Diesels, DPFs and short island journeys

The short version

Every modern diesel has a particulate filter (DPF) that traps soot, then burns it off on longer, faster drives.
Most island trips are too short and too slow for that to happen, so the filter can clog.
A blocked DPF can mean a garage bill of hundreds, or well over £1,000 for a new filter.
If you mostly drive short hops, a petrol, hybrid or electric car is usually the better buy. Check your own driving.

What a DPF is, and how it cleans itself

Diesel engines make soot. Since around 2009, new diesel cars have had a filter in the exhaust that catches it, so it doesn't come out of the tailpipe. The filter is a ceramic honeycomb, and like any filter it slowly fills up.

To empty it, the car burns the soot off as ash. This is called regeneration, and it needs the exhaust to be very hot for a sustained spell:

Passive regeneration happens on its own during a steady, fast run, typically at 40mph or more for a good while.
Active regeneration is when the car's computer sees the filter filling and changes how the engine runs to heat it up. It usually needs 10 to 20 minutes of steady driving to finish.

The filter traps soot on every drive. It only clears it when the exhaust stays hot long enough.

Why island driving is a problem

Most island journeys are short: Newport to Cowes is about five miles, Newport to Ryde about seven, and summer traffic keeps speeds low. A regeneration that starts on the way to the shops gets cut off when you switch the engine off in the car park. Do that every day and soot builds up faster than the car can clear it.

An illustration, not real data. Short trips push the soot level up step by step. One proper run clears it, but only if it happens before the filter gets too full.

Island tipThere's no motorway here, but you can still give a diesel a proper run: a steady loop on the island's faster A roads, such as the A3054 between Newport and Ryde or the A3020 between Newport and Cowes, at the speed limit for 20 minutes or so. Many island diesel owners make it a weekly habit.

Warning signs of a clogging DPF

A DPF warning light on the dashboard, usually amber, often a box with dots inside. Read the handbook for that car: most say to drive steadily at speed until it goes out.
The cooling fan running after you switch off, or a raised idle and a hot smell after a short trip. These can mean a regeneration was interrupted.
Stop-start stops working, or fuel economy suddenly gets worse.
Loss of power or "limp mode". The filter is badly blocked. Don't keep driving it: get it to a garage.
The oil level rising on the dipstick. Repeated failed regenerations can dilute the oil with diesel, which is bad for the engine.

Steer clear of "DPF delete"Removing a DPF is illegal on a road car, it fails the MOT, and it can void your insurance. Be wary of any used diesel with a "remapped" or "DPF-off" history.

Checking a used diesel before you buy

Ask how it's been drivenA diesel that did a daily commute on the mainland is in better shape than one that has only done school runs on the island.
Look for DPF work in the historyInvoices for a forced regeneration, cleaning or a new filter aren't always bad, but you want to know about them.
Start it from cold and watch the dashboardNo warning lights should stay on after a few seconds. Check the oil level too.
Watch the exhaustA DPF-equipped diesel shouldn't puff visible smoke. Smoke on acceleration is a red flag, and an MOT tester will fail it.
Check the MOT historyLook for emissions failures or smoke advisories. How to read an MOT history.

Does a diesel suit you?

Diesels still make sense for some island drivers: people who commute over on the ferry, tow a caravan or a boat, or cover long distances for work. For short local driving, petrol, hybrid or electric is the safer bet.

Island stock today: 330 diesels, 637 petrol, 99 hybrids and 34 electric.