Preventing a DMF failure

razorfox

Obukit wrote: »

Dual mass flywheels are designed to absorb vibrations from the engine, and so too much vibration, or too much heat, is what kills them. Most vibration tends to come at low revs, so it's important: -

- Not to try to accelerate from idle, or to pick a "gear of the day". Use the gears to accelerate. If the car is juddering, your DMF is beyond the maximum vibration it can absorb.

- Remembering not to ride the clutch, and to engage neutral and release the clutch rather than sit with your foot on the brake & clutch in gear at traffic lights, to avoid overheating the clutch and thus DMF.

I must admit I have felt that judder when I've ridden it hard but i I thought that may of been the DPF regenerating. I'll tone it down a notch or two!

mattyprice4004

Don't set off at very low RPM causing judder.
Don't ride the clutch.

Basically, a DMF is like anything else - looking after it will help but if it's going to fail, it'll fail.

Got 2 friends both with Mk3 Mondeo diesels and they're both on their 2nd DMF now - I drive a lot harder and I've never had to change mine despite having more power.

Some are more susceptible to failure than others - what car?

I'm on 190k on a Rover 75 CDTi, original DMF and no rattles.

razorfox

mattyprice4004 wrote: »

Some are more susceptible to failure than others - what car?

Ford SMAX 2.0 TDCI 58 Reg

Lum

(warning: post contains speculation, feel free to tell me I'm wrong)

I wonder if heat is the whole story here? Watching the videos of how a DMF works you basically get the output of the engine pushing against a couple of springs that then push on the next stage of the transmission i.e. the input to the gearbox.

Since a 4 pot diesel engine isn't smooth at all, this makes sense as the speed variations will be smoothed out by the spring, transmission turns smoothly, everybody is happy, until the repair bill arrives at least.

The problem I can see is when you lift off suddenly, now instead of the engine pushing a spring pushing the gearbox, you have the gearbox pushing a spring pushing the engine. At this point the spring will extend fully before being hit from the opposite end and compressed again. This process will happen again in reverse when you accelerate once more.

So basically you have a hot spring and every time you lift off you hit the end of it with the force of two tonnes of metal travelling at 70mph. Surely that will eventually make the metal brittle and prone to the dramatic DMF failures we've all seen on youtube.

If this idea, and I stress again that this is just an idea for discussion and in no way a statement of fact, is correct then things like double declutching before you lift off should prolong it. Riding the clutch wouldn't be so great as this will generate heat and increase wear on the thrust bearings.

The best way to avoid a failure would be to use a modern DMF/DPF equipped diesel car in the way it was intended, hauling up and down the motorway with the cruise control on and a boot full of pamphlets about fantastic lease details on photocopiers.

paddedjohn

I got nearly 200k out of my 1997 VW lt35 van before i had to renew the dmf.

andygb

This is quite a good animation of how a DMF works

http://www.youtube.com/watch?v=YnaXB8q3uzQ

They do seem to fail at indeterminate intervals, and although some manufacturers blame driving style, there are too many cases for that to be true.
The best advice if a DMF fails, is to have a SMF fitted instead.

Notmyrealname

Strider590 wrote: »

Don't know a lot about them in general, but I have a good idea of how they work and I know how i'd design one to work......

I would imagine they need to be "exercised", that means not driven around at 2k rpm for it's entire life.

So you in fact DON'T know how they work....

Driven around at 2k for their entire life would be a recipe for ensuring they never ever fail.

The most stress on a DMF is placed on them in 1st and second gear acceleration and when flooring it from low revs in too high a gear.

Town driving kills them the quickest so if you're mostly driving round town you don't really want a car with a DMF or a DPF either.

Other than that, just drive them normally. The Mondeo I had with one was on over 160,000 miles when I sold it and was still on the original clutch and DMF. It didn't do much around town at all in my ownership and judging by how blocked the EGR valve was at 90k when I bought it, had been a motorway cruiser company car.

Notmyrealname

C_Mababejive wrote: »

Why even fit them anyway? We have managed perfectly well for decades without them.

Because people want diesels but with the smoothness of a petrol engine.

movilogo

Town driving kills them the quickest so if you're mostly driving round town you don't really want a car with a DMF or a DPF either.

Well, that's the main point. You don't need to buy diesel unless you really do starship mileage (at least 20k/year).

Buy petrol and you don't have DMF/DPF issues. But most people nowadays can't think and only go by higher MPG figures of diesel.

mattyprice4004

Notmyrealname wrote: »

Because people want diesels but with the smoothness of a petrol engine.

Must admit, even my ancient diesel feels lovely and smooth thanks to the DMF.