retcav1sg

I've searched and read more than I care to but am still not sure how "big" a problem TBF really is. I spent the better part of the morning trying to find someone locally that could do the flex plate/end play check here in Chattanooga, bottom line, no one will touch it and the indies that have any Porsche experience haven't heard of the issue. Garage 928 down in the Atlanta area said they could but they're 2 hours away. I would do it myself but am not that trusting in my ability. I have access to a lift but am faced with 2 scenarios:

1. Remove the cover bolts (drop the exhaust for the rear 2) do the check and hope I don't do any ancillary damage when the exhaust has to be dropped and hope I did the check correctly ( a mm here and there makes a big difference), BTW, would have to purchase the proper measuring tools.
2. Remove 4 bolts and cut the other 2 and leave them out, I told one shop this and the owner laughed. Still have to hope I measured correctly when I do the checks and not do any damage along the way.

The forum is littered with stories of owners diving into their cars to fix/prevent something from breaking and winding up with many new issues that weren't present before they started the work. My car runs great, no symptoms of anything. Pulls hard, no rattle/squeaks, everything (except radio) works as it should. History and previous owner indicate the car was never driven "hard"

So I have 2 question...Are there symptoms to watch for as far as impending TBF failure goes or does it happen without warning? I've read many threads where owners have done the check only to find everything was OK, I've read just a couple where owners who've done the check found major issues. So second question is...are we looking at a 100 out of 50,000 problem here or is it you're going to have TBF if you don't check it (they all fail) issue?

TIA and please be gentle
Mike

davek9

Hello Mike, if the car still has the "stock exhaust" w/ the CATS, you will need to disco the O2 sensor before dropping the exhaust else risk ripping out the wires

So you are looking at removing the six nuts from the down pipes to header's and supporting the hanging exhaust in order to remove the lower bell cover. You could remove the CATS, but most likely they will be stuck on the pipes and require a bit of work.
edit: if the CAT's are still in place the AIR feed tube will need to be disconnected too.

Once you can access the flex plate, look at it and see if it is bowed, mark it on the shaft and release the clamp tension by the bolt and watch it move rearward, make a note how far it moves.
Re tighten the clamp to 66 ft lbs.
As far as you questions, they all seem to move and the tension should be relieved to prevent TBF.

Dave K

Bertrand Daoust

Mike,

Have you tried to remove the 2 back bolts with the exhaust in place?

You might be able to remove the 2 back bolts with the exhaust in place.
Try that first.
Once removed, don't put them back.

Then you will be able to check the flex plate and the crank end play (if you want).
Don't forget to also check the back bolt on the shaft.
For peace of mind, I would put two new bolts.

FredR

Mike,

TBF has a degree of randomness to it and not everyone agrees about exactly what happens. As an owner who was one of the first to conquer the slipping flexplate clamp saga and still has a viable motor some 19 years later, I have written on this matter frequently. No one seems to know just how many examples have lost their motors due to TBF but I have what may be a unique experience in that I know how many examples were imported over here and how many of them failed- the percentage is alarming to say the least but the problem i have is whether my knowledge is significant statistically speaking given the relatively low number of examples.

There are two potential causes for TBF problems- one is slippage of the rear clamp and the other is slippage of the front flexplate clamp. I have seen 6 engine failures and all of them were caused by slippage of the front flexplate. The commonalities that I have seen were distorted flexplates, clamps slipped well along the splined shaft and relatively low mileages generally in the region of 60k km to 100k km. There are other factors that may have been in play and fact is that over here historically folks tend to drive such cars more in the style Porsche intended and we also have a very hot climate wherein things get somewhat hot under the bonnet as well.

The earlier type clamps with the shimmed connection do not seem to problematic and the problem is thus specific to S4 and later automatics although I understand some earlier models may have the later clamp design and some early S4's have the smaller diameter 25mm drive shaft. Interestingly all the failures I have seen had the 28mm drive shaft with the tapered ends to accommodate the earlier splined shaft geometry presumably retained to avoid having to redesign the front and rear clamps.

The later 28mm shaft design has a bad reputation for failure - commonly snapping at the base of the taper at either the front end or the rear end of the shaft. This failure is commonly perceived as being caused by an inherent structural weakness due to the taper causing a stress raiser. I have a slightly different theory which I will shortly address. In the meantime what you might be asking is what has the shaft failure to do with TBF? The problem I have with theories about shaft failure is that if the failure was due to an inherent weakness then why is that that not all shafts fail and many go on to do huge mileages?

Back in 1999 some 4 months after I acquired my first 928 [a 1990 S4] the drive shaft snapped one fine afternoon in the middle of the Omani desert in 45C degree heat and no mobile phones! Analysis of the failure showed that the 28mm drive shaft had snapped at the base of the taper. I thus had to fit a new drive shaft at quite some $$'s. Before the problem I had noted an uncomfortable vibration at precisely 3050 rpm. After the new shaft was fitted the vibration disappeared - for two weeks! About that time I gained access to the internet and promptly found this community. It then became apparent about just how common engine failures on the automatic S4's were and how it was seemingly being caused by flexplate clamp slippage. I immediately returned the S4 to our local dealers and with the car on the lift, we dropped the inspection cover and after marking the position of the shaft relative to the splines, we released the flexplate clamp and it immediately popped back about 2 to 3mm. That caught my attention. The clamp was reset and the supervisor made sure it was torqued to spec and checked with two different torque wrenches. Took the car away and the vibration had gone- for about two weeks then it reappeared! Back to the workshop and the paint marks we had placed over the splines showed that the clamp had slipped. I then asked Porsche what they were going to do about it and that I would not drive the car unless they came up with a solution. The local agents immediately contacted Porsche and within ten minutes they came back with an instruction to replace the bolt and over torque it to 110% of rated torque. Germany were adamant there was no problem with their design but how come they could respond with a solution so quickly if there was no problem? We fitted the new bolt and two weeks later I was back at the main dealers with the vibration issue- the problem was not resolved. I discussed the problem with two of my rotating equipment colleagues in the local oil company, one of whom suggested chemically locking the clamp to the shaft. About the same time, one of our fellow listers [Earl Gilstrom] came up with a procedure involving the use of Loctite 290 - I immediately applied his procedure my 928 being the 2nd example to have this done- it worked and held until I lost the S4 in a big accident in the summer of 2005. A year later I purchased my current GTS with a TBF failed motor in it and transplanted my S4 motor into it. The car has now completed 100k miles and we secured the flexplate with loctite- it is still holding to this day. The engine has not failed, the drive shaft has not snapped and I have no vibrations at 3050 rpm.

Now to relate all this to your query- the front flexplate clamp cannot hold the splined shaft in a given number of cases- the exact percentage of clamp slippage is not known but I suspect it is well over 30% and may be higher. The next obvious question is why does the clamp slip? Many examples hold some do not- did Porsche get the clamp spec wrong or do some examples simply not hold as well as others? We know that increasing the clamping force enables the shaft to be securely held. The ultimate mechanical solution is Constantine's excellently designed taper clamp- his lab tests showed that his clamp can apply orders of magnitude more grip than the stock clamp. No one who has fitted one of his clamps has experienced clamp slippage. I have used the Loctite approach for 19 years and had no issues and I have not heard of anyone who has used loctite report that it did not work. There are other auxiliary clamps that also appear to work well. Factors like operating temperature [more expansion] may explain why some clamps fail as could driver style - foot to the floor implies more direct loading. Whereas such factors could play a role, my favoured theory is that it is a random failure most likely caused by differences in production but presumably within allowable tolerances. At some point in its life, some kind of fretting possibly takes place and the clamp [having an inherently marginal design] eventually starts to let go under load on many examples. As the shaft slips the drive shaft length increases and the thrust bearing then becomes a disc brake of sorts. The driver is just not aware of what is going on and eventually one of two things happen- either the thrust bearing fails or the drive shaft fails due to the superimposed compressive load. In my case it seemed as though the shaft would only slip 2mm to 3mm but it was enough to snap the drive shaft. On other examples it keeps on slipping and the thrust bearing fails completely wrecking the engine when the thrust bearing white metal seizes and spins the shells causing the crankcase to crack- game, set and match!

Another snippet of information is that the 28mm automatic shaft only has two bearings installed. This made me wonder if in fact the shaft is vibrating and this in turn causes the shaft to migrate through the clamp possibly compounded by shaft bearings that move due to vibration or simply migrate and cause a vibration. Interestingly enough, I recently asked Constantine if he had heard of any drive shafts failing that had his clamp fitted- the answer was no. My clamp is firmly restrained and my drive shaft is intact. Stop the drive shaft clamp from migrating and TBF will not occur. Stop the flexplate clamp from migrating and maybe it will prevent the 28mm shaft from shearing. My theory is what it is- it may be spot on or might be a load of bollocks but to date it is the only theory I have that seems to explain all that many owners have experienced.

Bottom line- run a 928 automatic with the later style clamp without mitigations to ensure the clamp does not move and it then becomes a crap shoot as to whether your example will survive - "feel lucky- well do you?'.

Checking for crank end float is quite straight forward- use a dial gauge on the flywheel and pry the crank back and forth. A good example demonstrates about 0.2mm of movement. The thrust bearing retirement end float is 0.4mm and if this shows one has to wonder why as that should not happen. A TBF knackered motor typically exhibits in excess of 1mm end float at which point the engine is well and truly goosed.

Mrmerlin

As far as dropping the exhaust you need to loosen the 6 header to manifold bolts and remove 2 bolts on each side.
NOTE leave the longest bolts in with the nuts screwed on a few threads ,
this will let you drop the front of the headers down enough to put a 13MM box wrench onto the rear bolts of the belly pan,
Pay attention to your O2 sensor wire as you dont need to disconnect it at the CE panel as it should have enough slack, but you do need to monitor it.
The header pipes just have to be moved a lil bit so the bolts can be loosened and then remove them with the pan.
NOTE I put anti seize on all of the header bolts prior to installing them back so the next time they will come apart easily,
if yours are rusted then buy a new set of them.
ACE usually has these nuts and bolts.

For the rear pinch bolt check you need to remove the bolts that hold the 2 rear heat shields and push them aside
Use a hi quality allen tool that about 2 and half inches long so you can reach the cap screw 66 ft lbs is final torque.

note how much force is needed to remove the pinch bolt i will bet its loose.