Cracked Flexplate / Loose Flexplate bolts?

1988 XJ Base, 4.0L, AW4, NP231, D35/30, 2 In BB

I have a knock that sounds like its coming from back of the engine. I havent crawled under it to check to see if its coming from the bellhousing, but it sounds like that general area when listening from the top. I've searched many times for: rod knock, lifter noise, rocker noise, pushrod noise, crank bearing knock, wrist pin knock, piston slap, and flexplate cracking/loose tc bolts. After reading all the threads i could find on the above subjects, the closest thing to my problem seems to be flexplate cracking or loose tc bolts. However, none of the threads said anything about a few of the quirks that my particular engine has developed.

About four months ago i was sitting in traffic and i heard this ticking. It sounded just like an old vw diesel. After pulling away from the light I realized it was my XJ. Thinking it was just a little lifter noise, i changed the oil. The noise didn't go away, so i decided to pull the valve cover on the weekend to check things out. I never got the chance, the tick went away before the weekend. For the next couple of weeks it was very intermittent, it would do it for a few minutes once every couple of days. Over time it got more and more common, and finally it became a permanent ticking. About two months ago its slowly starting going from a loud tick to a loud knock, sounds alot like rod knock. Throughout this whole time, it was just a noise. My XJ idled just like it did before the noise. The gas mileage stayed the same, the oil pressure never went down, and the oil from my oil change when this all started is still clean. Thats what makes me think its the flexplate. The engine shows no sign or internal problems other then the noise itself.

But now its gotten worse. As it gets colder (SE Michigan), the knock has changed once again. The knock itself is still there, but it is now accompanied by a metal on metal scraping noise, and a deep rumble underneath the shifter area, but the rumble only shows up at exactly 1000 RPM and at exactly 2000 RPM. When it does this rumble, the whole truck shakes pretty bad. Its very bad when the truck is in gear, but it also does it when its in park or neutral, just not to the same degree. Also, sometimes, usually only when cold, it makes a howling noise. Try to imagine wind noise from a bad dool seal, but the XJ makes this noise at idle, not moving. The noise isnt a consistent frequency, the pitch changes seemingly randomly.

I also think its important to mention that this rumble was always there, but only at idle and it was very faint, barely noticable, and it happened once in a great while. Also it will knock on acceleration all the way up to 1500 RPM, then it stops. Also on decel the knock is very sharp and only betwenn 2000 and 1500 RPM then it stops entirely until idle, then the knock will start again.

Now for the truly bizarre part. Most of the time on cold start (after parked overnight) it will knock right at startup. But once in a while it wont knock for 2 -3 miles, then it starts. Also, once in a great while, it will stop knocking entirely while running. Sometimes a blip of the throttle and it will come back, sometimes it takes a few minutes to come back, and sometimes it doesnt come back until i restart the engine. This happens about once a week.

It runs just fine, thats whats so confusing about this. At any rate, im paranoid that one of these days what ever is making this god awful noise will just fail, stranding me in rush hour traffic. Since the noises started, i only drive to and from work (~10 miles), and always in morning and afternoon rush hour. The last thing i need is for it to fail in traffic, resulting in many angry people and a tow bill. Any help your guys could provide would be greatly appreciated. Anyone in the Rochester Hills, MI area that would be willing to come down and check it out would gladly be compensated (Beer or Cash).

Anyone got any pointers for me?

Take the inspection cover off and look at the flexplate-to-torque converter bolts. There are four of them, and they frequently loosen up, causing oddball noises.

Edit: Thanks for posting up your specs too. Many of the frequent offenders could take that as a good example of how to ask a question.

Is there a specific torque rating for those bolts, or should i just reef them.

Thanks for your help.

There is...but it's a pain to get to because the whole assembly wants to spin while you're tightening them. I can't find the value right now in my FSM, I remember them being buried in there oddly.

MANY of your symptons are exactly the same ones I had. E-1 said it was the TC, swapped it out and thats when we discovered it was the flexplate. Replaced it, along with the TC ( had it out anyways and now had a spare one). Presto ...noise was gone. Amazing how that thing can make so different noise, actually go to the point where you could hear me coming down the street
Ditto on checking the bolts and get a pry bar or long screwdriver and pry (carefully) on the flexplate and see if you have any cracks or movement(front to rear). Good luck.

Tonight after work I'm going to check that flexplate out. Couple of questions real quick. Does anyone know the size of the FP bolts off the top of your head. Also, I'm assuming that if i find the flexplate to be crack, i gotta pull the tranny to replace, or at least put a tranny jack under it and slide it back as far as i can?

Thanks for your help

IIRC the bolts are 14 mm. You will need to use a box end wrench on them. if you use a socket and breaker bar on the front crank pulley you can rotate the engine so you can get at each bolt and tighten them. If it has a crack it will be up where it attaches to the crank. You will need a powerful flashlight and the aforementioned long screwdriver/prybar to find out. the crack will go around the crank and be very close to it. A little loctite on the 4 bolts may keep them from loosening agian real soon.
BTW Greetings from the West Coast of MI !

We pulled my tranny, but you can do it by sliding it back a bit.

Yucca-Man said:

I can't find the value right now in my FSM, I remember them being buried in there oddly.

Click to expand...

Hah, I get to help Yucca-Man out

FSM for 94, but should be close enough. Data found in the torque specs at the end of the 4.0 engine section (section 9 p89):

Flywheel/Converter Housing Bolts 38Nm (28 ft/lbs)

also, if rsa42784 needs to go here:

Flywheel/Crankshaft Bolts 143Nm (105ft/lbs)

I've managed to torque mine using a big long 1/2" extension right up the driver's side of the engine against the oil pan. Pull the inspection cover, and spin the crankshaft at the nose until a bolt head is visible. Fit the socket, and put some cardboard and a rag up against the oil pan and bear the wrench head up against that until the wrench clicks. Repeat for all 4.

I think some Loctite Red on those threads would be a really good deal.

If you have to pull the trans to replace the plate, you will want an extra set of hands to help hold the crank still while getting those flexplate/crankshaft bolts out. At 105 ft/lbs (6 of them), they're a real booger to deal with (and in my case, the engine was out, so it was easier). Also, check the timing slots in the plate very closely against the one that came out of it, HO and non-HO (mopar vs renix) are different, and dealers have been known to mix them up. You don't want to have to do it twice because of a non-start...

Good luck with the fix

So let me get this straight. From what you guys are describing and from other discussions on the board, the flexplate is inbetween the crank and the tc. and there are six bolts on the inner ring that bolt to the crank at 105 ft/lbs and there are four 14?mm bolts on the outer ring that bolt to the tc at 28 ft/lbs. The TC bolts are accessable from the bottom, through the inspection hole, but only one or two at a time. I need to turn the crank clockwise from the front of the engine on the pully to access the other bolts, which i can do with the tranny in park.

One more question: During this proceedure will I lose any tranny fluid, or is this area dry?

If any of the above is incorrect let me know. In about an hour im getting off work, and im gonna give it a shot.

Thank your for your patience, i know im asking alot of questions.

prob. lose more skin off your knuckles than tranny fluid, you might want to wear a pair of gloves,if you can, while doing this, wrench slipped while doing mine, OUCH!!!

rsa42784 said:

So let me get this straight. From what you guys are describing and from other discussions on the board, the flexplate is inbetween the crank and the tc. and there are six bolts on the inner ring that bolt to the crank at 105 ft/lbs and there are four 14?mm bolts on the outer ring that bolt to the tc at 28 ft/lbs. The TC bolts are accessable from the bottom, through the inspection hole, but only one or two at a time. I need to turn the crank clockwise from the front of the engine on the pully to access the other bolts, which i can do with the tranny in park.

Click to expand...

Does all of this sound correct?

Thanks SteveC - that's why it was so odd to find; it's not in the tranny section but rather an engine piece.

rsa42784 - The bolts in question are the four that are accessible from under the oilpan when you remove the inspection cover. The picture below is my AW-4 after pulling it out to replace the bellhousing. The torque converter is still in place, and the four welded fittings on the face of it are where the flexplate bolts tighten.



The flexplate is still in place on the rear of the motor, and the six bolts mentioned above are on there, holding the flexplate to the crankshaft.

You can tighten the four bolts as mentioned above with a box-end wrench; it's a tight fit and you need to think about the angles you're using when you tighten. With the wrench 90* to the converter, you'll turn the assembly, but if you run the wrench closer to the converter itself you'll tighten the bolt. At any one time, you'll be able to see two bolts at the most - one on either end of the open inspection cover.

Hope that helps.

Ok, well i checked things out, this is what i discovered.

The bolts are actually 15mm or 1/2in. One of the four wouldnt budge, two others tightened about 1/8 of a turn, and one tightened about 1/4 turn. I checked for cracks, i didnt see any visible, but then again its not expected to be visible. However, and i dont know if this is normal, but when i checked for cracks (long screwdriver method) i discovered that the FP moves about 1/8" - 1/4". I dont know if it should be solid and not move at all, or if, as the name suggests, it should be flexible a little and moving as much as it did is normal. I checked for cracks after tightening the bolts to make sure any movement wasnt because of loose bolts. I checked it at eight places. Once under every bolt, and once exactly halfway between bolts. The movement varied randomly.

At any rate, i noticed on the 10 mile drive home that the knock has changed after screwing with the FP. The knock is completly gone at idle when its cold. The knock is also gone when its hot, but its been replaced with the faint tick that started this whole thing. Sounds just like lifter tick. Also the knock isnt regular anymore. I will try to explain this as best i can:

Before the knock was at a regular frequency : knock-knock-knock-knock-knock-knock-knock-knock-knock-knock ,

Now the knock is random : knock-knock-silence-knock-silence-silence-silence-knock-knock-knock-silence-knock-knock

It knocks only once per revolution of the crank, and now it skips revolutions randomly

Also, it used to knock on decel, now its doesnt at all, but instead knocks on accel all the way up to 2500 RPM instead of the 1500 RPM it did before.

The vibration i had at 2000 RPM has diminished a lot, i can barely feel it now, and the vibration at 1000 RPM is still there, but its random now.

Thats all i got for now, hopefully someone out there has an idea or something i can do to test it. I dont know what to think now.

Thanks for all your help guys, its very appreciated.

Anyone got any ideas? TTT

If you can pry the plate and see movement near the crankshaft flange, the plate is probably cracked. You really shouldn't see any movement in there.

I replaced mine just on general principle during an engine swap last month. Local dealer price was $105 (HO in my case).

Before you go and buy a new flexplate, contact me. I have a good one sitting in my garage. Remember the spacer that goes between the flex plate and the bolts. The flex plate bolts are not the same length as the bolts on a flywheel. If you use flywheel bolts, they will be really close to the block and shroud it not touching. Use loctite on the bolts.

SteveC - Thats the thing, i didnt really see the plate really move in relation to the crank flange (i couldnt see it very well, combination of light and the contorted position i was in at the time) but i in relation to the bellhousing it moved about 1/4IN in some places.

old_man - Thanks for the offer. If i need a new flexplate i will definately contact you.

Just some general information. When looking at the flexplate from underneath through the inspection hole, i noticed what looked like a ridge in the metal running from the center ring to the outside ring (radially) at two of the bolt holes. It almost looked like casting flash from when it was manufactured. I dont know if thats normal, but it could be a crack. As i understand it, the cracks dont form radially though, so i dont know. I think im gonna crawl under it again with the front end on ramps so can see everything better, and im gonna take some pictures this time. Hopefully with the extra room to work and a better light i will be able to see the FP better.

I also started the engine with the inspection cover off and crawled back under it. I didnt see any abnormal movement of the flexplate, but the knock was definatly coming from that general area.

If I understand the role of the flexplate, it shouldnt move at all, whether in relation to the crank flange or the bellhousing. But im not entirely sure, im hopeing one of you guys can answer that.

Thank you for your help.

rsa42784 said:

I think im gonna crawl under it again with the front end on ramps so can see everything better, and im gonna take some pictures this time.

Click to expand...

A photo would help.

Some movement at the edge is normal, at the center isn't. The radial ridge you referred to is curious. My observation was that the plates are stamped, not cast, so a ridge is suspicious.

rsa42784 said:

So let me get this straight. From what you guys are describing and from other discussions on the board, the flexplate is inbetween the crank and the tc. and there are six bolts on the inner ring that bolt to the crank at 105 ft/lbs and there are four 14?mm bolts on the outer ring that bolt to the tc at 28 ft/lbs. The TC bolts are accessable from the bottom, through the inspection hole, but only one or two at a time. I need to turn the crank clockwise from the front of the engine on the pully to access the other bolts, which i can do with the tranny in park.

One more question: During this proceedure will I lose any tranny fluid, or is this area dry?

If any of the above is incorrect let me know. In about an hour im getting off work, and im gonna give it a shot.

Thank your for your patience, i know im asking alot of questions.

Click to expand...

The bolts holding the torque converter to the flex plate are a LOT smaller than 14mm. So are the bolts holding the flexplate to the crankshaft.

I did one on jackstands by using one of those $49 Harbor Freight tranny jacks and just sliding the transmission back as far as it would go. Nuisancy job, but quite do-able. My job went a lot easier because I borrowed a tool that locks onto the flywheel ring gear teeth to hold it in place while you tighten the bolts.

If you buy new bolts from the dealer, they come with Loc-Tite already on them. I would not recommend red -- that requires heat to remove. Loc-Tite blue and the recommended torque should do the job quite nicely and still be capable of being serviced without using heat to take it apart.