348 Engine flywheel refurbish

Black and red, my favorite combo.

 

Good good you did not out grease on the center section.

 

Cool post, thanks


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Hard start after hot soak is usually caused by other things. How did you diagnose this one?

 

It is also caused by lack of, or very low grease in the flywheel on a 348. This was an accurate diagnosis.

 

Not really that many things on a 348 at least. Out of balance FW (kluber gone) often tripple seal related and need for starter relay are the 2 big reasons, are there more reasons? Starter voltage is easy to detect.

 

So, the engine was slow to crank, or the engine cranked fine but wouldn't fire?

 

Because of the unbalanced flywheel, the engine would crank erratically, which causes the ECU to think that it is not spinning fast enough and it does not enable ignition.

 

Hello

I take benefit of this topic since I was fancying regreasing my Mondial T flywheel because the hot starts were quit difficult.

However, after replacing both crank sensors and coil packs (because of misfire) it looks like the problem is solved.
I have also seen that the big rear flywheel ring nut has already been removed at least once so my grease may not be so bad.

I am having a slight rattling noise when stopping the car when hot but don't know if it is anormal or a sign of a bad flywheel grease.
I would like to know if you would recommend me based on the video regreasing it now or leaving it as is, because if it ain't broke, don't fix it

Here is the hot start/stop video:



Thanks!

 

Hey, I've got a '94 348 spyder. How do i know if i've got single or dual clutch setup?? Nice thread here.

 

It's single

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Can someone explain? Why only when hot? And why in general would imbalance lead to hard starting? As an engineer who once worked with rotating systems I'm not getting this at all.

 

If the crank sensors detect a jiggly cadence, they can't tell what the module should do next. Has to do with deciphering #1 trigger to get the counts going.

 

That doesn't explain why hot, not cold. Plus, there is already a "jiggly" cadence due to cranking speed variations as the pistons come up on compression. Imbalance won't result in jiggly cadence. It will just shift the center off mass off the rotational axis. Flywheel imbalance would be more detrimental at high RPM where vibration is an issue. I simply can not see how flywheel imbalance which doesn't shake the car to death at 6000RPM would have any effect on starting let alone be different when hot or cold. It's like saying an imbalanced tires makes it harder to push the car when the tires are hot.

[Edit; I'm assuming jiggly cadence means variation in rotational speed.]

 

When I bought my 348 (dual clutch), it had a rattle from the "pumpkin" on hot engine switch-off and the engine was difficult to start when hot. However, it would hot-start fairly easily if I engage a gear and keep the clutch pressed.

When I removed and opened up the flywheel, it was low on grease and there was a lot of gearbox oil mixed with it (leaking triple seals). This "mass" was probably able to dampen the flywheel when cold but not when hot when it gets "thinner".

After regreasing the flywheel, I no longer had difficult hot starts. Although the information is not available as to how exactly the difficult hot start is caused on the 348 by low/diluted flywheel grease, the fact is that the problem is solved with regreasing. It seems that the flywheel needs to be kept well dampened at all temperatures since the grease used for it is the type with very high stability at temperatures (stable viscosity).

What John a Barnes said above is probably correct. Difficult starts are also experienced on other cars with dual mass flywheels when something goes wrong in them. It appears that it is something to do with the resonant torsional vibration which is altered when the grease leaks out. If the resonant torsional vibration (which is much stronger than the normal crankshaft torsional vibration) falls within the crankshaft rotational speed at starting (cranking), this may produce abnormal induction in the crank sensors - instead of a "still" tooth passing the sensor, you have a vibrating tooth. This vibration induces additional higher frequency electric signal from the sensor, on top of the usual smooth sinusoid produced by a non-vibrating passing tooth and could confuse the ECU-s. In addition, it is known that the resonant frequency generated by a faulty (insufficiently dampening) dual mass flywheel can also prevent the crankshaft from spooling-up from the cranking speed - the flywheel and its "pulses", instead of helping the spool-up, act onto the crankshaft in the opposite rotational direction.

 

That may be a reasonable explanation and I did consider it, but a rotational resonance is not a balance problem. Perhaps we need to be a little more specific with the language used. Yes, I'm nit picking as it is obvious form the start that this is a damping problem not a balance issue. May I suggest that Tony said "out of balance" when he meant "undamped".

Moving right along, does this result is a spark issue or an injection timing issue, or both? The ECU is supposed to be able to compensate for a noisy signal from the crank sensor.

 

Here is the important thing, the only important thing. "Did it correct the hard start hot issue and was the the only thing done."
Hard start hot but starts fine when cools = Repack Voith?
I know the answer but recording it for posterity is important.

 

No. I would like to know why it corrects the hot start issue. There are plenty of cases where owners DIY repack the flywheel. I doubt they have the ability to balance them precisely afterwards. The damping issue makes more sense.

 

I was not referring to your post, it was more general, its always good to close a problem thread with the resolution.
I hate finding a thread with a problem I have where someone says they will try X,Y and Z then never went back to update if any of of worked worked.

But yes the dampening makes more sense than balance but when the grease is hot or infected with hydraulic fluid it will be more liquid and settle into a puddle and thicken as it cools. One would think when cold it would have the same balance/dampening issue or worse. It is odd. Its not as if liquified grease somehow spreads back out evenly, the Voith is vertical not horizontal. I still don't get it myself.