Flex plates rarely ever crack. I had one crack because the motor started to separate from the trans. It made a squeak sound at idle but nothing at rpm. You may want to post a video of the noise you are hearing. Post it on youtube and link it here.

 

I will disagree, while not everyday common, cracked 4.0 L flex plates do happen. A cracked flex plate is much more likely than bottom end 4.0 L engine issues.

I would be sure to have more than one shop diagnose the symptoms. Don't bias anybody by saying what you think, or another shop thinks is the problem, simply explain the symptoms. If you drive in and say "My flex plate is cracked", they will gladly replace it even if the flex plate may not be the root cause of the symptoms observed.

Have you ruled out cracked piston skirt or piston slap issues ?

 

Honestly, I'm not sure how to rule those things out. I've just been doing a ton of searching and reading through various sites, forums, youtube, etc., and so many people keep pointing to the flex plate so I started leaning towards that.

Any tips on how to rule out (or in) a cracked piston skirt or piston slaps?

There aren't any clicking or knocking noises coming from the top of the engine anymore. When I first purchased it there was a click coming from the top front area of the engine but I treated the oil with Sea Foam, changed it, and added Lucas oil treatment and that noise has totally gone away.

Also, I should note that there was no sound when I first turned it on this morning. It sat cold all day and night so first thing in the morning it was gone. After I revved the engine a few times it showed up. If that helps.

 

Let me take a quick video and I will post it.

 

Ok, here is a link to the video:

Would love your opinions!

 

going to be hard to say if it the flex plate without doing an R&R of the trans to have a look where that plate bolts to the crank. when they go bad they tend to spider crack at the bolt holes in the plate and just go from bad to catastrophic.. I have seen it even on fly wheels but on HP BBCs. once they are seen they must be replaced as when that lets go unless it behind a scatter shield it like a buzz saw at 7k+...

That oil could be from up top valve cover or a bad main seal. If that sound is say something on the crank. sometimes with good gauge you will see the pressure jump around. a short bit of hose can be used as a stethoscope to pin point the sound better.

 

Does the noise go away if you put it in drive with the brake on?

 

It doesn't seem to. It's harder to hear it once the vehicle gets moving or the engine revs up, but I'm not sure if that's just because it's spinning faster so you can hear the clanking anymore. But today as I drove it to the shop I could definitely hear it when I let off the gas.

Honestly, I was pretty terrified to drive it to the shop! The noise was loud! But we made it there safely so we will see what they can find out.

 

So I got it back from the shop today (had the diff gears changed, that's another problem in another post, lol), the tech swore up and down that it isn't the flexplate. He said he has a Wrangler and the sound just happens on these engines and his has a sound like it and he has been driving it for 60k miles without problems.

I don't know, it just didn't feel right. He said the way he knew it wasn't the flexplate was because the sound didn't "change" once you put it in gear. Can someone explain that to me? Is that a legit thing?

 

Hello The Goblin,
We are certainly sorry to hear about your recent troubles. As the Customer Care Team, we rely on the diagnosis of the dealer. If you still believe that there is a concern with your vehicle, we recommend visiting a second dealer for another opinion. In the event that you do visit another dealer, please let us know beforehand. We can certainly have a case escalated for additional assistance.
Laura
JeepCares

 

Is there any way to rule out the piston slap? Or broken skirts or anything on piston 6?

I used a stethoscope on it while it was in park and I can definitely hear the "tapping" sound in the bell housing of the transmission, but I can also hear it when I put the stethoscope up next to the top of the oil pan on the what I think is the bottom of the engine. Really hard to tell any difference of where it's coming from.

Suggestions? If I unplug the spark plug to cylinder 6, would the sound go away if it was actually piston slap from that cylinder?

 

Shorting out a cylinder will change the sound of a piston slap.

 

How do you short out a cylinder?

I just talked to the tech at the shop that is working on the Jeep at the moment and his assessment is that it is either piston slap or a bad main bearing.

So, anyone's thoughts on how bad this is? Sounds like piston slap is no big deal and pretty common with these engines, right?

What about a bad main bearing? Anyone have experience replacing one without rebuilding the whole engine?

 

Hey guys. I know it's been a few weeks since I last posted so I thought I would give an update and see if anyone has any advice.

I went ahead and drained the oil to see if there was anything in it and I found some metal shards along with some "glitter" in the filter. After getting some advice on another forum I went ahead and pulled the oil pan to check the rod and main bearings. Everything was tight. I couldn't find any play in any of the connecting rods or the crank shaft itself. Comforting, but confusing at the same time. I went ahead and removed the caps for the rod bearings on cylinders 5 and 6 and they looked a little worn, but everything was super smooth, both the bearings and the journals (thank goodness!).

Here are a few pics. Would you recommend changing them anyway? Or just put them back on and move to the next step in trying to diagnose this? I would think that if it was a rod bearing, due to how loud the sound is, there would be a pretty obviously loose connecting rod. But as I said, everything is tight.

A couple of people suggested that the metal actually came from the SeaFoam I put in the oil as it cleaned up 18 years of carbon deposits that released 18 years of tiny metal filings that were stuck in the sludge. A couple of short-interval oil changes would show if the problem is current, or if it clears up. Thoughts on that?

upper bearing (some streaking and discoloration, but super smooth), lower bearing (smooth, not a lot discoloration really), and the underside of the connecting rod where the bearing sits (discoloration, but super smooth).

The one thing that seemed a little odd to (although I have never done this before so I don't really know what's normal), is that the bearings were loose in the caps when I took them off. They just sort of slid around in there instead of clicking in tightly. The backs of the bearings were covered in oil as well. What are your thoughts on that?

 

flex plate to see cracks you would need to pull the trans and inspect it or engine what ever is easier, motor out now?. I have seen fly wheels get spider cracks on BB chevs that were used hard. i had a flex plate on a 7.3 TD do it from age and heavy loads. replaced it when the trans was built but it did not fail just had the spider webs starting.

to know how well those bearings fit you should use plastigauge, not just it feels tight. stuffs only a few bucks.

if you looking for a knocking sound i would think you in the end should gauge all rods and mains first. so you have better picture of the whole. before just replace them all. see where you are before you just go wack a moiling

the crank have the same lines/scratches? thats from dirty oil, you said sea foamed the block. yea that could send junk down the line.

metal in the filter is not good and shards?

 

flex plate to see cracks you would need to pull the trans and inspect it or engine what ever is easier, motor out now?. I have seen fly wheels get spider cracks on BB chevs that were used hard. i had a flex plate on a 7.3 TD do it from age and heavy loads. replaced it when the trans was built but it did not fail just had the spider webs starting.

to know how well those bearings fit you should use plastigauge, not just it feels tight. stuffs only a few bucks.

if you looking for a knocking sound i would think you in the end should gauge all rods and mains first. so you have better picture of the whole. before just replace them all. see where you are before you just go wack a moiling

the crank have the same lines/scratches? thats from dirty oil, you said sea foamed the block. yea that could send junk down the line.

metal in the filter is not good and shards?

The engine is still in the vehicle, I'm just working on the bottom end so unfortunately I can't get a look at the area where the bolts connect the flexplate to the crank. I may have to pull the trans and take a look, if I don't find a source of the knocking in the engine.

I went ahead and got some plastigauge as you recommended. Used it on the connecting rods for cylinders 6 and 7, both perfect. I pulled the rear main bearing cap as well since I need to change the rear main seal and the plastigauge says the gap is perfect there too. So I'm feeling pretty good about the bearings.

All the journals look great, super smooth with no scoring. The main bearing did have some crazy scratches in it over on the side by where it bolts on, like some sand got in there or something. The good thing is that it looks really recent and the crank feels just fine, so whatever it was, it got in and got out before damaging the journal, thank goodness.

I did reach over and check the timing chain and it is WAY loose so it looks like I will be changing that too! lol. I don't think that would be the source of the knocking though since the sound is coming from the back of the engine and not the front. But, best to change it anyway. It could be the reason for the metal slivers and glitter I found in the oil. It's pretty darn loose.

I got a recommendation on another forum to check the thrust bearing so I'm going to check that today. The crank is really tight and has no apparent play, but I'm going to check it a little more thoroughly today and maybe even pull the the thrust bearing and see how it looks.

So I'm left with a few plausible options for the knocking:
-cracked flexplate
-lifter noise

Someone did suggest a possible cam bearing, any recommendations on how to check cam bearings? I did reach up in there and grab a hold of the cam to see if it was loose at all, same as everything else, nice and tight.

At least I'm narrowing it down! Lol!

Thanks for the help!