How a DPF clears itself
On a sustained highway run the filter burns its soot off on its own. When that doesn't happen, the engine computer starts an active regeneration, adding a little extra fuel to heat the filter to around 600 degrees. RACQ says that typically takes five to 25 minutes at a minimum of about 80 km/h, and once it starts it needs to finish.
Cut it short often enough and the filter clogs. Power drops, the DPF and engine lights come on, and the car can drop into limp-home mode. From there it usually needs a forced regeneration by a mechanic, sometimes with an oil and filter change, because fuel can end up in the oil.
Diesels and DPFs around Brisbane
Where the soot builds up
A diesel ute doing school drop-offs and short hops around the suburbs may rarely hold 80 km/h for long enough. RACQ suggests drivers who mostly sit in slow city traffic think hard about whether a DPF diesel suits them at all.
Where it gets cleared
Brisbane has the roads for a proper run. The Gateway Motorway alone is 48 km from Drewvale to Bald Hills. RAA suggests a weekly drive of 20 to 30 minutes at highway speeds, and a motorway run outside peak hour is one way to fit that in.
Deleting it is illegal
Queensland's vehicle standards regulation makes it an offence to drive a light vehicle with its exhaust modified in a way that reduces its effectiveness, and it gives removing the diesel particulate filter or the silencer as examples. The maximum penalty is 20 penalty units. A blocked filter gets cleaned or replaced, not removed.
Petrol exhausts have their own troubles
On a petrol car, the part to protect is the catalytic converter. RACQ says misfires, rich fuel mixtures and oil burning from a worn engine can ruin one, and a hard hit from a speed bump or road debris can destroy it. A misfire is worth fixing before it takes the converter with it. Rattles, blowing joints and a louder exhaust note are all worth a look too.
What to ask about a blocked DPF
- Is it the way the car's driven, or another fault stopping it from regenerating?
- Will a forced regeneration do, or does the filter need cleaning or replacing?
- Does the oil need changing after the regeneration?
- What should I change about how I drive it so it doesn't block again?
What we'd do
When the DPF light comes on, we'd follow the owner's manual first, which often means a steady run at speed. If the light stays on, or the engine light joins it, we'd stop trying and get it to a mechanic before limp mode turns a clean into a new filter.