Unknown engine knock?

4.0/ auto in 96 XJ with 123000 miles.
Last fall I started hearing a knock that sounded as if it was coming from low in the engine. Its not the lifters, or injectors. The knock can be heard inside the passenger compartment as well as outside. It increases and decreases with engine RPMs and it only knocks when the engine is cold, most noticably when the temp is real cold like on a winter morning. When the weather is warmer it can hardly be heard and after warmup, it cant be heard at all. The oil pressure stays steady at 50 when cold or above idle and 30-40 when warm at idle. Fresh oil doesnt change the sound. I went around the engine with a dowl but cant seem to isolate the noise although it does sound loudest when the dowl is against the bellhousing but now that its warmer out, its harder to hear. I pulled the inspection cover today and checked the flywheel/TC bolts and they were all tight (one of them was not as tight as the others but wasnt real loose either). I checked the flywheel well, by prying on it and looking closely with a powerful light, for cracks and found none. I also checked that the flywheel/crank bolts were tight, as well as I could without removing the tranny, by prying on it and then watching it for trueness while it ran and didnt see anything unusual.
I havent completely ruled out an exhaust manifold crack but Ive looked it over well and felt over it while running and havent felt any exhaust escaping.
Thats the history and situation.
My questions now are: If its a rod or piston slap, will I be able to isolate it easily with the dowl and exactly where should I listen and would it disappear when warm? Ive always thought that a rod knock got louder as the engine warmed up and oil got thinner.
If the manifold is cracked, where exactly do they tend to crack?

Sorry this is so long, but people (myself included) tend to get upset when only given part of the story. I did search but couldnt find my symptoms or an answer to my questions

Thanks

Either the flexplate bolts have worked loose or the flexplate itself is cracked. Remove the bellhousing inspection cover and take a look.

I looked at it today. The bolts between the flex plate and TC are tight. I inspected the FP well for cracks and didnt see any. I also pryed on it a little to see if it moved or was loose and it was tight. I also ran the Jeep with the inspection plate off the the FP is true and appears to be tight to the crank.

Ray H said:

4.0/ auto in 96 XJ with 123000 miles.
Last fall I started hearing a knock that sounded as if it was coming from low in the engine. Its not the lifters, or injectors. The knock can be heard inside the passenger compartment as well as outside. It increases and decreases with engine RPMs and it only knocks when the engine is cold, most noticably when the temp is real cold like on a winter morning. When the weather is warmer it can hardly be heard and after warmup, it cant be heard at all.

Click to expand...

This could be piston slap. As the pistons get hot and expand, the piston to bore clearance gets smaller and the ticking sound goes away. It's always worse on cold days when the engine's cold.
Worn rod or main bearings would cause a low oil pressure and a knock that's louder when the engine's hot.

mechanic buddy told me I had a bad motor so I decided to run it until I could get it fixxed, turns out I had bad carbon build up causing predetonation a bit when I first put the motor in, an over loaded trip west with 33" on 3.55's and the pedal to the floor the whole time cleaned out the motor good and its just now starting to make that slap/knock sound again...3 or 4 years later...I hope thats your problem too...try a good decarb system

That sounds reasonable. Can you explain exactly what piston slap is and what causes it? Is it the side of the piston hitting the rod or cyl wall?
I suppose theres not much I can do with that short of digging into the engine.

IIRC piston slap is when the piston wallows abit in the cyl...newer xj's have teflon(?) coated piston skirts...everyone used to say the noise was piston slap but more and more I think its carbon, I'm not a mechanic though.... HTH

I have piston slap, it only last a minute or so when cold, sounds kind of like a rod, but its "slap"

Ok, Ive read up on piston slap. I now have a better understanding of what it is. I feel its likely that piston slap is my problem. It would seem theres not much, short of tearing the engine apart, boring and fitting a new piston, that can be done. Im not going to worry with it.
I actually found a sound clip of piston slap in a GM V8. You can tell its a V8 but the slap sounds identical to what I hear from my 4.0.

So can you give then link to the audio clip?

Hers the link. It worked last week but I get an error message now.
http://www.pistonslap.com

Those years (94-96?) are bad about the piston slap issue. To me it kind of makes the engine sound like a diesel. It is "normal", and should not adversely effect engine life or peformance. I think the factory issued a bulletin about the matter, but it's been a long time ago and my memory ain't getting any better!