FYI I'm cross posting this here. It looks like a lot more eyes on the cherokee forum than grand and I believe the engine is the same in both.

I recently got a free 1997 Grand Cherokee. Its only problem is a loud knocking that sounds like it's coming from the #5 cylinder. It quiets down at about 1700 rpm. It was parked as soon as it started to make noise. It is a 4x4 automatic with a 4.0 210,000 on the motor. Trans and transfer case were fairly recently rebuilt (about 20,000 miles ago). I thought it would be a rod bearing but oil pressure is perfect. It seems to drive and idle decent. I only drove it around the block though. I pulled the plug wires one by one while it was running and the knocking wasn’t affected. Don’t you usually see a drop in oil pressure with a worn bearing? Doesn’t stopping the cylinder from firing usually quiet a rod knock? I did a forum search and saw a lot of people had issues with the flex plate either being cracked or loose. I haven’t checked it yet. Could it be a lifter, valve spring? Something else? I’m going to do a compression test this weekend. What else should I check? Thanks for your input. ---Rich

 

The problem is a flexplate or a rod knock sound alot alike,like someone inside with a hammer trying to get out,lifters are more of ticking noise. I just changed the flexplate in a 2003 malibu and the crack was so small I had to use a magnifying glass and steel wool to see it,but it was loud. Disconnecting the plug wire may have a little effect but not much.

 

Does the flexplate noise stop as RPMs increase? I would think it would continue thoughout. My noise pretty much stops around 1700 or so. This is in park.

 

i had my flexplate in my xj only make noise when I accelerated,if the noise goes away at 1700 rpm I would point at the flexplate. It is possible though that as oil pressure increases it can cause a bad bearing to get quiter. One thing you can try is run a heavier oil and see if it makes any difference.
You can see your flexplate easy enough and if it has any cracks,mine started on a bolt hole and worked its way out.

 

If it goes away above 1500rpm then its a valve train issue. How do I know this? Because mine did the same thing.

I just went through this. My first thought was the flex plate, then rod knock and even a bad lifter issue. So common mechanical sense told me the flex plate would make noise through out the rpm range. Rod knock would increase with rpm as well. Thats where the lifters came into play, above 1500rpm my engine makes near 80psi for oil pressure, more then solid, and the noise drops off. Has to be lifters at this point.

What I did was switch to Amsoil 5w30 (its cold here) and add a qt of Marvel Mystery Oil. Before the switch I ran a half can of Sea Foam through the crack case for half hour of driving right before the oil change.

Now it has been getting progressively quieter.

 

Make sure the flexplate bolts are tight.

 

How can you identify a bad lifter? Visually? Is there a test I can do? Will it show up in a compression or leak down test?

 

How difficult is changing the flex plate, you have to drop the transmission?

 

I'm having a very similar knocking issue, it definately seems like a rod knock when I tap the accelerator and it progressively increases with the RPM's like a chatter. Not familiar with the flexplate though. Where is it located and look like? I'm new to jeeps so sorry for the dumb question

 

A bad lifter is just a tick kinda of like tapping a screwdriver on the valve cover,a rod or main bearing and a flexplate sound like someone is beating on the block with a hammer. There is a significant difference.

 

The flexplate is located between the trany and engine,it is bolted to the crankshaft,the flexplate bolts to the torgue converter,the flexplate is also what the starter uses to start the engine,it has teeth on it for the starter,it is the same thing as a flywheel in a manual except a flywheel is much thicker so it can handle the heat caused by the clutch friction. To remove it you have to remove the trany.
But you can inspect it by removing the dust cover from the front of the trany and using a flashlight you can see if there is any cracks,look from the center or bolt holes for cracks.

 

True for a lifter that is lets say half working. But a collapsed lifter, meaning its not building any pressure, will "knock" because it does not have the proper pressure to adjust to valve train movement.

Like I said before rod knock will increase with rpm. No way will a worn rod bearing quiet down with rpm. There is more stress being applied to the bearing as rpm increases.

To the op. Try process if elimination saving the flex plate for last. I would try a different oil with a detergent treatment before I go pulling the pan to look at rod bearings. To change the flex plate you need to remove the trans.

 

Try 20/50 to shut it up. Good Luck.