Can I have some opinions on this knock?

88 4.0 AT 206,000 miles

https://www.youtube.com/watch?v=H_Tgptlh64g

Through a stethoscope, I can hear it loudest on the oil pan below the main seal. All the flexplate bolts are tight, and I can't see any cracks in it.

Thoughts?

That sounds like a rod knock or the thrust bearing is gone.

Thank you. I think I'll pull the pan off and take look as soon as I can find the time.

I'd pull the pan take the connecting rod caps off (mark them for position and direction). Visually inspect the bearing. Look at the top bearing also. Not hard to raise the piston (spark plugs out). You can inspect some of the cylinder wall for scuffs. Plasti gauge all the connecting rod bearings top and bottom. Check and see what is in the bottom of the pan. The hardest part is getting the pan off and cleaning the bottom of the motor. I use half a dozen cans of brake cleaner and let it drip over night. Check the side of the block, distributor side, for the codes for off size bearings. If you replace the connecting rod bearings, too tight is worse than a tad loose.
Even after all the work and you still have a knock you will at least raise the oil pressure 50%. With a T type cylinder mic you can do a decent measurement from the bottom and get an idea of what you have for cylinder wear. I've seen some sorry looking main bearings work just fine, I don't mess with them unless I'm doing a complete rebuild. Connecting rod bearings with black scorch marks means you have an oiling issue, worn through the coating in places is normal, scoring has to be severe to be an issue. IMO the 4.0 is pretty forgiving and was designed to last, not for race car tolerances.
Side note I've found some connecting rod bolts that were pretty darned loose, not much tighter than finger tight, always made me say Oh wow to myself. I've also found two number three caps in one motor, that was kind of a head-scratcher.

What is your oil pressure like, a mechanical gauge not a dash gauge?

Definitely use a mech gauge, I have one in tandem with the factory one and they rarely agree. I would like to ask a question along these same lines. I do have some slight rod knock, it sounds just like the last junkyard motor before a rod let go, im running rotella but as it gets colder my knock gets worse, I think the oil is just too thick when it's cold. My pressure is 60 highway, 20 hot idle, I was thinking of switching to a lighter weight to see if it helped. Any suggestions? It's something op could try as well.

I'd suggest a couple of things;
1. Piston rings stuck closed on the pistons can cause some piston knocking. Pull the spark plugs and spray Gumout carb cleaner down each cylinder to free any stuck-closed piston rings. Use one can of Gumout on the engine, let set overnight, replace spark plugs, fire up to loosen all piston rings. Proper functioning piston rings tend to center the pistons in their bores and reduce knocking.
2. If everything is OK, after you pull the rod-caps and measure bearing clearances, etc, replace ALL the rod-cap bolts with NEW bolts. Rod-cap bolts are subjected to "reverse-cycling loads", i.e.the rod-cap bolts fatigue. Therefore, once rod-cap bolts exceed their "fatigue-life" they start to permanently stretch and loosen up.

Best regards,

Thanks, guys. Great info.
I'll pick up a mech oil pressure gauge. Dash gauge indicates the oil pressure is fine, but, I agree, the gauges on the dash are pretty vague at best.

I was thinking I'd do a leakdown test before I take the pan off. This motor has 207k on it, and judging from the sludge under the valve cover, it wasn't always treated so nice.

I have a few more short trips before I can shut it down for a bit to work on it. I'll post up with my findings.

Oil pressure looks fine after testing with a mechanical gauge.

Cold 50 psi at 750 rpm
Cold 60 psi at 2000 rpm
Warm (180) 22 psi at 750 rpm
Warm (180) 50 psi at 2000 rpm

I guess I'll do a compression and leak down test next.

Hot oil pressure looks good as well.

Looking at the boss on the passenger side of the block just forward of the distributor, I read "707M31".

Does the M indicate .010 undersize? Or something else?

Hi,
Double check the flexplate. Those cracks are hard to see with it all bolted up. One way I was able to make the noise change was to accelerate with the transmission in D and then switch to N or R and I noticed noticeable difference in the noise. It ended been the flexplate.

Thanks. I looked at the flex plate very closely with a flashlight. I'll try your suggestion.

I answered my own question above. That is the engine day build code.

I'm starting to think that maybe it really is the flexplate. I put it in drive and hold the brake down, and over 1,250 rpm the knock fades out. Does that sound like a flexplate issue?

It's definitely louder and more prevalent the lower the rpm.

My compression numbers. Engine was warm.

Cyl. 1 135 dry
Cyl. 2 135 dry
Cyl. 3 121 dry
Cyl. 4 145 dry
Cyl. 5 133 dry
Cyl. 6 120 dry

Those numbers are not very good but most likely not the knock itself, you would have to have severely wasted pistons for that.

Factory service manual says 120-150 psi with max 30 psi variation. You're in there, but barely and on the low side.

I had a difficult time seen the flexplate crack. Take another look with a powerful flashlight. Look closely at the area by the bolts that go to the crankshaft. Keep us posted.

Thanks. I'd like to run a leak down test at least on cyl 3 and 6.

I also picked up a good condition OEM flexplate. Debating on whether to tackle that job this weekend or not.

I got the transmission off yesterday, and wouldn't you know that the aforementioned good condition oem flexplate that I picked up looks nothing like the flexplate in my jeep. The timing windows are completely different. I haven't pulled the old flexplate off for inspection yet. I hope to get to that later today.

Mine is 88 4.0, and the one I picked up is supposedly out of an 87. I'll post up pics later when I get a chance. I'm curious what jeep the flexplate I have came out of.

While I have it apart, I figured that I'd go ahead and replace the seal at the input shaft. Is that seal commonly referred to as torque converter shaft seal? I only see one option on RockAuto.

l always called them front transmission seal.