Dual-Mass & Lightweight Flywheels poor running and why??

The reason the dual-mass flywheel (DMF) is standard in the 993/964 is to address the engine‘s ignition-induced rotational speed irregularity cause-of-vibration in the driveline. At certain speeds ignition timing matches the natural vibrations of the driveline amplifying the vibration causing transmission rattle and body boom.

Dual Mass Flywheel failure is a less common issue for street-driven cars.  - IF your car seems to idle a bit too rough and you have been experiencing:

• The check engine light and;
• A reoccurring non-cylinder or;
• Multiple named cylinder “emission relevant misfire OBDC code;
• Some experience a rattle at engine shutdown or other noises.

AND you have addressed the other less invasive sources of this such as:

• Run the quick, unplug-the-primary-distributor, belt test,
• Verifying there is not a vacuum leak on the engine,
• Cleaning the idle valve,
• Replacing the distributor caps and rotors and,
• Addressed any cylinder-specific reoccurring misfire codes, wires, plugs, and injectors (done after the items above and a clearing of the codes with the misfire code still returning).

Then you may have a problem with the dual mass feature of the flywheel (DMF).

Testing the dual-mass feature of the flywheel:

I am told the litmus test for a failing DMF is to use the PSTII (Porsche tool), put the car on a lift, and with the car in gear, read the speed sensor directly. I'm told that it will rapidly slow and speed up out of sync with the engine RPM. Also, LuK offers a DMF testing tool that physically tests the flywheel once the transmission is removed. If the flywheel needs to be replaced lightweight non-dual mass flywheels are widely discussed on this board as a replacement option.

Lightweight non-dual mass flywheel replacements:

I researched the non-dual mass lightweight flywheel option and found that if you replace the flywheel with a lightweight single-mass flywheel be aware that you should expect a good bit of additional clutch chatter and driveline noises, per Luk testing data and technical social media postings. Some who have done this like the concept and throttle feel of the simpler flywheel and feel it is a good tradeoff for a perception of improved performance. Others have found the noise bothersome. Luk testing has shown installing a single mass flywheel can accelerate wear on engine/driveline components due to increased torsional impact loads to the engine and transmission. Also be aware replacing the clutch may be required when any flywheel other than the OE configuration is installed.

Because of the lower store of kinetic rotational energy from a lightweight (lower mass) flywheel engaging the engine from a stand-still without stall-out takes a bit more skill and attention. 

Some have experienced stall-out problems with the lightweight flywheel when taking their foot off the accelerator and the engine suddenly drops below idle and stalls requiring DME software changes to fully address. Certain 1995 and earlier models have a Motronic system that can not resolve this problem as configured from the factory.

Medium weight dual-mass & single mass flywheel replacements:

In addition to the above options, there are now available medium-weight flywheels in both dual-mass and single-mass configurations offered by either FVD, NineMeister or Aaseco These potentially offer a middle option between a light-weight and the OE weight flywheel and in the case of a dual-mass design an option to mitigate the noise issues associated with a single-mass unit.

In conclusion:

There is a reason why even the standard RS from the factory came equipped with a dual-mass flywheel. My gut tells me that if your car is tracked regularly a single mass flywheel or refreshing the DMF flywheel during a clutch replacement may be worth considering. However, street driven cars that run well but seem to throw a lot of random misfire codes, are not exhibiting a rattling sound when the ignition is switched off, and have passed the definitive test above should retain your existing installed OEM DMF at clutch servicing and look for other causes of the misfire related Check Engine Light.

Flywheel wear-out:

Beyond dual-mass feature issues, a flywheel can be worn out. It can be measured for this. The surface radial runout limit is 0.10 and the axial runout limit is 0.20. Typically the flywheel is good and need not be replaced at the first clutch replacement for most folks this is between 60K and 100K miles on the odometer. It is a common and possibly unneeded upsell item when having your clutch replaced so if you are asked to replace it with the clutch refresh quote be sure to ask the technician as to why as the need to replace it for wear can only be determined after it has been removed from the vehicle and measured.

Here is what Luk, an original equipment manufacturer of our cars' flywheel says about servicing dual-mass flywheels:

• "Dual-Mass Flywheels (DMF/DFC) - New retaining bolts should always be used when installing DMF/DFC, since they are stretch bolts.  
• Worn parts should not be reused, since the bearing race may be damaged by wear on the mating parts.
• Clean the mating surface of clutch pressure plates with a degreasing agent prior to installation. 
• Make certain that the clearance between the speed sensors and the DMF’s sensing pins are correctly set. - 
• Machining of the facing surface of a DMF is not recommended. 
• Using the incorrect bolts for securing the clutch pressure plate will cause noisy operation or failure of the pressure plate (scoring on the primary mass). 
• Also ensure that the locating dowels have not been forced inward, since this could also cause the before mentioned problems.  
• Check the engine timing sensor for damage. 

The following is allowed on some vehicle makes and models and have no effect on the operation of clutch components:  

• A small amount of axial movement is allowed between the primary and secondary assemblies, 
• The secondary assembly may be free to rotate about its axis when not under load and may not automatically return to its original position. 
• There may be traces of grease on the rear (engine side) of the DMF, extending outward from the sealing caps."

The original Luk source document for the above notes can be found here.

Be aware clutch and transmission servicing can be done on a 993 without removing the engine. In many cases, the labor cost for a full engine out can be avoided. If your technician is quoting an engine-out procedure to service the clutch, or transmission ask as to why as taking out the engine incurs a good bit of labor and involves disconnecting a lot of interfaces that may be better left alone in a 26+-year-old car.

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