Hello to all,
I am a newbie and hope someone can help me. I own a 1989 s4 automatic and I just found my flex plate cracked again. This is the third one I have installed and I am becoming so frustrated. I have installed all three in accordance with the workshop manual...tightened all bolts to proper torque, tightened the front and rear pinch clamps to the recommended 66ft lbs and the last one I installed I even coated the spline with loctite. The previous two I had creeping so using loctite I thought would finally solve my problem. When I developed the dreaded sound a day ago I could only assume I had a torque tube bearing failing because when I dropped the bell housing I had no creep. But with further examination there was a crack in the corner of the flex plate. The car only has 85k on the clock and is in great running condition with the help of Roger Tyson and Tom at 928int. (replaced motor mounts, new MAF, both fuel pumps, new plugs & wires) so the car is running smoothly. Any suggestions or thoughts would be so appreciated!

 

I have NEVER heard of this happening... I don't recall a SINGLE thread on this...my guess is you are not doing something right...or you are REALLY putting out a TON of HP!

 

Could something be misaligned?

 

HAS to be something like that... 3 flex plates in less than 100k miles...hell, ONE flex plate in the LIFE of the car...a flex plate is strong enough to pull the crank through the motor! How to you break 3 of them?!

 

Maybe the bearings in the torque tube have migrated to different spots in the tube causing vibrations at certain RPMs putting more stress on the flex plate than it was designed to handle?

Dan
'91 928GT S/C 475hp/460lb.ft

 

You've ruled out the possibility of movement of the front and rear pinch clamps which could cause/allow flexplate movement so it leaves the possibility of axial movement forward of the flex plate (crank end float) or to the rear of the back coupling (transmission). Easy enough to check your crank float to rule that one out ... and you wouldn't think it would be transmission, because to transmit end float from there the torque tube bearings would have to be shot and probably the transmission would be complaining. The fact that you had the last two installs with creep of the couplings seems to point toward a problem of constant back and forth of thrust load on the driveline ... which could be crank end float ... hope not!! Just trying to think logically ... you have to rule out axial movement from elsewhere before you start thinking about misalignment .... which would have to be fairly severe I would have thought, and therefore fairly obvious to see. I'll be really interested to hear the outcome on this one.

 

Maybe that too. More likely than misalignment.

 

I've seen one broken flexplate (2 of the 3 arms broken) in a 928 that put down 538HP. Interestingly, we just found that its forward TT bearing had migrated all the way to the rear of the tube, although we didn't discover that until a couple of years after we replaced the flexplate.

 

Static force I suspect the flex plate is very strong, wiggle, and eventually metal cracks.

 

Another thought:

Could the engine go into limp mode every now and then - maybe unnoticed?

This would cause some serious vibration that has destroyed the shafts before - why not a flex plate in this case?

 

My money's on ...
1. TT bearing migration.
2. Crank end float.
or 3. Both.

Nicole ... you'd think the OP would have noticed limp mode but I guess that's a possibility also, as I understand it produces destructive harmonics, like TT bearing migration can.

 

What does it sound like (just for reference)?

In which corner (relative to the crank position and rotation)?

 

Hey Troy,

I have a dial indicator you can borrow to check the end play, but I need the thing that holds it in place. If I can find one on my way home tonight, you could come over tomorrow and check your end play. Then again, I don't know if you want to drive around too much with a broken flex plate. Either way, the offer still stands.

That sound was hard to describe. Basically a rat tat tat noise in sync with the motor, not noticeable while cracking, and was intermittent. Sounded like something rotating hit something stationary with each revolution and kind of tinny like tang, tang, tang, tang.

At first it wasn’t very load like an exhaust leak, rattling heat shield, or really noisy injectors, but later on it became much more noticeable.

 

That description of the noise sounds like I would expect from the two sides of the cracked section flexing back and forth against each other with each revolution .... which you would not think would happen with a flywheel that is true ... why would the plate flex with each revolution??

Just to rule that possibility in or out, as much as its highly unlikely, while you have the dial indicator it's worth checking the flywheel for run-out ... because that could definitely cause cracking of the flex plate.

Can't imagine how a flywheel could have or develop run-out and not have some other symptom ... but this is a weird problem, and you just never know ... and it's easy to check and rule out before you do the end float check.

 

Sounds a lot like a rod is ready to leave the block.

My bet is you have some kind of harmonic issue going on, it takes a lot of HP to just break one.