Bunk Flexplate vs. Exhaust Leak

Hi everyone.

I've recently been running into a pretty irritating issue on my XJ that won't go away. I'm hoping someone has some insight on which direction I should pursue next.

<Symptoms>
1. "tick" at idle
2. hard tapping/vibration under load (1500-2500 rpms), every gear except OD.

<Assumptions>
1. Bad flexplate
2. Exhaust leak (manifold/gasket/flange gasket etc)
3. I'm paranoid and just need to let this go.

<Findings>
I bought this Jeep in Nov/2015, and never noticed this noise/vibration until after I installed my SYE this past March. Almost instantly, I noticed that while under load (1500-2500 rpms) and in every gear but OD, there was this really apparent tap/vibration from right beneath my feet. As soon as I let off the gas though or as soon as it kicks into the next higher gear the vibration subsides a little. Then, completely subsides once in OD.

My initial thought was incorrect driveline angles with the install of the SYE, but the rear pinion yoke is correctly aligned with the TC side yoke. Just a week ago, I happened to come across a thread about loose flexplate bolts and cracks.

So, I pulled the cover this last weekend. All bolts were tight and I didn't notice any cracks. I actually pulled the bolts out, and re-tightened with loctite to spec. I did notice though, the holes in the flexplate that the 4 bolts go through are somewhat 'ovaled'. I didn't know if this is how OEM flexplate comes, or if those 4 holes should be as tight as the o.d. of either one of the 4 bolts. There was a slight bit of play in the flexplate (fore and aft) if I pryed at the flexplate teeth with a wrench (maybe 5-10mm of play). Also, I let the Jeep run with the cover off and there's no visible wobble - it seems centered.

I do notice that the tapping sound goes away a bit after warmed up or a long drive, although the vibration/tapping feeling never subsides. It only goes away when engine load goes back to 0 or once finally in OD.

<Question to the Forum>
So my question is, what do you feel is the next best step?

I originally was deadset on replacing flexplate and torque converter, but now am having doubts. I don't mind the work to do this job, but only if I know it's going to fix the issue. Seeing as this sound has sometimes just been an exhaust leak for others, I'm not sure replacing flexplate will do anything (although the fact that I have a vibration in my floorboard leads me to believe it is the plate).

...How would you determine if the culprit is an exhaust leak? I don't know of any "smoking gun" ways besides finding visually some crack etc.

...Can the flexplate still be bad, even if no visible signs of cracks or warpage?

...if I happen to pull the transmission, what other items besides torque converter, flexplate, trans oil pump and rear main seal are worth inspecting and/or replacing?

Maybe both issues are there and I need to fix each, but I can't stand to spend money and time fixing something only to find you fixed it for no apparent reason :banghead:

Thanks for the help!

I believe one location to crack at is the TC to plate bolts. Another location to look at is plate to crank bolts, I remember reading somewhere that one guy had is sheer complete and had extremely horrible driving problems. His shear completely.

Does the vibration tapping go away when you power brake?
Also Jeep 4.0 uses an old school style rear main seal. It's housed inside the rear main cap. Access by dropping the oil pan. Not your traditional tranny removal.

Dragonlich1961 said:

I believe one location to crack at is the TC to plate bolts. Another location to look at is plate to crank bolts, I remember reading somewhere that one guy had is sheer complete and had extremely horrible driving problems. His shear completely.

Does the vibration tapping go away when you power brake?
Also Jeep 4.0 uses an old school style rear main seal. It's housed inside the rear main cap. Access by dropping the oil pan. Not your traditional tranny removal.

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The vibration is only there when moving and accelerating in first 3 gears. So power braking it isn't noticeable, although the sound is still there. I did notice that power braking helps the tapping sound go away but it's hard to tell because the sound of the engine seems to drone out the tapping anyways.

I noticed as well that the tapping (sound and vibration) is very pronounced when in reverse. If I pop it in N, it goes away. Out it back in R and the sound is there again.

Now I'm really starting to lean towards a new plate.

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As it started immediately after installation of the SYE, I'd be looking at driveline issues first. Is the rear driveshaft new? rebuilt? used?

Also, did you have a TC lowering kit in it before the SYE that is now removed? If so, might want to take a look at engine mounts and/or transmission mounts.

The fact that it only does it while moving in gear (not in neutral or under power braking) also points to drivetrain issues.

Flyfisher said:

As it started immediately after installation of the SYE, I'd be looking at driveline issues first. Is the rear driveshaft new? rebuilt? used?

Also, did you have a TC lowering kit in it before the SYE that is now removed? If so, might want to take a look at engine mounts and/or transmission mounts.

The fact that it only does it while moving in gear (not in neutral or under power braking) also points to drivetrain issues.

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It's a Tom Woods driveshaft, with AA SYE kit.

There's no TC drop, although transmission and engine mounts are new (I just replaced in January).

I was thinking there might be some issues with the front DS pinion yoke, maybe I didn't torque it down correct upon re-installation of the SYE. I'll check this soon just to be sure it isn't something this simple.

Well, I have some updates. I'm happy I now know for the most part what the issue is and why. Not happy that I only got 50% done this weekend and now have no way to get to work tmrw lol.

I got the tranny dropped, and in doing so found that the one E14 bolt was looser than just finger tight. The other, completely missing. Also, one of the 17mm bolts that threads through the alignment dowels was completely missing. So I'm fairly positive this is why the flexplate took a crap. It's got a pretty noticeable crack around the center.





Question for those at home.....I'm wrapping up for the night and leaving the tranny disconnected with torque converter intact. Likely I won't get to this again until maybe Wednesday. Is it bad for the torque converter to sit intact with the tranny?

There's a slim chance I may not get to this again until Memorial day too being it that I'm spending Thur through Sunday in Indianapolis for the Indy500, so trying to plan ahead.

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If your flexplate is cracked, it will not get any better with time, so replacing it is pretty much a slam dunk. As for the torque converter you'd only replace that if it was showing symptoms. I am dealing with torque converter replacement right now. I too have limited time, energy and a gravel driveway to work with, so work is slow going.

My flexplate is fine, except that the backing plate/reinforcement plate was totally missing (my fault) when I took it apart. I was also missing a couple mounting bolts on the trans and one was working itself out pretty good...like just sitting there.

Since you have loose and or missing bolts on the TC/flexplate, that would explain both the cracking and the noises.

Not sure what you mean about leaving the trans and torque converter intact. The TC just slides out with very little effort as the only thing it's bolted to is the flexplate.

ParadiseXJ said:

If your flexplate is cracked, it will not get any better with time, so replacing it is pretty much a slam dunk. As for the torque converter you'd only replace that if it was showing symptoms. I am dealing with torque converter replacement right now. I too have limited time, energy and a gravel driveway to work with, so work is slow going.

My flexplate is fine, except that the backing plate/reinforcement plate was totally missing (my fault) when I took it apart. I was also missing a couple mounting bolts on the trans and one was working itself out pretty good...like just sitting there.

Since you have loose and or missing bolts on the TC/flexplate, that would explain both the cracking and the noises.

Not sure what you mean about leaving the trans and torque converter intact. The TC just slides out with very little effort as the only thing it's bolted to is the flexplate.

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I had read some information not too long ago that leaving the TC intact with the transmission puts stress on the input shaft and bearing when not directly connected to the flexplate.

I'm just a little paranoid lol (as mentioned in the original post), but it's probably not a big deal. I should have this all wrapped up by Memorial Day, or so I Hope.

Guess I should I add that I was planning to leave as much intact as possible to avoid any issues in putting back together. I.e., I wasn't sure how hard it is to get the TC splines to line up with the input shaft on the transmission. So I was planning to leave both attached for now...I guess I'll find out.

ParadiseXJ said:

If your flexplate is cracked, it will not get any better with time, so replacing it is pretty much a slam dunk. As for the torque converter you'd only replace that if it was showing symptoms. I am dealing with torque converter replacement right now. I too have limited time, energy and a gravel driveway to work with, so work is slow going.

My flexplate is fine, except that the backing plate/reinforcement plate was totally missing (my fault) when I took it apart. I was also missing a couple mounting bolts on the trans and one was working itself out pretty good...like just sitting there.

Since you have loose and or missing bolts on the TC/flexplate, that would explain both the cracking and the noises.

Not sure what you mean about leaving the trans and torque converter intact. The TC just slides out with very little effort as the only thing it's bolted to is the flexplate.

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Did you ever get a reinforcement plate?

Lol.......

TheSloMoShow said:

Lol.......

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WOW! Made in Canada!!

cruiser54 said:

Did you ever get a reinforcement plate?

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I did, yes. The OP says he has one...but it looks a little rusty.

Doubles