Response to whittingename/car juddering at high and low speeds

Answered your reply on post below .


" If the fault doesn't occur on roadtest or show on diagnostic , it's hard to fix or know where to start .You dont mention mileage of car ? Is it used for short journeys ? "

thankyou for your second response car done approx,
15,000 miles, mailnly used for short trips.
used for longer trips on holidays abroad 2-3 a year or occassional says out, thanks for your help, clare

If you do short journeys , sometimes " a Ferrari tune up " i.e a blast down the motorway at high revs for about 1/2 an hour works wonders .You can get problems with the DPF as explained here ,
though if bad , warning light should come on .


Diesel Particulate Filters (DPF)
Reducing diesel soot emissions by 80%

If you're buying a new car and plan to use it mainly for town-based, stop/start driving it would be wise to avoid a diesel car fitted with a Diesel Particulate Filter (DPF) because of the possible hassle of incomplete 'DPF regeneration'.

The exhaust emissions standards for new cars have effectively required fitment of a DPF in the exhaust of diesel cars since 2009 when the 'Euro 5' standard came into force. In fact, many cars registered before 2009 will have had one fitted too in anticipation of the change in standards.

Standards aim to deliver an 80% reduction in diesel particulate (soot) emissions but the technology's not without problems – AA patrols are regularly called to cars with the particulate filter warning light on indicating a partial blockage of the filter.

Even if your driving isn't mainly urban/stop-start, changes to driving style may be required to get maximum benefit from these systems.

How do they work?
Diesel Particulate filters (DPF) or 'traps' do just that, they catch bits of soot in the exhaust.

As with any filter (think of the bag in your vacuum cleaner) they have to be emptied regularly to maintain performance. For a DPF this process is called 'regeneration' – the collected soot is burnt off at high temperature to leave only a tiny ash residue.

Regeneration may be either passive or active.

Passive regeneration
Passive regeneration takes place automatically on motorway-type runs when the exhaust temperature is high. Because many cars don't get this sort of use car manufacturers have to design-in 'active' regeneration where the engine management computer (ECU) takes control of the process.

Active regeneration
When the soot loading in the filter reaches a set limit (about 45%) the ECU can make small adjustments to the fuel injection timing to increase the exhaust temperature and initiate regeneration. If the journey is a bit stop/start the regeneration may not complete and the warning light will come on to show that the filter is partially blocked.

Hope this helps .