Dual mass flywheel symptoms: how to spot a worn DMF and what to do next

The typical dual mass flywheel symptoms are noises: a rattle at idle that changes when you press the clutch, a metallic clunk when you start or switch off the engine, and vibration through the floor when you accelerate from low revs. A worn dual mass flywheel (DMF) is replaced rather than repaired, and usually together with the clutch.

That last part is where most of the money is decided. To reach the flywheel the gearbox has to come off the engine, so you want to be sure the noise really is the flywheel, and you want to replace everything that is worn in one visit. At ATR Transmissies in Montfoort we separate flywheel, release bearing and gearbox noises with a test drive before anything is taken apart, and we measure the flywheel once it is out.

What does a flywheel do?

A flywheel smooths out the engine’s power delivery. A piston engine does not turn evenly: every combustion stroke is a push, and between those pushes the engine slows itself down. Without something to even out those peaks, a four-cylinder engine would shake rather than run smoothly at low revs. The flywheel is a heavy disc on the end of the crankshaft that stores energy during the power stroke and gives it back in between.

It also has two practical jobs. Its outer edge carries the starter ring gear, the teeth the starter motor engages to crank the engine. And its flat face on the gearbox side is the friction surface that the clutch plate is clamped against. So the flywheel is a vibration damper, a starter gear and half of your clutch at the same time.

What is a dual mass flywheel and why does your car have one?

A dual mass flywheel is a flywheel split into two halves with springs in between, so that engine vibration is absorbed before it reaches the gearbox. A conventional single mass flywheel is one solid disc. That worked well for engines that delivered their torque fairly softly and high in the rev range. Modern diesels and small turbo petrol engines do the opposite: they produce a lot of torque from just above idle, and it arrives in hard pulses. Dual mass flywheels were developed to deal with exactly that rise in torque at low revs.

The construction, as ZF Sachs describes it:

• The primary mass is bolted to the crankshaft and carries the starter ring gear.
• The secondary mass carries the clutch friction surface and sits on the primary mass on plain (slide) bearings, so the two halves can turn slightly against each other.
• Between the two sits a spring damping system filled with grease: soft springs for smooth engine starting and stopping, and firmer springs to decouple torsional vibration while you drive.

The result is that the torque pulses are cushioned before they reach the gearbox input shaft, gears and synchromesh. Think of it as a deliberate weak link that protects the rest of the drivetrain and keeps the cabin quiet. Like every weak link, it wears.

Dual mass flywheel symptoms

Most dual mass flywheel symptoms are sounds and vibrations, not warning lights. These are the four we hear most often in the workshop.

Rattle at idle that changes when you press the clutch

This is the classic symptom. With the engine idling you hear a rattling or clattering noise from where the engine meets the gearbox. Press the clutch pedal and the noise disappears or clearly changes character. On some designs it is the other way round: it rattles with the pedal down and goes quiet when you lift your foot. Which one you hear depends on the type of flywheel and where the play is. The point is the change: a noise that reacts to the clutch pedal comes from this area.

A clunk or knock when you start or switch off the engine

When an engine starts and when it runs down, it passes through its resonance range. A worn DMF with too much play then hits its internal stops audibly. It sounds like loose metal parts knocking together, short and sharp, exactly as the engine fires or stops. In our experience this is one of the most reliable signs.

Vibration through the floor at low revs

If you accelerate in a high gear from low revs, the flywheel does its hardest work. When the springs are worn out, the torque pulses reach the gearbox unfiltered and you feel a drone or vibration through the floor, the pedals and the gear lever. It often comes with a dull, low-frequency booming noise that fades as soon as you change down. If your car shakes or stutters in other situations too, find out what your symptom could mean in our guide to a car that shakes or stutters.

A dull rumble when you press the clutch

A worn bearing between the two flywheel halves makes itself heard when you operate the clutch. Not a sharp squeal, but a dull, metallic rumble that rises and falls with engine speed. Do not confuse it with the release bearing, which sounds different (see below).

Dual mass flywheel, release bearing or slipping clutch?

These three are mixed up all the time, and the diagnosis decides what gets replaced. A simple way to separate them:

If you suspect slip, our article on a slipping clutch explains a simple check. The release bearing and pressure plate have their own article too.

Keep the gearbox itself in mind as well. Rattling can also come from worn gearbox bearings or a worn input shaft. If in doubt, compare with the symptoms of a failing gearbox. In the end there is only one conclusive check: the gearbox comes off and the flywheel is measured for free play, rock (side play) and the condition of the friction surface. Those are also the two wear characteristics flywheel makers give limits for.

What kills a dual mass flywheel?

A dual mass flywheel is a wear part. Everything it does costs it a little. Some of that wear is unavoidable, but a lot of early failures have a cause you can name:

• Worn springs. They are compressed and released with every power stroke. Eventually they sag, break or wear into their guides.
• Lost grease. The springs run in a grease-filled chamber. Heat makes the damping grease ineffective, and once the springs and sliding shoes run dry, wear accelerates quickly.
• Heat from a slipping clutch. ZF Sachs lists clutch misuse, such as letting it slip excessively, as the cause of an overheated secondary flywheel, recognisable by heat cracks and blue annealing colours on the friction surface.
• A rough-running engine. This is the one owners rarely hear about. Sachs names irregular ignition and fuel injection, differences in compression between cylinders and faulty injectors as causes of DMF damage. If that cause is not fixed, a new flywheel is under the same attack.
• Driving at very low revs and frequent stalling. Labouring the engine in a high gear and stalling are on the same Sachs list. Towing, lots of short trips and engine tuning without matching drivetrain parts also cost a DMF visible life.
• Excessive load on the bearing. A release system that runs rough or a foot that constantly rests on the clutch pedal wears the bearing between the two halves.

That is why we do not just swap a rattling flywheel. We check why it failed: is there a misfire, an injector fault or a clutch that slips? Otherwise you pay for the same job twice.

Can you drive with a bad dual mass flywheel?

Usually for a while, yes, but it will not get better. The risk is that the secondary mass gets so much play that internal parts break loose or the friction surface cracks. ZF Sachs describes how, under extreme strain, destroyed internal parts can wear through the housing of the primary flywheel, leading to total failure. Then the car stops, sometimes with extra damage to the bell housing or starter ring gear. Have it checked as soon as the noise is clearly audible, and certainly before a long trip.

Can a dual mass flywheel be repaired or skimmed?

For cars, the practical answer is no. Most DMFs are sealed, welded or riveted units that were never designed to be serviced. Some companies open them and renew the springs and bearing, but balance, grease fill and sealing are critical, and this part sits where you do not want to go back a second time. ZF Sachs states plainly that the secondary flywheel friction area must not be reworked, so skimming a DMF is out. Replacement with a new flywheel is the standard route.

A conventional single mass flywheel is different: its friction face can often be skimmed flat, provided enough material remains and the starter ring gear is undamaged.

Why the flywheel and clutch are replaced together

To reach the flywheel the gearbox has to come off the engine, and that is the job. Once the gearbox is off, replacing the flywheel, clutch plate, pressure plate and release bearing adds relatively little time. Renewing only the clutch on a flywheel that already rattles means paying for the same labour again before long. The reverse is also true: an old, bedded-in clutch plate on a brand new flywheel wastes the new friction surface.

ZF Sachs goes further and advises always replacing the dual mass flywheel when the clutch is replaced for the second time, because the torsional damper in the flywheel is also a wear part. If the clutch is worn for the first time and the flywheel measures within tolerance, it can stay. Measuring decides. Our article on clutch replacement cost explains what the whole job involves.

Solid flywheel conversion: when it makes sense and what you give up

For many models there is a conversion kit: a solid single mass flywheel plus a clutch plate with stronger damper springs in the hub. The appeal is clear, because a solid flywheel has no springs or bearing to wear out. Still, it is not a free upgrade:

• Damper springs in a clutch hub cannot match the arc springs of a DMF. You get noticeably more vibration and gear rattle at low revs, especially on diesels. Even clutch makers that sell sprung clutch discs say they reduce gear rattle rather than eliminate it.
• The torque pulses the DMF used to absorb now go further into the gearbox and drivetrain.
• The car feels different: harsher take-up, more noise in the cabin, sometimes resonance at certain engine speeds.

Our view: on an older, high-mileage car used mainly for work, where comfort comes second, a conversion can be a defensible choice. On a car you want to keep for years, or an engine with a lot of low-down torque, we normally fit an original-type dual mass flywheel. The drivetrain was designed around that damping.

What decides the cost of a dual mass flywheel replacement

Most of the price is the labour to remove the gearbox, and that varies a lot between cars. The main factors:

• Access to the gearbox, including whether the car has four-wheel drive or a subframe that must come down.
• The flywheel itself, which differs a lot by engine and design.
• What goes with it: the clutch kit, a leaking crankshaft oil seal, the slave cylinder or new flywheel bolts (Sachs advises always renewing the crankshaft bolts).
• What we find once it is apart, such as an oil leak that has contaminated the clutch.

Contact us for a price indication based on your registration number and complaint.

How ATR diagnoses a rattling flywheel

We start from the complaint, not from the part. In practice:

1. You book your car in with the registration number and a description of when the noise happens: at idle, when starting, with the pedal up or down, at low revs.
2. We do a basic diagnosis with a test drive, listening to how the noise reacts to the clutch pedal and engine speed, and a read-out of the engine and gearbox control units. That rules out a misfire or injector fault that would also destroy a new flywheel.
3. You decide. Only after the diagnosis do you get our advice. Once the gearbox is off, we measure the flywheel and check the clutch, seals and release system, so only what is worn gets replaced.

The same applies to dual clutch gearboxes. Transverse DSG gearboxes, for example, are coupled to the engine via a dual mass flywheel, so a rattle or judder in a DSG car can be the flywheel and not the gearbox. Read more in our article on DSG clutch replacement. If the cause turns out to be deeper in the gearbox, we are a gearbox specialist and repair that too.

Book your car in and describe when you hear the noise. That alone often tells us where to look.

Do you run a garage and want a second opinion on a drivetrain noise or fault code? ATR Remote offers diagnosis at a distance for garages.

Frequently asked questions about dual mass flywheels

What are the symptoms of dual mass flywheel failure?

The main symptoms are a rattle at idle that changes when you press the clutch, a clunk or knock when starting or switching off the engine, and vibration through the floor and pedals when accelerating from low revs. A dull rumble when operating the clutch can point to the flywheel’s internal bearing.

Is it okay to drive with a bad flywheel?

For a short while, usually yes, but the wear only gets worse. A dual mass flywheel with too much play can break up internally or crack its friction surface, and then the car stops. Have it checked once the rattle is clearly audible, and before a long trip or towing.

What kills a dual mass flywheel?

Heat and irregular engine running kill most DMFs early. A slipping clutch overheats the secondary mass and dries out the grease. Misfires, faulty injectors and uneven compression hammer the springs. Labouring the engine at very low revs, frequent stalling, towing and tuning shorten its life as well.

Is it worth replacing the clutch and flywheel together?

Usually, yes. The labour to remove the gearbox is the same, and a worn clutch on a new flywheel or a new clutch on a worn flywheel tends to lead to a second repair. If the flywheel measures within tolerance at a first clutch change, it can stay in.

How long does a dual mass flywheel last?

There is no replacement interval. Driving style and use decide most of it: short trips, pulling away at very low revs, towing and engine tuning shorten its life, calm driving with enough revs extends it. In practice many cars need one at some point, often around the same time as the clutch.

Can you test a dual mass flywheel without removing the gearbox?

Partly. An experienced technician gets a long way by listening, linking the noise to the clutch pedal and ruling out the release bearing and gearbox. Hard proof needs the gearbox off: then free play and rock can be measured and the friction surface inspected. ZF Sachs notes that a full functional check is only possible on a test rig.

Does a worn flywheel set a fault code?

The flywheel is not monitored, so there is no fault code for it. On engines where the crankshaft position signal is read from the flywheel side, a lot of play can disturb that signal and cause misfire or rough-running codes that do not point at the flywheel directly. Our guide to transmission fault codes explains what codes can and cannot tell you.

Should I convert to a solid flywheel?

Only if you accept more vibration and gear rattle in exchange for a part that does not wear in the same way. It can suit an older work car. For a car you plan to keep, or a high-torque diesel, we recommend an original-type dual mass flywheel because the drivetrain was designed around its damping.

Hearing a rattle you cannot place?

A rattle at idle or a clunk when you switch off does not fix itself, but it is also not automatically a flywheel. Let us find out first. Book your car in at ATR Transmissies in Montfoort and you will know what is wrong before you decide on a repair.