The dual mass flywheel, or DMF, is designed to protect the driveline from the torque pulses and torsional vibrations of the engine. DMFs are fitted to cars and trucks equipped with a manual gearbox and is located at the end of the crankshaft where a regular solid flywheel would be. They are fitted mostly to diesel and high torque gasoline engines, and engines that don't run smoothly. Ultimately, its aim is to make starting the engine, idling, and gear changes smoother. Then can also smooth out surging when lugging the motor in too high of a gear.
Dual mass flywheels consist of two separate flywheels that are independently of each other. One is attached directly to the crankshaft, the other is the friction surface for the clutch to contact. Between the two flywheels, connecting them, are a series of springs which dampen the engine vibrations and prevent torque spikes from reaching the gearbox.
How reliable are dual mass flywheels? This all depends on your driving style.
Lots of stop and go driving, for example in traffic, puts more stress on the DMF because it’s working hardest when the engine is at low RPM and when moving off from a dead stop. However, allowing the engine to lug at low RPM in high gears also puts a strain on the DMF.
Symptoms of a worn or broken DMF include increasing vibrations, especially through the clutch pedal. A bad DMF can make the motor appear to have a misfire and rough idle as well. You may also hear a rattle, which will disappear when you depress the clutch. DMFs can last more than 100,000 miles but may go bad in as little as 20,000 - it depends on the quality of the part and especially your driving style. Drivers who are not smooth in their clutch engagement cause serious amount of wear every time they get behind the wheel.
Many mechanics recommend changing the dual mass flywheel whenever the clutch is replaced, because it involves no additional labor, and just the cost of the DMF to be replaced. If there have been enough miles accumulated to wear out a clutch, you have gotten plenty of miles out of the dual mass flywheel. There is really no telling when a DMF will go bad, as it typically happens suddenly. To leave it in place and change only the clutch is a false economy, because when it does go bad the labor alone to replace it is hundreds of dollars.