Cold engine knocking??

My 96 4.0 with 122000 on it has slowly developed a knock over the past 20000 miles or so. It sounds like its coming from the block. Its not the normal lifter, valve tap or injector click that Ive heard for years. This is lower in the engine and sounds like a deeper metal to metal knock. The thing is, it only does it when the enigine is cold, much worse during cold winter startups. Once it warms up to operating temp, it goes away. Rod knock does the opposite and gets louder as the engine warms doesnt it? And piston slap is always there right? I havent ruled out a cracked manifold yet. Its hard to tell but it sounds like the noise is louder on the pass side and lower. The engine maintains good oil pressure at all times and runs well. My next step is to break out the stethescope some cold morning to try to pin point it better. Am I missing anything? Something I havent thought of?
I did a search but but didnt really find much concerning "cold" engine knocks.

Is your XJ an automatic? My '97 had some knocking sounds coming from the bottom end when I bought it. While pulling the engine to replace bearings and grind crank I found 4 loose flexplate/torque converter bolts. Torqued the bolts, hooked up everything else and voila, no knocking noise. An easy thing to check.

Sheepdog said:

Is your XJ an automatic? My '97 had some knocking sounds coming from the bottom end when I bought it. While pulling the engine to replace bearings and grind crank I found 4 loose flexplate/torque converter bolts. Torqued the bolts, hooked up everything else and voila, no knocking noise. An easy thing to check.

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Yes it is an auto. I'll give this a look see. Would a loose fleplate only knock when cold though?

Ray H said:

Yes it is an auto. I'll give this a look see. Would a loose fleplate only knock when cold though?

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Not sure if it would only do it when cold. If the bolts are only slightly loose the clearance between the flexplate and torque conv. could shrink as the metal expands.

Good luck.

strange. mine knocks when its hot outside or i've driven it a lot in city traffic. i thought mine did it because the oil is probably getting thinner as the temperatures go up, but who knows.
do you have plenty of oil pressure?

My pistons slap some, when it's cold. It mostly goes away after it heats up.

Could be piston slap...mine has it 'til it runs a couple of minutes....but I did have the flexplate issue....sounded just like the lower end or TC....'til the cracks got worse, then it did it all the time, super noisey.

What causes piston sap. I just took out my XJ and it was knocking pretty good. But after it ran for about 5 minutes it sounded like it completely went away. I'm pretty new to XJs as I just bought a 98 sport, automatic, with the rubicon 5.5 lift with deaver coils and rubicon leafs in November and have been tinkering on it with the help of this board.

If you are talking about the diesel sound in the morning, then i should say most 4.0 engines have it. I call my jeep the panzer in the mornig bc its so loud. As it gets warmer it should get a bit less noisy, tho its still a bit loud.

I think what you are describing is just that.

pete

Matt_S said:

What causes piston sap. I just took out my XJ and it was knocking pretty good. But after it ran for about 5 minutes it sounded like it completely went away.

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Piston slap is caused by the piston cocking in it's bore during the power stroke. It's aggravated by pistons that have short skirts (like most modern engines have), coupled with slightly generous clearances between the piston and bore.

As the piston heats up, being made of aluminum, it expands more than the cylinder, and the fit of the piston to bore becomes tighter. This, in turn, lessens the slap.

On the 4.0, pistons can slap annoyingly loud indefinitely with no harm to the engine. (Please don't infer that this means that ANY 'knocking' noise in a 4.0 is OK, but piston slap in a 4.0 is nothing to worry about.)

http://www.pistonslap.com/whatisit.htm
Apparently GM motors have it so bad they started a whole website for it...

Sheepdog said:

Is your XJ an automatic? My '97 had some knocking sounds coming from the bottom end when I bought it. While pulling the engine to replace bearings and grind crank I found 4 loose flexplate/torque converter bolts.

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I agree that's the most likely diagnosis and it is indeed an easy thing to check.

Not sure if it would only do it when cold. If the bolts are only slightly loose the clearance between the flexplate and torque conv. could shrink as the metal expands.

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That does sound logical.

Most likely piston slap and RESTORE available at most parts stores will take care of a lot of it. STP and Lucas did not help one bit. Restore claims to cut down or stop oil burning by filling in the worn spots on the engine but the same process lessens the slop between the pistons and bores. I had a 96 and was at Advanced Auto parts and while ideling in the parking lot I started to pour it into the engine. When half the can was in the engine it stopped knocking! Call it snake oil if you want but it really does work. That was almost two years ago and it still works but must be added at every oil change.
http://www.restoreusa.com/restorer.html

JoBo

Are there any draw backs to using the RESTORE product? Also can you use it if you also use a product something like ZMAX?

Matt_S said:

Are there any draw backs to using the RESTORE product? Also can you use it if you also use a product something like ZMAX?

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I don't know what Zmax is so I can't say about that. As far as drawbacks there's only one and it's minor and that's the price! It comes in 3 sizes, 4, 6 and 8 cylinder size cans and the price is around $7 for the 6 cylinder size. I read a article on a atv group talking about using it in a Ford ranger truck that used oil bad. They added Restore each time they changed the oil and the oil usage kept going down to not needing any between changes. The next day I tried it on my diesel sounding 96 XJ with 140k on the clock and was amazed. It still knocks a little at start up but completely goes away after a very short while.

JoBo

You can try MMO (mystery marvel oil) its been used since the beginning of time. Its realtively cheap, pur some in your crankcase and it should help. Remember this is an inherit flaw of the engine so whatever you do you will never get rid of piston slap completely.

pete