Jeep 4.0 knock when cold

Tripp

New Member
Joined
Feb 16, 2010
Location
Lincolnton, NC
New to the forum. But I have a question on a Jeep 4.0 that has me a little baffled. I just picked up a 96’ ZJ for a project. The engine runs great and makes good power for what it is. But when I start it up cold it has a knock in the engine. Sounds to me like a rod knock. But once the engine is warm the knock goes away. Oil pressure is fine idles at just under 40psi and rises to about 45psi or so at highway speeds. My only guess is maybe a lifter is not pumping up as I should. But I am baffled. Any ideas?
 
sounds like a happy and healthy 4.0 to me. Mine started doing it last year, and now it is pretty noisy all the time. doesn't burn much oil and has excellent oil pressure... just a I6 thing, no worries.
 
my 88 4.2 does samething. worried about it for a while but then thought if it takes a crap that would be a good time and excuse to go v8.that was 4 years ago and still no problem.
 
It worries the crap out of me. If I knew it was a lifter I would relax a little. I'm going to have to replace the rear main so when I drop the pan I'll have a little look see at the rotating assembly.
 
knockin 4.0

I have 3 yj's with 4.0's and they all knock for first 15 to 30 seconds depending how cold it at the time
 
I've had three Cherokee's with the 4.0 and just about all of them had 200+k miles on them with a bottom end rod knock. The last one had 195k on it when it was purchased and it had the same 4.0 knock. I just ran 15w-40 rotella and ran the piss out of it. It held up for 20k more miles until it was sold.
 
yeah i have a 92 yj 4.0L and it knocks when i 1st start it all the jeep 4.0l i have had did that its just a jeep think lol
 
My dad has had 4 different jeeps with the 4.0 & everyone of them did that & he never had any problems with them. One of them had over 300,000 on the original motor when he sold it & it's still running around town with the same motor.
 
our xj is the same way. when you first fire it up, it knocks for 20 seconds or so, then no more.
 
You could throw in a heavier weight oil. I have also heard that using MMO (Marvel Mystery Oil) works really good as well. but like everyone else is saying... 4.0's have always been pretty noisy engines and lifter tick is just something that is going to happen.
 
Typically that cold morning knock is "piston slap", which means your cylinder wall clearance is getting a little high. The piston swells back up with heat and the noise goes away and a rod would typically make noise all the time. Run it till it breaks or until your tired of hearing the knock.
 
I noticed what sounded a bit like a valve chatter when I did my last oil change and switched to autozone oil. I dumped it a couple hundred miles later and used mobil high milage instead (not full syn) and the noise was gone.

Might be worth trying some higher quality oil to see if it helps. In my case is was very clear the noise started in the mornings right after it's oil change though.
 
newcomerracing said:
Typically that cold morning knock is "piston slap", which means your cylinder wall clearance is getting a little high. The piston swells back up with heat and the noise goes away and a rod would typically make noise all the time. Run it till it breaks or until your tired of hearing the knock.
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Sounds about right. These engines typically wear out the rings long before they develop any bearing problems, does it have any significant amount of blowby in the air filter?

"She'll go or She'll blow" run it until it dies!!!
 
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