Doctor Diesel

Six Questions

Letter one - Mazda6 (2)_cropped1I have a Mazda 6 2.2, 09 plate, Diesel Sport 180bhp. I do 25,000 miles a year, composed of 320 miles on the motorway every seven days, but then only ten miles a day to work.

This is the first car I have owned with a DPF (diesel particulate filter), and a mpg read out. My last car was an Audi A3 2.0 TDI on a 2004 plate and at 160k miles, it was traded in. Back to the question though!

My mind is in conflict, as if I drive to the best mpg figures, the DPF will not get hot enough (so I believe) to burn off the soot. Already I have had the DPF light on twice in 10,000 miles, and both times it had to go to the garage for a £130 regeneration.

Hence, I am asking, what is a person meant to do? Drive with the DPF in mind, or to the best mpg? Also, I would like to improve the motorway mpg, which averages 45mpg at present. Could the engine be chipped, and would this affect the DPF operation? What chipping would you recommend?

Paul Lockwood

There is, I suppose, some significant conflict between economy driving and exercising the engine well enough to avoid DPF problems.

Letter one - Mazda6 (1)_cropped1But you don’t say whether your two garage trips for DPF forced regeneration were when you were doing your present routine that includes 300-odd miles on the motorway every week.

If so, I’m somewhat surprised, and so I’m wondering if you have changed your habits in some way and that the DPF is now looking after itself, or whether this all happened when you were not doing such regular motorway trips. But there are ways of helping your DPF stay healthy without compromising your mpg much, if at all.

When you are accelerating up to speed, give the engine a good workout through the gears, taking it up to 2,500 to 3,000rpm in each gear. This approach particularly applies when you are maybe negotiating a series of roundabouts. When you are on the motorway, accelerate positively whenever overtaking, to get past things quickly, and give the engine some decent exercise, which will build up temperature in the DPF and keep the soot build-up down.

One of the problems is that, with 180bhp available from your engine, you are rarely using much more than a third of that power, and the engine, the turbo, and the exhaust system all generally run fairly cool, which will tend to generate more soot to bung up the DPF.

Whenever you stop on the motorway (at service areas) again get back up to cruising speed rapidly and use such an opportunity to use more engine power. As for “chipping”, there is a prospect of some small gain in fuel economy, but I doubt very much that you’ll save enough fuel to repay the likely cost of a £350 to £500 conversion that quickly.

Most of the tuning advertisers in Diesel Car have been around for a few years, but I would find one that has a fitting agent close to you, so you have the prospect of good after-sales service. Hope this helps your thoughts.

Doc Diesel

Paul came back….

The garage trips were during the motorway runs, but at 70mph the engine was only doing 1,900rpm, so that may well be the reason. I’ll follow your advice and hopefully put an end to the DPF problems.

I didn’t actually go out to buy a 180bhp car, but my long time source of used wheels, Audi, had become very pricey for low mileage three year old cars, so when my A3 needed replacing and I saw this 2009 car with 35,000 miles on the clock for £9000, with all the toys I needed for the motorway, and “bargain” I thought. I’ve decided I’m not going down the chipping route. Thanks again,


Confused of Connah’s Quay

Letter two - Kia Sportage_cropped1I have owned diesel cars for over twenty years now. For most of that time I have been a regular reader of Diesel Car magazine.

I am presently the contented owner (from new) of a two-year old Kia Sportage 2.0 CRDi AWD manual (in KX-3 SatNav trim). These days I cover some 4,000 miles per year of mixed, mainly rural motoring.

Although not the paragon of fuel efficiency, it returns 37mpg in the summer and 33mpg in the winter. My driving style has remained unchanged, using the engine torque at what I call comfortable engine speeds.

This receives the apparent approval of the computer display, which prompts gear changes (for economy) at pretty well the same engine speeds as my brain. So you may not wonder at my confusion on reading Doctor Diesel’s rpm recommendation for gear changes (reply to Ian, “Only available filter-tipped” in issue 312) of 2,000 to 2,500rpm.

Does this not go against the very reason most people buy diesels in the first place – fuel economy – facilitated by effortless torque at low to medium engine speeds? My 2.0-litre Sportage is quite comfortable (on the flat) at 1,500rpm, and with gear changes at or just above 2,000rpm. Does this mean that unless I change my driving style – and use more fuel – I will sooner or later run into DPF trouble?

If this happens, will the Kia seven-year warranty cover this? As if the above is not enough, we are now told about Euro Vl. Surely pumping (crystalline) urea into our exhausts will simply block up our DPFs with carbon twice as quickly? Perhaps all manufacturers will commit to assuring us this will not happen, and adjust their warranties accordingly?

David Sissons, Flintshire

The Editor has passed me your letter for reply. I’m afraid that, whilst diesel engines do have a lot more low-down torque than petrol engines, the engine speeds and amounts of torque often quoted are something of a fallacy, and the manufacturer quoted figures are rather optimistic.

They are optimistic not so much in terms of the actual maximum torque delivered, but the engine speeds at which maximum torque actually arrives. I am referring to a graph produced by the well-known and respected tuning company of Superchips (, for the 2.0 CRDi engine in a Hyundai Tucson tested on their rolling road dynamometer, before and after a Superchips tune.

The engine is very similar to the engine in your car, if not maybe quite identical, on account of the individual engine fine tuning done by Kia and Hyundai, which may differ slightly. These graphs are not atypical of most turbocharged diesel engines, and I’m also referring to one for a Volkswagen 2.0 TDI engine as a further example.

As you will see, the peak torque for the 2.0 CRDi engine is delivered, both standard and tuned, at around 2,500rpm plus. I could say “I rest my case” and leave it at that, since you will know that Kia quote their current max torque figures as “at 1,800 to 2,500rpm.” It is quite apparent that this is something of a romantic idea of the reality for, as you can see, the torque at 1,800rpm is around 225Nm (166lb ft) whilst it peaks at 290Nm (214lb ft) at around 2,700rpm.

As for the VW 1.6 TDI, you can see from the graph how little torque there is at 1,500rpm, and how Volkswagen’s figures that say that max torque is delivered at 1,500rpm are similarly somewhat difficult to accept. If you drive your Sportage between the 1,500rpm at which you say it is comfortable, and “just above 2,000rpm” when you seemingly normally change up a gear, you are not, I’m afraid, making the best of the engine, David.

I have been told by a highly qualified turbocharger engineer that most turbochargers blow hardest, and give maximum engine efficiency, at between 2,000 and 2,500rpm, and that the strong turbo boost at such engine speeds help keep DPF filters clean, while also helping to generate enough heat to make DPF regeneration work well. The change of driving style that you envisage will not use more fuel, and may even use less.

If you accelerate more briskly, as you probably will, if you follow my advice, get up to cruising speed quicker and spend less time accelerating, so whilst you may be using fuel at a faster rate, it will be for a lesser time. Engines are often best thought of, dare I say it, like the human body; eat little, walk slowly everywhere, and you’ll probably die earlier than if you get some regular aerobic exercise and get those muscles working!

So please keep an open mind David, try my suggested strategy, and see if you use any more (or probably even less?) fuel. Keep the engine generally between, say, 1,800 and 2,500rpm.

Accelerate with more gusto than usual, where it is not unseemly, and possible, and save your light foot touch for steady cruising speeds, and see how you get on. I’ll be very interested to hear from you, either way, and would appreciate an update after a couple of months, if you would be so kind.

I will conclude by agreeing with you about the annoying constraints of some emissions equipment though, although I can assure you that AdBlue urea type additives will not block up anyone’s DPF!

Many thanks for writing, whether we agree or not!


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