeep engine failures he was aware of. I only know about mine first hand. The mechanic who replaced my engine said they had done a few (indy shop) so they are uncommon but not unheard of.

 

[jeepman3071]

Of the failures I posted above, only about half of them resulted in catastrophic engine failure. The failures were usually from a broken/bent rod (I think yours was a bad rod after the piston broke?) and one engine the cylinder wall was too scored and needed to be honed out. The owner of that Jeep just swapped in a junkyard engine instead of rebuilding.

The others found the problem before the engine failed. They heard a loud knocking and pulled the oil pan to find pieces of piston. These owners were able to put a new STD piston in place of the piston with the broken skirt and continue to use the engine without issue.

Donald, do you remember which piston had the broken skirt on your engine? I find it interesting that of the failures I have seen, the failed piston is usually a #6 or #1 piston. I wonder what makes those pistons more likely to fail? Maybe the difference in water temperature between the front (near the T-stat) and rear of the engine block.

A guy on the Jeepstrokers.com website did a study on the wear numbers between 30 and 40 weight oils in the 4.0L. I'm going to look back and see if the 40 vs. 30 weight made any difference in aluminum wear numbers. Maybe the 40 weight oil provides a bit more cushion between the piston and cylinder wall. I know that the 4.0 in my Cherokee seems to run quietest on PYB, which has a fairly high moly content. Moly is known for coating metal parts with a protective layer, like ZDDP would.

 

[Chris142]

The broken pistons seem to be most common on the TJ forums.

 

[SwampSurvivor]

Originally Posted By: 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... ;-)


Ride in mine. Pickup front springs, heavier rear springs and load range E truck tires.


Originally Posted By: 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.



I never thought of the pre-cat causing problems. What year did TJs get the 0331 head and two pre-cats? A friend of mine had a 01TJ start knocking, we pulled the oil pan and sure enough, there was a piston skirt hanging out in the oil pan.

 

[440Magnum]

Originally Posted By: Miller88

I never thought of the pre-cat causing problems. What year did TJs get the 0331 head and two pre-cats? A friend of mine had a 01TJ start knocking, we pulled the oil pan and sure enough, there was a piston skirt hanging out in the oil pan.


Well, the pre-cats definitely increase heat-soak AFTER shutdown, I'm not sure how much they raise temps during operation when the fans are running. Probably a bit though, that's a lot of hot metal radiating heat at the side of the block and oil pan. And of course the cast iron manifold holds WAY more heat to soak things after shutdown than the thin-wall header used up till 99.

As for the TJ getting the331 head? I'm not positive, but it would either be 99 or 2000. The 331 head casting came about (partly) to provide mounting bosses for the distributorless ignition coil rail, and the dual pre-cats and multiple O2 sensors came with the engine controller that runs the DIS. So its a bit of a package deal.

The 331 head casting started to be phased out and replaced by the 331 TUPY stamped casting in 2001. The TUPY stamped casting is the desired one with the cracking issue resolved.

FTR, it seems like the Wrangler application is harder on engines, either by design or by average use, than the Cherokee/Grand Cherokee application. Even the new 3.6 Pentastar saw more of its issues (something lik half, IIRC) in Wranglers as opposed to all the other vehicles it goes in.

 

[rslifkin]

My mom's 99 Grand Cherokee 4.0 had the 0331 head crack a few months ago at about 110k miles. No sign of pistons skirt issues, but it's always had a little bit of mild piston slap during warmup (it starts around 120* coolant temp, goes away by 160* and it's only audible if you're standing next to it with it idling).

 

[wsar10]

the 0331 casting were the one WITH issues.
I also have to add that I too have been around many 4.0's and involved in the JEEP world since I was a kid, and since 2000 I have "heard" of MANY "piston skirt failures"(another good reason to run a HDEO!) in forums although I personally have never had the issue. It seems that most guys think its an extreme version of the "famous" 4.0 tap and it turns out to be this, piston skirts and OPD failures seem to be the most common "major" failures for 4.0's 2000 and up.

 

[Donald]

Originally Posted By: wsar10
the 0331 casting were the one WITH issues.
I also have to add that I too have been around many 4.0's and involved in the JEEP world since I was a kid, and since 2000 I have "heard" of MANY "piston skirt failures"(another good reason to run a HDEO!) in forums although I personally have never had the issue. It seems that most guys think its an extreme version of the "famous" 4.0 tap and it turns out to be this, piston skirts and OPD failures seem to be the most common "major" failures for 4.0's 2000 and up.


How would an HDEO help with piston skirt failures?

 

[Donald]

Where is the casting number for the 331 head located?

 

[jeepman3071]

It is right under the valve cover on the driver side above the exhaust manifold. If your engine has a heat shield on the intake you might need to push it back a bit to see it.

 

[440Magnum]

Originally Posted By: jeepman3071
It is right under the valve cover on the driver side above the exhaust manifold. If your engine has a heat shield on the intake you might need to push it back a bit to see it.

The casting number is outside the valve cover gasket, but if its the improved "TUPY" casting, that indicator is under the valve cover as can be seen here. You might be able to see it down the oil fill hole- not sure on that. This is the "better" head that was used from mid 01 onward and isn't likely to crack. It may not be as good as the pre-331 heads, but its definitely better than the non-TUPY 331 heads.

 

[wsar10]

maybe it wont, maybe it will.....
just an added level of overall protection.
How can it hurt ?
Originally Posted By: Donald
Originally Posted By: wsar10
the 0331 casting were the one WITH issues.
I also have to add that I too have been around many 4.0's and involved in the JEEP world since I was a kid, and since 2000 I have "heard" of MANY "piston skirt failures"(another good reason to run a HDEO!) in forums although I personally have never had the issue. It seems that most guys think its an extreme version of the "famous" 4.0 tap and it turns out to be this, piston skirts and OPD failures seem to be the most common "major" failures for 4.0's 2000 and up.


How would an HDEO help with piston skirt failures?