Why Short Journeys Damage Diesel Particulate Filters and Exhausts

Moisture builds up inside the exhaust

Burning fuel produces water vapour as well as other gases. On a long drive the whole exhaust gets hot enough to push that moisture out as steam. On short trips the pipe and silencer stay cool, so the water condenses inside and sits there.

That moisture, mixed with acidic by-products, attacks the metal from the inside. The silencer box is usually worst affected, because it sits at the cooler end and has the most room for water to collect. Small drain holes in some silencers help, but the rust still gets a hold. It can lead to leaks and failures well before the outside looks tired.

Cold starts add to the problem. When the engine is cold it runs richer, which produces more condensation and more soot, and the first few minutes after starting are where much of the damage is done.

Why a diesel particulate filter needs sustained driving

A DPF traps soot, and has to burn it off from time to time through regeneration. To do that, the exhaust has to reach a high temperature, and the engine management usually needs the car to be at a steady speed for a good stretch of time.

If you only do short, stop-start trips, the regeneration gets interrupted. The system tries again and again, soot builds up, and eventually the warning light comes on. In some cars, repeated interrupted regenerations can also push extra fuel into the engine oil.

• Short trips never get the exhaust fully hot
• Stop-start traffic interrupts regeneration
• Soot slowly fills the filter
• A warning light or limp mode may follow

Town driving and Tyneside conditions

Local trips between towns and across the Tyne, through traffic and lights, are exactly the stop-start pattern that suits neither filter nor exhaust. Cold winter mornings make it worse, because the engine takes longer to warm.

If most of your driving is short, a diesel may not be an ideal match, and it is worth thinking about this before buying. That is a general observation, not a rule, and your own use is what counts.

Anyone who does a high number of short trips in one vehicle may find an alternative for the longer runs helpful, but that is a personal choice and depends on your circumstances.

A longer run now and then

In general terms, a longer drive at a steady speed once a month or so, with the engine fully warm, gives the DPF a chance to regenerate and dries out the exhaust. A dual carriageway or motorway is the easy option. Check your handbook, as some manufacturers give guidance for their cars.

Do not rev the engine while parked, as it is not an effective way to regenerate and wastes fuel. If the warning light comes on, follow the handbook and get it checked if it stays on.

Keep other maintenance up to date too. A tired battery, a lazy sensor or an old air filter all make regeneration less likely to complete, so regular servicing supports the filter as much as the driving does.

When it has already gone wrong

If the filter light is on or the exhaust is blowing, the answer depends on diagnosis. At Banks of the Tyne in Hebburn we can run DPF diagnostics, carry out DPF cleaning and repairs, and handle exhaust repairs or replacement where corrosion has done its work.

Catching it early usually leaves more options open. A filter that is only moderately clogged is easier to deal with than one that has been driven in limp mode for weeks.

Frequently asked questions