ive got a 2004 jeep grand cherokee 4.0
i hear alot about piston skirt issues i was also told that in 2004 they changed the pistons
in any case i get this noise kinda like a diesle at idle once the engine starts to warm up doesnt do it cold
oil pressure is good

 

my 2004 tj does the same thing its quit annoying and it only has 99k miles on it

 

i am starting to wonder because the more i drove it over time it seemed to make the noise more often and im starting to wonder if maybe a couple cracked the skirts and small chunks may be starting to clog up the pick up screen i took it off the road so i could pull the pan and do an inspection to see whats going on
funny thing is 1 month later i start the engine and the noise is very faint so im thinking the screen was getting clogged
i was told they revised the pistons in 04 but ive yet to confirm this,, in any case im pulling the pan tommorow and going to take a look and see if i have anything in the pan or pickup screen
then take a look at the skirts
ive yet to hear anyone wether online or at the shop give an explanation of why it almost sounds like a diesle at idle.

 

Worn pistons may slap at idle but this usually goes away once warmed up. A broken skirt however will be constant. Since you're pulling the pan you should know soon enough if you see a broken skirt. They break on the passenger side usually. How many miles?

 

Are you sure it's not lifter rattle? That's normal for a 4.0. I'd hate for you to pull the pan for nothing.

 

If he pulls the pan and looks it might not be for nothing. With slap, he should be able to rock the offending piston(s) at bottom dead center. Have seen where guys could demonstrate the slap this way. Or might even see actual damage to the bottom of the skirt. I am kind of waiting to see what he finds. Jeep famously uses the short, light weight pistons.

 

Well, that is the only true way to know for sure that's the problem. What's funny is I have the same problem in cylinder #5 with mine. The body is good enough to contemplate a replacement engine. I don't have the time or place to go thru it.

 

There is another way to know if that's the problem. Drive it till it blows up, that's my plan, which it may or may not do. Mine is fine when warmed up. It's such a known problem. Anything past about the 1996 years, is most likely just simple cylinder and piston wear, which will likely still drive for 100k miles that way. imo

 

The reason the piston skirts tend to crack is they use slipper pistons so the skirts clear the crankshaft counterweights at the bottom of the stroke. The cylinder barrels are short enough so the skirts come a long way out of the barrels without support. Every time they start back up, they tend to wobble until they get guided by the barrel. The only way to cure this is to make the block taller or decrease the stroke, which nobody want to do. What I don't understand is how a stroker stays together! Forged pistons might be better but then you'd have to rebalance the engine.