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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
Hi Mike, welcome to Rennlist.
You're already on the right path, and you've got plenty of information above to read and digest. Don't let it scare you - its just good knowledge to have about 928's.
In terms of your problem, my recommendation is to first, measure your crank end play , while you've got the lower bell housing and flex plate undone. The wear limit specified by Porsche is 0.4mm, so anything less than that is fine, although if you're close to it, I'd be measuring it at every oil change.
If the end float is within spec, then replace your flex plate with one that isn't cracked - see Dave's post above (Worf), and see if that fixes the noise/vibration. Measuring the distance to the front bearing in the tube is also a useful idea, but first I'd start with fixing the known broken bits (i.e. flex plate), rather than just diving in and removing the torque tube without further diagnosis.
Don't worry about anything else until you've done the above two steps.
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.
Another good way to check the MMs is to see how much space is between the oil pan and the crossmember.
You should be able to fit a finger in there. If not, they are "squashing."
If you can just barely slide a sheet of paper in there, then they are in need of replacement.
A possible cause is lack of manifold vacuum to the shift-firmness regulator on the transaxle causing all shifts to be neck-snappers. Add in excessive harmonic vibration due to torque tube bearing migration. Maybe some engine misfiring or perhaps some 4-cylinder mode running adding more weird vibration.
While you're under there measuring crank end-play and replacing the flex plate:
- you'll have to remove the pinch bolt on the front coupler. Once the plate is out slide the coupler forward and look closely at the drive shaft in the area where it is clamped.
- remove the rubber plug at the rear of the torque tube and inspect the drive shaft were it goes into the rear coupler.
Look for signs of damage on the drive shaft where the splines start. That's where the shafts fail.
Next, check the vacuum line to the shift modulator. It starts at the 7-way connector near the Mass-Air Sensor and snakes down, over the length of the torque tube and then to the transaxle. Make sure it is connected. Make sure it will hold vacuum. Make sure the 7-way has manifold vacuum (once you can operate the engine.)
A cracked flex plate is probably a symptom of other things being wrong. If the plate has broken due to excessive and/or weird forces on it, then unfortunately, the drive shaft is probably ready to snap too.
After I read polecat's response per finding a new engine from Mark Anderson, I got pretty nervous. I couldn't stop thinking about that... as another poster put it mildly "that scared the $h!t out of me" LOL. Anyhow, I just did the crank end play check exactly as Dwayne's Garage website/procedure describes and got 0.15mm of play. I did the check about 4 times making sure I applied enough pressure with my pry bar, held the micrometer correctly... I got 0.15mm consistently. I have to admit that I nearly had a heart attack the first time the measurement came back because I thought the play was supposed to be 0.011mm to 0.031mm to meet spec versus 0.11mm-0.31mm and I was 10x worse LOL. Jeez what a roller coaster. Anyhow, the stress is over and I'm relieved we are healthy.
So onto the other stuff I was able to do. I stuck a tape measure into the TT and took some photos as well.
A very promising start and what I expected to read this morning.
As I carefully advised you in my notes take it one step at a time and do not jump to conclusions as to what has or has not happened but it is important to understand the possibilities if the essential measures are not taken either by the previous owner or yourself as the current custodian. You will have a number of decisions to make as to how far you want to go preemptively and I strongly recommend you get as many inputs as you can.
The crank end play is perfect [0.015mm? - ha ha] so you can pretty much rule out the possibility that TBF has already occurred. Your shaft photo shows no signs of delaminations - you will just have to take a careful look all round the surface but again- looks good.
I do not know off the top of my head where the front torque tube bearing should sit but the distance shown on your tape does not look unreasonable- get someone to tick that box for you. If you do not know the history just note that at some stage the torque tube bearings will go and age is a major factor in this as will the torque converter bearings- they may have been changed already.
You would do well to investigate why the flex plate has failed. If the vacuum supply to the modulator on the gearbox is disrupted gear changes become very harsh as does the change if the modulator adjustment is set to fully stiff. As I advised in my earlier notes, with a good flex plate and a new torque tube my motor vibrated at exactly 3050 rpm when the clamp creeped that 3mm. Whether this type of vibration could cause the flex plate to fail remains to be seen- I simply could not tolerate to drive the car this way. Your motor mounts may well be shot- that typically induces vibrations at 1400 rpm and 2800 rpm and similarly the gear box mounts also fail with time- that is relatively easy to check for by visual inspection.
If all you have to do/want to do is replace the flex plate as I understand, this can be done with nothing else needing to be removed but I will leave that to others to confirm. Your next decision will be how to secure the clamp - there are a few options to consider. I use the Loctite method, it is not a perfect solution but it has served me well for the last 16 years.
Thanks for your replies - very good information. Let me make sure I understand correctly:
- The drive shaft end points (front/rear) need to be carefully checked for cracks/excessive wear. What would wear look like? Would it be obvious like cracking or something more subtle?
- Manifold vacuum to the shift-firmness regulator. This seems very interesting. I have no benchmark as to what 928 shifts should be like - this is my first time. We have an Audi A6 and a Volvo XC90 as our other cars, and both shift smoother than the 928. Come to think of it, the shifts are occasionally a bit clunky. The interesting thing is that while under the car I noticed a disconnected hose (maybe a 1/4", not sure as I didn't inspect the hose thoroughly) on the passengers side hanging down from under the MAF general area. I couldn't reach to trace it back but I will investigate today from the topside maybe take a photo. Seems suspect now that you mentioned the transmission shifting.
Thanks for your replies - very good information. Let me make sure I understand correctly:
- The drive shaft end points (front/rear) need to be carefully checked for cracks/excessive wear. What would wear look like? Would it be obvious like cracking or something more subtle?
- Manifold vacuum to the shift-firmness regulator. This seems very interesting. I have no benchmark as to what 928 shifts should be like - this is my first time. We have an Audi A6 and a Volvo XC90 as our other cars, and both shift smoother than the 928. Come to think of it, the shifts are occasionally a bit clunky. The interesting thing is that while under the car I noticed a disconnected hose (maybe a 1/4", not sure as I didn't inspect the hose thoroughly) on the passengers side hanging down from under the MAF general area. I couldn't reach to trace it back but I will investigate today from the topside maybe take a photo. Seems suspect now that you mentioned the transmission shifting.
Regards,
Mike
Mike,
The vacuum modulator line runs via one of the black nylon vac lines that sit by side of the MAF emanating from the multi way connector [7 point?] that sits in that location. The vacuum line can become detached and this results in a somewhat harsh shift that changes gear with quite a thud. The modulator is adjustable depending on whether you like softer but longer shifts or firmer but quicker ones. Mine is set a bit on the harsher side for firmer shifts.
Per the vibrations, its strongest in the 2900-3100 range, but not too bad. The vibrations would rattle a quarter on a flat plate if I had to describe it. I think this vibrations is causing the exhaust to rattle somewhere, hence the metal rattle I mentioned earlier.
You mentioned 1400 as well, and yes, there is slight vibration at that rpm as, although not as noticeable as at 3000. 800 - 1300, 1500 - 2700, and, 3100 and above is all really nice - smooth and solid -no vibrations at all. The idle point is stable (will have to get back to you on the exact number), but it is still a bit rough. Note that I haven't done much other than some lucas fuel injector cleaner plus a drive through 1 tank of gas. I believe the car sat for about 3 years prior to that so the injectors may have to be cleaned professionally.
Your description of the vibration would have me looking at the motor mounts - Roger does a good after market line in these [a Volvo spec?] at reasonable price. Porsche ones being quite a bit more expensive. Motor mounts would not IMHO explain the flex plate failure though.
You are probably going to find quite a bit of deferred maintenance will be required and the list of potential candidates can be scary but many of these items can wait until you have the prime suspects taken care of and generally they do not all go at once.
Besides your immediate issue with the flex plate and securing it, the other prime candidates [of course] are the water pump and more specifically the timing belt- most of those in the know recommend both items be replaced at the same time- I have put a new belt on a previously used water pump with no problems but I knew its [relatively short] history. With the timing belt if you do not know when it was replaced just do this now. If you are a dab hand at DIY consider doing it yourself- plenty of help/advice/tutorials etc on hand- just read the current thread from Adrian [Stratford Shark] in the UK. You really need a crank locking tool to do this.
It takes quite a lot of effort to sort a 928 but once you get there it is very rewarding.
Short list of candidates to be checked over and above those covered:- ignition wires, rotor arms/caps, fuel pump [plus in tank fuel pump -if you have one?], 3 key relays [type 53B], temp 2 sensor, MAF [accuracy], fuel pressure regulator/dampers, knock sensors [2], Hall sensor, crank position sensor, inlet manifold hoses, fuel lines [another biggy- the fire risk], accessory belts, engine wiring harness [they degrade with age/heat], water hoses/thermostat, and whatever else I cannot think of at the moment. if all these items are in tip top condition great but if they are it would be most unusual. Plenty written about this in many threads.
The other common failure is the LH computer if it has not been refreshed to date. These things generally either work or do they do not and when they fail you go nowhere.
There is a good chance about 25% of these items may need replacing now or in the very near future
What you have seen is typical of the pre-cursor to very serious total failure of the engine-
Originally Posted by FredR
Based on what you have told us initially I would think that you are in what I call the "pre TBF phase".
Originally Posted by FredR
A very promising start and what I expected to read this morning.
As I carefully advised you in my notes take it one step at a time and do not jump to conclusions as to what has or has not happened...
(Oh really!?)
Originally Posted by FredR
Your description of the vibration would have me looking at the motor mounts -
Fred,
I know you're trying hard to be helpful, but I think you need to take a step back on this one. It's like you see this as a "who can dump as much knowledge as possible in a single post contest." You win... but it's not on point, and worse, I'd be having a heart attack about now if I were the OP.
Your track record on this topic, so-far has not been good and self-contradictory. Let's not overlook the obvious... the FLEX-PLATE is cracked. Let's fix that, then move on to other things, if needed.
As you're working to figure out the issues with the car, please feel free to use our DIY Tech Articles as a resource to help you with your fixes. Good luck!
I just ordered a used flex plate from 928 international.
From the picture of the flex plate, it looks as if it comes with the clamp. I've read that the clamp will not come out easily, and, that one has to keep the blades with the clamp as they are balanced. Here is the thread:
Just recalled how a bunch of my rattles went away. New MMs will change the geometry of the exhaust system and move it away from heat shields, under carriage, suspension, etc.
Glad to see this thread move away from certain TBF death. When I joined the forum I was hit with the same TBF warnings and every damn rattle or creak had me fearing the worst. I'm well into year 5 doing proper maintenance and the TBF boogey man no longer scares me.
I had the same thing happen on my 87 S4. Cracked flex plate was from migration towards the engine. Checked everything, changed flex plate, put porken clamp in place and no issues. Changing the flex plate required sliding the transmission back towards the rear of the car. Good luck! Hopefully no serious damage. My flex plate caused a rattle. Rattle stopped with used flex plate.
04-29-2015, 09:28 PM
