20/08/2026
Modern diesels….if you own one, do you know what’s going on in your engine bay or under your car, in it's exhaust?
Remember years ago when diesel engines used to be really sooty and plume black exhaust smoke out the back? In an effort to reduce that, be more environmentally friendly, and meet government emissions targets, manufacturers developed a new exhaust system that pumped out less black crap.
It’s called a Diesel Particulate Filter - a DPF. It’s only fitted to diesel cars, and looks like a large hotdog type muffler in your exhaust. Inside, it is constructed from porous ceramic (with silicon carbide and aluminium titanate, but let’s not get too technical lol) in a honeycomb shape, and inside the little honeycomb holes is where the exhaust soot gets trapped. Think of this as the bit where the black sooty particles get caught while the exhaust gases go through and out the exhaust.
There are pressure sensors - one before and one after the DPF - and these do exactly what their name suggests. They measure the difference in exhaust gas pressure before the DPF and then after, in what is flowing out of the DPF. The car’s computer uses this data to gauge how much soot is trapped in the DPF and knows when to ‘clean’ it.
So what does it do to ‘clean’ it?
That’s when you get that little whooshie symbol like there’s a wind blowing in your instrument cluster lol. This is called a DPF Regeneration. In very, very simple terms, depending on the the system of fuel delivery, a bit of fuel is sq**rted through the fuel injectors (during normal operation) the excess fuel then enters the hot DPF and burns off the soot, turning it to gas and ash. The gas flows out the end of the exhaust and the ash stays in the DPF but is a lot smaller than the amount of soot it previously was.
There are 3 types of Regeneration that can happen:
* Passive regen:
This happens while you’re driving, usually on the freeway or highway at 80 kph, higher gears; the exhaust gases get to around 350-500 degrees to burn off the soot without you even noticing.
* Active Regen: Based on DPF pressure sensor data, your cars computer says ‘that’s enough soot thank you very much’ and triggers this regeneration. It’ll inject extra fuel into the exhaust to boost the temps up to 600-650 degrees and bam, have yourself a regen. You’ll know this is happening because your cooling fans may run after you park the car, you’ll smell a hot burning smell under the car, it may idle higher and fuel consumption may go up temporarily.
* Forced Regen: this is when we are involved, the mechanics (or in some commercial car or ute, they have a manual switch on the dash). If your car gets stuck in a cycle where it needs to burn off the soot but can’t/is interrupted too many times/has an issue/is too soot clogged for a regular active regeneration, we can plug in the scan tool. The scan tool will guide the car (whilst stationary) through a burn.
Some times, in high kilometer cars, the ash content is too high - lots of regenerations have happened over time and that ash now needs to be cleaned out. We can remove the DPF from the car and have it cleaned out by a specialist (high pressure flush). In some worse case scenarios, replacement is required, but this is rarely the case now, since removal and proper high pressure flush cleaning works quite well.
There are issues that are out of your control that can stop your car from doing a regeneration - boost hose and air flow issues, DPF or temperature sensor issues, faulty EGR valves, injector or fuel issues - but there ARE things that you, the driver, can do to help your car:
* avoid lots of short trips (under 20 mins) that mean your engine doesn’t get the chance to reach it’s operating temperature.
* take your car on the highway or freeway for 20-30 minute drives every 1-2 weeks.
* don’t turn your car off if it’s part way through a regeneration. Let it complete it!
* don’t ignore a DPF warning light - get that sorted before limp mode becomes an issue for you.
Stay tuned for the next installment - oil dilution! What it means for your engine.
