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Car Buyer's Guide: Stretch your diesel’s legs regularly
By Jack Biddle • 05/09/2015
Think carefully about purchasing a diesel-engined vehicle
A mate did what he thought was the decent thing recently and offered to occasionally start a family member’s near-new diesel-powered SUV while they were on an overseas holiday.
He didn’t want to, or feel the need to take the vehicle for an extended drive in their absence. Instead he went to their house weekly, started the vehicle, backed it out of the garage to stop unpleasant diesel fumes drifting through the house, and left the engine to idle for around 10 minutes.
The keys were handed back with the battery in good shape when the owners finally returned home.
A short while later, however, a warning light stayed on, alerting the owner to a problem with the Diesel Particulate Filter (DPF), part of the vehicle’s exhaust and emission control system.
It turned out the problem was caused by repeated short spells of the engine idle-running only. The DPF had become heavily contaminated with soot particles, a by-product of diesel combustion.
Cold, short runs combined with extended periods of idling are not ideal operating conditions for any diesel engine, especially modern, common-rail diesels fitted with DPF systems.
The DPF’s job is to trap and burn unwanted and unhealthy soot particles, but it must go through a regular regenerative process controlled by the engine electronic control unit (ECU).
If a vehicle is driven regularly at open road speeds and at normal operating temperatures, owners have little to worry about. The regenerative process is taken care of automatically, usually by the ECU injecting small quantities of fuel into the engine after combustion, which increases exhaust temperatures and allows the soot to safely burn off.
With repeated short cold runs or long spells of idling, however, it becomes the owner’s responsibility to ensure the car gets a good open-road outing occasionally.
Left unattended, the result could be a heavily blocked DPF system that is beyond help, which can mean high repair costs.
Not all diesel-powered vehicles have DPF systems fitted.
The current requirement for vehicles entering New Zealand is to meet a certain engine emission standard only. So think carefully about diesel ownership.
Not only will you pay more for a new equivalent-size vehicle fitted with a diesel engine, it will also be more expensive to service, you pay road user charges and there is a chance that the DPF system, if fitted, will eventually create unwanted issues if most of the driving is chugging around town on a cold engine.
For many transport applications and personal needs, diesel-fuelled engines are still the only practical and viable choice. But when fitted into vehicles that are used for constant cold running and limited use, you do have to wonder if there are better alternatives.
We shouldn’t see the fitment of the DPF system as the problem either.
Instead, we should be encouraging manufacturers to install them to ensure tailpipe emissions are as low as possible.
It really is more about buying a vehicle that is totally fit for purpose.