Isaacsracing

I've been trying to figure out the cause of major roughness in my 1992 968 track car. After a successful season of multiple days on the track with no issues, the second session of the day a few weeks ago I got major roughness on track that felt like I had belt coming through a tire, but immediately stopped when in neutral - so it wasn't a tire rotation issue. It seemed like an engine issue with misfiring. Inspection has found little other than a fuel injection wire that wasn't well insulated. I've changed plugs and wires and distributor which were all maintenance things that were good to do anyway. No change.
BUT here's the current symptoms.
1. If the car is not moving and the clutch is in (pressed), then the car is fine. Runs to redline as smooth as can be.
2. If the car is not moving and the clutch is out, then the car runs rough and gets considerably worse with higher RPMs.
3. If the car is moving and the clutch is pressed AND the car is in neutral, the car is fine.
4. If the car is moving and engaged, then it gets increasing rough with higher RPMs
5. If the car is moving and the clutched in pressed BUT the transmission is still in gear, then the car is also very rough.
I'm deducing that anytime the driveshaft is spinning, either driven by the engine, or driven by the transmission, then the car is really rough. Could the actual shaft be damaged? Or the LSD or the transmission?
Any suggestions to diagnose further would be really helpful.

951and944S

It's confusing whether you are describing an engine performance or a course "feel" that makes the incidents of "roughness" that you are describing.

When you depress the clutch pedal, you are decoupling the entire drivetrain and isolating the engine as an individual component thereof, so if everything is ok in this condition, it pretty much rules the engine out of the equation with maybe the exception of the disc itself causing some kind of misfire by being freed from the normal clamping of the pressure plate that would make a damaged portion of the disc's metallic base interfere with the timing sensor/flywheel position signal.

With engine running and the pedal released, the disc that's engaged with the flywheel via clamping force of the pressure plate, drives the shaft from the clutch disc spline all the way to he transmission but when the transmission is in neutral only the input shaft is spinning, not the pinion shaft or differential so that may at least rule out that question for you.

What you might try is, remove the rubber inspection plug (the largest one) at the rear aluminum flange that the transmission bolts to.
Remove the clamping bolts on the cast drive sleeve that joins the torque tube drive shaft to the transmission input shaft and slide it backward enough to decouple the two shafts.
If you run engine in this condition and release the clutch and you still hear/have the roughness, you have eliminated the transmission completely and narrowed down the problem to the torque tube's internal driveshaft and/or bearings or the clutch disc itself.

T

indes

I have a similar issue as described in #4 - When you're driving at a reasonable speed and take your foot off the gas, do you get a pulling back feeling?
Car feels fine while either the clutch is depressed or the car is rolling in neutral?

Possibilities;
(a) My torque tube bearings are going - I could hear them making noise when I spun the tube by hand and can hear them making noise while driving.
I think it may be causing drag when the transmission/clutch is engaged and in gear.
(When in neutral, the torque tube is spinning but not connected at the transmission, when the clutch is depressed, the trans axle drives the shaft at the geared ratio of the rear wheel speed)

(b) The transmission carrier bracket is deteriorated, causing the gearbox to sway when torque is applied and removed, this causes an oscillating "pull back" feeling as the shaft slides around

I don't really have issue #5 -- If the car is moving and the clutched in pressed BUT the transmission is still in gear, then the car is also very rough
That seems to indicate your trans axle needs attention. When was the last fluid change?

Mr Exotherm

Could also be a broken clutch center.

Van

I think it's a clutch disk problem.

Isaacsracing

Thank you, everyone, for your input. I replace the Trans fluid this season, so it has about 30 hours on it. The fluid I removed looked reasonable. The plug did have some metal on it. Here's a picture of the magnetic plug from the transmission at the last change:

T - I'm going to try to follow those directions to access the back of the torque tube. It's certainly an area of the car I'm not that familiar with.

I have been having trouble with the clutch, especially that last few days leading up to this particular failure/issue. Hard getting into reverse and second. I bled the clutch with no big improvement. Lately the clutch has engaged lightly right near the floor and then the rest of the engagement happens near the top of the pedal. So the clutch is certainly suspect. It's a SPEC Stage 3 clutch which shouldn't be worn out yet, but it makes sense that the "center" may be broken. Would the "center" be considered the Throw out bearing?

The clutch center seems like a pretty hard thing to get to. Is there any way to inspect this, or do I just head straight to the Clark's garage procedure, which starts with removing the exhaust and then the entire transmission in order to slide the torque tube to the rear?

indes

If you can't see anything from the little inspection hole where the slave cylinder drives the release fork -- you gotta remove the bell housing, which means moving the torque tube back, which means dropping the trans axle... (been delaying replacing my torque tube for a while myself because of the extra step of having to remove the transmission carrier, etc...)

.. that also seems like a lot of shavings in my opinion, but I guess it depends how long you go between fluid changes and how hard you are on the gearbox

Isaacsracing

Those shavings were the prior owner's. I changed the fluid in the gearbox upon purchase, so I don't have any idea how long had been. The transmission seems to be the most overlooked piece of this car in my opinion. The engine and suspension are great. The brakes are ok other than needing a better master cylinder. But the transmission hasn't had much done to it and the LSD is worn considerably. I'm imagining the torque tube is original.

What I can see in the inspection hole near the inspection cylinder is the spacing to the lever which gives a good idea of clutch thickness and it indicates that the clutch is the thickness of nearly new.

Is there anything else I should do before I rip the whole drivetrain out? That's where I think I'm headed.

Hey Van - do you have a video of this procedure? If not, want to make one? I'll bring you the car!

Van

This is probably a clue...


Unfortunately, yes, it's hard to get to. Not "difficult" to get to, but it takes a lot of steps.


You're in luck! Your clutch disk will look a little different, because this is a factory rubber-center disk. But I think you're going to find some damage on your disk that has caused it to go out of balance (like a chunk is missing, or one of the center strap rivets has come out...)

928FIXER

Just curious, did anyone read the full 1st posting by the OP?He does say that it is a 968.Clutches in 968's are easy.No trans to pull or tube that has to be pushed back.Kind of like a 928.

Van

Good point - I missed that. I still think the problem is with the clutch disk.

Isaacsracing

I just saw this thread about replacing a 968 clutch.: https://rennlist.com/forums/968-foru...placement.html
I find so much more procedural things for the 944, that I default to looking for 944 procedures. The clutch on the 968 looks very manageable! Thanks so much. I'll pull it apart and let you know what I find.

Isaacsracing

I have the next question. This time about the Torque tube. Mine looks pretty bad just from the outside. I saw a picture of one for sale on ebay that also has a seam and tape running down it. Please confirm from my pictures that mine is totally shot? This again is a 968, not a 944. But to remove the torque tube doesn't the rear suspension and transmission need to be removed, even on a 968?
Does this look like a factory failure or a rebuilt failure? Does a factory unit have those weld marks and tape? Is black Sea R&D the way to go for replacing this or are there other suggestions out there? Should I still be looking for a clutch failure also? Here's a link to a quick video too: https://photos.app.goo.gl/2rTN1mce8xasTyTx7


968 torque tube near the clutch end.


968 torque tube. exhaust and heat shield removed.

Mr. Toasty

To me, it sounds like the torque tube is the culprit. Still worth checking the clutch disk, of course, but aren't the split-case torque tubes known to have issues? I can't confirm one way or the other whether the tape is from the factory or not, though I suspect it is.

Isaacsracing

I started the process of removing the clutch today and found a few big issues.
This first picture shows the race for the front of the starter shaft and the ring that the starter uses to crank the motor. The first issue of the weekend when the car started its vibration problems, was that the starter wiring had a short and the starter ran for a 20-minute track session. The housing where the starter shaft sits looks damaged, but the damage looks old. The cog, however, has fresh silver showing. I wonder if the running starter caused that and some small imbalance that led to more problems.


Further along in the process, the procedure for a 968 clutch removal has you loosen this retaining bolt that holds a sleeve into the front of the torque tube shaft. That retaining bolt has clearly new damage as it has rubbed on the inside of the torque tube.


Once that bad bolt was removed the sleeve on the front of the torque tube shaft slides back and the spindle comes out of the center of the clutch. It's in pretty bad shape.


The torque tube shaft has lots of play once it is not supported by the clutch. Thoughts: Is this a clear torque tube bearing failure or is there more to it? Is something else misaligned in some way? The engine mounts and everything forward of the clutch look perfect. I will pull the clutch tomorrow and am thinking the throwout bearing will be damaged, but if it is, I'm not sure if that's a cause or effect.