msteiner805

Hello All,

Background:
Last month I bought a 1991 928 S4 with 90k miles knowing the car had been neglected and not driven much for several years. The maintenance history was there from day one, but, no details were provided regarding what was replaced and when - just stamps from dealers in the service booklet. I've always wanted one of these cars and decided to take the plunge into a potential project. I enjoy doing the work myself and I'm already having fun, I love learning about these cars. That said I have 2 small children and a great wife, so my time to do the work is limited.

About the current condition:
It runs fairly well, shifts smooth. When the car arrived off the truck, the engine started but was a touch rough after not being driven very much in the last three years. After a basic tune up (plugs, filters, injector cleaner) and cleaning the MAF, it is starting to smooth out. There is still quite a bit of stuff left to tackle (I'm planning on taking apart the intake, service the A/T, timing belt, etc etc).

The issue:
Last night, while checking the flex plate tension I came across something that is concerning, the flexplate is cracked. I took of the flywheel cover and noticed the top flexplate had cracks along 2 of the 3 the outer-most edges - all the way across. The bottom plate appeared healthy. I released the tension and got about 2-3mm's of movement. From being under the car and seeing the condition of the housing, I don't think the tension had ever been checked before.

Another symptom:
There is a noticeable vibration at 2800-3100 rpms along with a slight metallic rattle. This vibration happens in park, neutral, D, 3, and 2. Always in the same rpm range, hot or cold.

After doing some basic research I'm starting to suspect that the cracked flex-plate and harmonic vibrations are indicative of an underlying problem with the torque tube? Being a total novice with these cars, I'd like to get on the right diagnosis path

Regards,
Mike

granprixweiss928

check your exhaust that its not hitting the body or heat shields as well.

NC928S4

Flex plate definitely needs to get fixed. Vibration at those RPMs is probably Motor Mounts if they haven't been replaced.

FredR

Mike,

What you have seen is typical of the pre-cursor to very serious total failure of the engine- a problem endemic to automatics that we call TBF. The clamp cannot hold the shaft and as the motor spools up it twists the shaft and this in turn wants to pull the shaft out of the clamp albeit it small increments. Once it puls out it cannot go back in and eventually the thrust bearing in the centre becomes a disc brake and it melts the white metal of the thrust bearing that in turn can seize the centre main bearing to the crank, the bearing then spins in the case and wrecks it- game set and match.

By your description I would say you are in the early stages of this process similar to the position my motor was in back in 1999. The flex plate is toast and needs replacing and you will also need one of the established methods to stop the new clamp from moving.

The first thing you need to do is stop using the car and check the end float of the crankshaft that should be in the region of 0.2mm. The factory limit is 0.4mm- if you have more than this you may have problems.

Replacing the flex plate is not a massive job but it requires a significant effort. There are several alternatives/supplements to the existing clamp- I use the cheapest- Loctite because that is what I have done since the problem first emerged.

Back then I knew when my flex plate clamp had moved [precisely 3mm] because I had an annoying vibration at exactly 3050 rpm. In my case the drive shaft failed- probably due to fatigue caused by this vibration that had probably been going on long before I acquired the vehicle.

I think you have caught this in time- do let us know what you find. You should also look at the drive shaft just behind the splines at the end of the taper- you may possibly see signs of delamination- a possible pre-cursor to shaft failure [ but hopefully not].

Regards

Fred

msteiner805

Thanks for the quick replies - this forum is such a good resource.

Granprix,
Yes, will do. FYI I had to remove some heat shields and saw off the back 2 bolts of the flywheel cover to get the cover off (I didn't want to drop the exhaust). Sawing those bolts was a total pain in the a** LOL.. my wrist is sore this morning... but, in the process I saw that the exhaust was actually in decent shape. Possibly some minor leaks and loose bolts towards the back. I'll do a better inspection tonight.

NC,
Is there a way to check if the MM's are good/bad visually? I do intend to replace them, but, of what I could see last night, they seemed to be holding together and not squashed or broken. Not sure what to look for.

Mike

msteiner805

Thanks Fred - very good info and I'm hoping I caught this in time! But, yeah, no more driving until these issues are resolved.

I'm going to check the end float of the crankshaft and report back. Out of (morbid?) curiosity, if the end float is out of spec, what are the consequences?

I will do some research on replacing the flexplate along with a clamping mechanism.

"You should also look at the drive shaft just behind the splines at the end of the taper- you may possibly see signs of delamination- a possible pre-cursor to shaft failure [ but hopefully not]. "

Would you mind elaborating on that last part? I'm trying to visualize what this check entails, but, I'm not familiar enough with the drive shaft to understand what to look for.

Regards,
Mike

OTR18WHEELER

I replaced my cracked flex plate last year, not a terrible job, just labor intensive. Check the front bearing in the TT, it May have migrated rearward, stick a tape measure in alongside the shaft, the front edge of the bearing should be no more that about 4 inches from
The face of the square flange of the
TT.
Also locate a used flex plate, new one's are near $600.

OTR18WHEELER

I suspect the front TT bearing has slid back, if it's not separated. The lack of frontal support of the shaft will allow harmonic vibration, along with the pressure against the flex plate, will crack the flex plate.

polecat702

I'd be on the phone to Mark Anderson, for another engine. Keep all the usable parts off the one ya pull.

NC928S4

You can visually look for compressed MMs but it's not obvious since you've got nothing to compare it to. A good indicator is seeing if the intake has sagged below the Transverse Strut across top of engine. I have relatively new MMs and the top surface of the strut is level with top of intake. Hope this helps and you don't have TBF.

FredR

Mike,

Based on what you have told us initially I would think that you are in what I call the "pre TBF phase". This basically means you need to take action immediately to stop the Full Monty from occurring and if you do you will save the engine and a $5k or whatever replacement engine being needed.

My expectation is that you will find that your crank end play is OK. The 3mm movement generally does not seem to do harm and presumably so because the flex plate does what it is designed to do -it flexes. However, as with all things mechanical it has its limits and when this happens something fails- in this case the blades. This suggests to me that at some stage the extension has been more than 3mm. Maybe the PO experienced this and decided to reset it and then cut and run without saying anything hoping the next buyer would not know about this to even ask for an inspection of this point before purchase.

For sure something has caused the flex plate to fail but at this stage we do not know what and this is what concerns me in your case. To be clear when a TBF failure occurs the flex plate is usually completely fubarred and the two go hand in hand. To be sure that no major damage has occurred you need to do the crank end float check I mentioned. Once these things go they do so rapidly and more often than not the first the owner knows about it is when he finds his engine will not start because the white metal has fused and solidified on cooling down locking the crank. I very much doubt your motor was ever in that state but we need to eliminate that possibility from the investigation.

If you find the crank end float has taken off then that will be bad news indeed but let's not even think about that at this stage. Motors that have suffered terminal TBF very quickly grind out an end float of over 1mm as the thrust bearing and some of the crank thrust face dissappear then the crank starts to take thrust on the crankcase webs and by that time the thrust bearing has gone, the crank has gone and the casing thrust bearing journal has deep cracks in it rendering a repair not financially viable to all intents and purposes. Engines can still run like this but they are usually rather rough to say the least.

So, let's find out what end float you have and not worry any further until we know that status. Taking this measurement can be done in about 30 minutes with a dial gauge and a good mechanic. Until then anything else is speculative.

My involvement in this area started shortly after I acquired my first 928- the 1990 S4 that I sadly lost in a big crash due to some tw*t on a mobile phone some 9 years ago. That motor powers my GTS chassis today. The first I knew of any of this was when the drive shaft snapped behind the splines and with the help of our new to Oman Internet, I managed to make contact with this list and some of the top names in the 928 fraternity. This was when I learned of this appalling problem with automatics. I secured a brand new replacement drive shaft and following good advice, replaced the torque converter bearings whilst the transaxle unit was out. Not sure that this has to be removed to replace the flex plate but the job lot has to be displaced axially by several inches to replace the flex plate I seem to remember by which time dropping it out altogether is not that much more work [doubtless the troops will stomp on me if I have that wrong]. For someone who knows what they are doing I believe they can complete the job in a day. If the transaxle is removed the torque converter can be removed and the bearings replaced.

Given the flex plate cracked it did so for a reason and the question is what caused it to fail? It could be because of the migration you saw, it could be because of previous more severe migration before your purchase or it could be because of something else. Despite driving a 500 hp monster racing machine Mark Anderson once told me that no matter what he inflicted on the 929 manual transmission the one thing he could not break was the torque tube yet my automatic managed to do this at the end of the taper just before the splines start and apparently this used to be quite common- I saw this happen on two other 928's here before mine went. I figured that maybe the increased length of the assembly was transmitting this vibration at 3050 rpm and causing the stress raiser to fatigue fail. Many folks prefer the older type narrower drive shaft without the taper even though the shaft is more slender. If your case is anything like mine was, then you should examine the surface of the shaft at this transition point to see if there are any signs of cracking. if there is you will likely see a number of parallel hairline cracks on the surface that run axially a mm or two apart- again hopefully nothing will be evident.

Finally, whilst you have this lot apart it is also an excellent opportunity to consider whether to replace the torque tube bearings. Constantine [Black Sea Developments], besides offering an excellent clamp modification, also does a refurbishment kit for the two torque tube bearings - these are ball bearings races carried in a rubber sleeve. After many years the rubber fails, the bearings can and do migrate and that could be the cause of the vibration that did for the flex plate. irrespective of whether your bearings have failed or not this will be an excellent opportunity to consider this pre-emptive piece of maintenance *** upgrade. I would say you are looking at upwards of $1k in parts plus a similar amount in labour costs if you cannot do this lot yourself. Indeed I believe Constantine now does a replacement shaft as well but that increases the cost somewhat and hopefully not necessary.

If the thrust bearing has gone the examples that are comprehensively insured have a strange habit of catching fire.

Welcome to the world of 928 ownership! Trust the above helpful but do not be too disheartened [at this stage]. There is a cheap fix for the slipping flex plate clamp and you should try to pick up a used flex plate from 928 international or any of our heroes who may happen to have one lying around. You also need to check to ensure that the rear clamp [at the torque converter end of the shaft] is fitted correctly- plenty of posts on that topic.

Rgds

Fred

worf928

Don't scare the $h1+ out of the OP for no reason. Maybe I missed something but I see no symptom of TBF has been reported.

OP, measure your crank end-play and report back.

OTR18WHEELER

No doubt, all good, and true info, but don't automatically assume the worse.
You may luck out as I did with mine.
I did not know of this forum before I bought my 928 either, if it's happened to, or been done to a 928, someone here has it documented.
Also take several PICS of any repair area before and during dismantling
. digital film is cheap.

drwhosc

Kind of cool that you checked the flex plate, An important thing to do, just under the timing belt. You should consider dropping your whole torque tube and checking the rear pinch clamp as well. You might be able to fix the flex plate without dropping the torque tube, but you should check the condition of the rear pinch bolt, and replace it. A good seal back there is a good thing.

Also agree vibration is most likely motor mounts.

I too bought a neglected car. Lots of fun to work on, but have to have patience.

Good luck, and one thing I learned, don't assume the worse case, fix the simple more common stuff first.

worf928

Flex plate is easy to change. Unbolt the coupler from the flexplate. Unbolt the flexplate from the flywheel. Slide it out. Once you have the lower bell housing cover off there's nothing else to remove.