Low Revs and Preventing DMF Failure

Ultrasonic

Recently I have been doing some thinking and reading in relation to the effect of driving style on the likelihood of a DMF failure. One of the common suggestions is to not only avoid accelerating at low revs such that the engine is obviously struggling but also to avoid low revs in general (although what constitutes low revs is not usually quantified). My question is, if you drive a car at low revs but under conditions where the engine seems entirely happy (no juddering, rough sound etc) could this still be putting extra stress on a DMF and therefore increasing the likelihood of it failing? My suspicion is that it might be, but then how do you judge how low revs is too low?

To put this in context, my own car is a Skoda Octavia 1.9TDi. Once fully warmed up it feels perfectly happy pottering along a flat road at a steady (not accelerating) 30 mph in 5th gear (at about 1000 rpm), and doing so gives the best fuel efficiency under these conditions. But if doing this might increase the chances of a costly DMF failure then the fuel economy saving would clearly be a false economy.

DaveF327

I'd be most interested in the answer to this too. Since I bought my first diesel (and I'm as tight as a duck's !!!! when it comes to spending money on fuel), I also do the 30mph in 5th thing, as well as 50mph in 6th with my finger never far away from the cruise control. Occasionally, gradients cause it to judder ever so slightly before it recovers - it's a powerful 1.9 turbo multijet.

Is it also true that diesel particulate filters (don't know much about them) can be harmed by this style of economical driving?

Lum

General advice seems to be that the engine needs to get nice and hot to trigger a DPF regeneration and de-clog itself, and if this doesn't happen for long enough then the DPF will get so clogged up that it can't fix itself and will need to be replaced.

Because of this, people who drive diesels solely in a frugal fashion in city centres are experiencing DPF failures.

If you take it on a decent motorway run once a month or so, e.g. to visit parents, then you won't have to worry about doing anything special to look after your DPF.

vaio

I don't visit my parents that often, what can I do to avoid DPF troubles?

Ultrasonic

DaveF327 wrote: »

Occasionally, gradients cause it to judder ever so slightly before it recovers - it's a powerful 1.9 turbo multijet.

I think I'd be tempted to avoid that happening by changing down to 5th before a hill if I were you. (My car has a 5 speed gearbox so I've never experienced exactly this.)

My car has cruise control but I never use it. I can get better fuel economy without, presumably because I can anticipate when I may need to slow down, as well allowing my car to slow a little going uphill if I choose to. I also feel it is a little safer to not use cruise control, partly as I am in full control of the speed at all times, but also because my right foot is closer to the brake pedal. Although I'm sure if I used it more this would become less of an issue.

My car is old enough (2005) to not have a DPF so I don't have to worry about that thankfully. DMF and turbo issues are still quite enough to worry about though!

jaydeeuk1

Lum wrote: »

General advice seems to be that the engine needs to get nice and hot to trigger a DPF regeneration and de-clog itself, and if this doesn't happen for long enough then the DPF will get so clogged up that it can't fix itself and will need to be replaced.

Because of this, people who drive diesels solely in a frugal fashion in city centres are experiencing DPF failures.

If you take it on a decent motorway run once a month or so, e.g. to visit parents, then you won't have to worry about doing anything special to look after your DPF.

Didn't realise you answered post no. 2 rather than the OP.

Had a nice sarcastic reply lined up too

atrixblue.-MFR-.

2 things you should watch out for on a diesel EGR valve and DPF failure to regen and either failure can cause problems to both over a period of time.

mazda diesels are ones to avoid, a failed regen will put the excess fuel required for the burn back into the engine oil (who thought of doing that at mazda i will never know) and cause engine damage and failure.

theory DPF will regen at a certain point the DPF fill up, town driving or not as soon as the required level of soot is in the DPF it will start a burn, but town driving and low mileage prevents a full burn due to stop and start, and low end revs slow cruising will also prevent a full burn, the ECU reads these failures from first attempt to regen and throws a fault code after so many failed attempts to regen, by this time it can be so clogged that the DPF needs to be replaced.

many try and prevent this by booking a day or two a month for a motor way blast, a regen can take upto 15 mins to complete, driving over 50MPH at over 3500RPM at a steady pace but this doesnt garentee that it will do a full regen cyclle. manufacturers should put a DFP regen light on the dash in my opinion, so you know when its doing a regen so you can carry on doing what is required to complete it.

Ultrasonic

atrixblue.-MFR-. wrote: »

2 things you should watch out for on a diesel EGR valve...

Could you explain a bit about the EGR valve issue please?

Notmyrealname

Ultrasonic wrote: »

[/SIZE][/FONT] To put this in context, my own car is a Skoda Octavia 1.9TDi. Once fully warmed up it feels perfectly happy pottering along a flat road at a steady (not accelerating) 30 mph in 5th gear (at about 1000 rpm), and doing so gives the best fuel efficiency under these conditions. But if doing this might increase the chances of a costly DMF failure then the fuel economy saving would clearly be a false economy.

It won't help the DMF accelerating from 1000RPM in 5th.

Driving around anywhere at 1000RPM is just plain stupid because as soon as you touch the brakes, the anti-stall function of the ECU will fuel the engine to stop it stalling, thus fighting the brakes.

In addition to that, engine load has more of a bearing on fuel economy than engine speed and the load on your engine doing 30 in 5th with the usual urban varying speeds will be more than mine in 3rd.


I drive around town in my diesel Mondeo in 3rd. Far more flexible, fewer gearchanges, less stress on the DMF, no worse fuel economy.

Ultrasonic

Notmyrealname wrote: »

It won't help the DMF accelerating from 1000RPM in 5th.

I never do that. My question is specifically about travelling at a steady speed.

Notmyrealname wrote:

Driving around anywhere at 1000RPM is just plain stupid because as soon as you touch the brakes, the anti-stall function of the ECU will fuel the engine to stop it stalling, thus fighting the brakes.

Not if you depress the clutch pedal at the same time. Also, I didn't think anti-stall kicked in until idle revs were reached, which is about 800 rpm in my car. I've never had a problem with the situation you described FWIW.

Notmyrealname wrote:

In addition to that, engine load has more of a bearing on fuel economy than engine speed and the load on your engine doing 30 in 5th with the usual urban varying speeds will be more than mine in 3rd.

I'm well aware of the relationship of revs, engine load and fuel consumption (i.e. BSFC). However, the fuel efficiency in mpg (as distinct from engine efficiency, a measure of the power produced for a given amount of fuel) is higher at 30 mph in my car in 5th gear than 4th (or 3rd or 2nd or 1st). This is easy to confirm with the trip computer.

Notmyrealname wrote:

I drive around town in my diesel Mondeo in 3rd. Far more flexible, fewer gearchanges, less stress on the DMF, no worse fuel economy.

I seriously doubt that the fuel economy part of that sentence is true. Have you tested it with your trip computer (assuming you have one)? I would be very surprised if you don't get better fuel efficiency in 4th gear at the very least. The debate re. flexibility is one I'm aware of but if possible I'd rather not take this thread off topic with that.

Trying to get back on topic, I know that I get best fuel economy with low revs in high gears (and FWIW VW recommend exactly that here), what I am trying to work out is whether, with the engine showing no sign of distress, if it may still be causing long term harm.

atrixblue.-MFR-.

Ultrasonic wrote: »

Could you explain a bit about the EGR valve issue please?

By feeding the lower oxygen exhaust gas into the intake, diesel EGR systems lower combustion temperature, reducing emissions of NOx. This makes combustion less efficient, compromising economy and power. The normally "dry" intake system of a diesel engine is now subject to fouling from soot, unburned fuel and oil in the EGR bleed, which has little effect on airflow but can cause problems with components such as swirl flaps, butterfly valves , where fitted. Diesel EGR also increases soot production, though this was masked in the US by the simultaneous introduction of DPF's.EGR systems can also add abrasive contaminants and increase engine oil acidity, which in turn can reduce engine longevity