If your car rattles at idle, judders when you pull away or knocks when you switch the engine off, the dual mass flywheel may be the culprit. It's a part many drivers have never heard of until a garage mentions it alongside a large repair bill.
Here's what a dual mass flywheel does, the symptoms of one wearing out, and what your options are.
What a dual mass flywheel does
The flywheel sits between the engine and the clutch. It's a heavy disc bolted to the crankshaft, and the clutch presses against it to connect the engine to the gearbox.
A dual mass flywheel, or DMF, is made of two separate halves connected by springs and dampers. The springs absorb the vibrations and pulses from the engine before they reach the gearbox, so the car feels smoother and quieter, especially at low revs. They're common on diesel cars, and many modern petrol cars use them too.
Over time, the springs and bearings inside wear. The two halves develop more free play, and the flywheel stops absorbing vibration properly.
Symptoms of a failing dual mass flywheel
Rattling at idle
A rattling or knocking noise at tickover, which often goes quiet when you press the clutch pedal, is one of the classic signs.
Judder when pulling away
A juddering or shaking through the car as you pull away, especially in first gear or reverse, can come from a worn DMF. It can also be caused by a worn clutch, so it's often hard to tell the two apart without inspecting them.
A knock when starting or stopping the engine
A heavy clunk or knock when you start the engine or switch it off can point to excessive play in the flywheel.
Vibration at low revs
Increased vibration through the pedals, gear lever or floor when driving at low revs in a high gear is another common sign.
Difficult gear changes
A worn DMF can make gear changes feel notchy or harder, although this has other possible causes too.
A slipping clutch
A failing DMF can sometimes cause the clutch to slip, or wear quickly. If you're replacing a clutch, the mechanic will usually check the flywheel at the same time. Our guide to a slipping clutch covers those symptoms.
What wears a DMF out
Like a clutch, a dual mass flywheel's life depends heavily on how the car is driven. Things that shorten it include:
- Driving at very low revs in high gears, which puts big pulses through the springs
- Lots of stop-start town driving
- Towing and heavy loads
- Harsh launches and aggressive clutch use
- Engine faults such as a misfire, which cause extra vibration
How is it checked?
A garage can check the flywheel's free play with a special tool, or by measuring the movement between the two halves once the gearbox is off. Some cars can show flywheel problems on diagnostic equipment through crankshaft speed readings. In many cases, though, the flywheel is only properly inspected when the clutch is replaced.
Replace it with the clutch?
Because the gearbox has to come off to reach both the clutch and the flywheel, it makes sense to replace them together if the flywheel is worn. That avoids paying the same labour twice. Our guide to clutch replacement costs explains how a flywheel affects the total bill.
What about solid flywheel conversion kits?
Some clutch kits replace the dual mass flywheel with a solid, single mass flywheel and a clutch plate with extra springs. They can be cheaper than a new DMF. The trade-off is that more engine vibration can reach the cabin and gearbox, and you may notice more noise and rattle, especially at low revs. They're not available for every car, so check what's offered for your model and talk it through with your garage.
Is it something else?
Rattles and judders can also come from worn engine mounts, a worn clutch, a failing release bearing or exhaust parts knocking against the body. A good garage will rule those out before recommending a new flywheel.
If the clutch pedal itself feels spongy or sinks to the floor, that points to the hydraulics rather than the flywheel. See our guide to clutch master and slave cylinder symptoms.
Finding the right parts
We stock a wide range of clutch kits, master cylinders and slave cylinders for most popular makes. Clutch parts are specific to the engine and gearbox, so match the part number on your old parts or the OEM number for your car. If you're not sure, send us your VIN and we'll help you find the right one.
This article is a general guide only. If you're unsure about the cause of a noise or vibration, have the car checked by a qualified mechanic.