If you drive a late-model diesel ute or 4WD, sooner or later you'll see the DPF light. A lot of people assume that means a new filter at dealer prices. Usually it doesn't. Here's what's actually going on and what to try, in roughly the order we'd try it.
What the DPF does
The diesel particulate filter sits in the exhaust and traps soot. Every so often it needs to burn that soot off, which is called a regeneration or "regen". On a long highway run the exhaust gets hot enough to do some of that on its own. When it doesn't, the engine computer runs an active regen. It adds extra fuel to push exhaust temperatures up for 10–20 minutes.
Problems start when the regen keeps getting interrupted, or when the engine makes more soot than the filter can burn off.
Signs your DPF is struggling
- A DPF warning light, or a message telling you to drive to complete a regen
- Regens happening far more often than they used to. Clues are a higher idle, the cooling fan running after you switch off, a hot smell, or worse fuel economy
- Loss of power or limp mode
The usual causes
- Short trips. School runs and 10-minute commutes never let a regen finish. This is the big one.
- Switching off mid-regen. Do it often enough and soot builds up faster than it burns.
- A dirty intake and EGR. Diesel intakes build up a thick, oily carbon sludge from the EGR and crankcase breather. The engine breathes worse, makes more soot, and the DPF pays for it.
- Injector problems. A dirty or worn injector makes more smoke, and that smoke ends up in the filter.
- The wrong oil. Oil that isn't the low-ash spec your engine needs leaves ash in the DPF. Ash doesn't burn off. Check the manual. Most DPF diesels need a specific low-SAPS oil.
- A faulty pressure sensor. Sometimes the filter is fine and the sensor that measures it is the problem.
What to try, in order
1. A proper highway run
If the light has only just come on, take the car for 20–30 minutes on the highway at a steady speed in a gear that keeps the revs up a bit. That's often enough to let a regen finish. It's free, so always start here.
2. Clean the DPF on the car
If regens keep failing, an on-car clean is the next step. A kit like the JLM DPF cleaning kit uses a spray gun with a long probe. It goes in through the pressure sensor or temperature sensor hole in front of the DPF. You spray the cleaning fluid into the filter to break down the soot, then the flush fluid, then do a regen drive. It's a job a handy home mechanic can do with basic tools in an hour or so. Follow the instructions in the kit closely, especially the soak and drive times.
3. Fix what's making the soot
Cleaning the DPF without fixing the cause just buys time. If the intake is caked in oily carbon, the JLM Diesel Intake Extreme Cleaning toolkit cleans the intake, EGR and turbo inlet without a full strip-down. A bottle of JLM Diesel Extreme Clean through a tank helps clean up the injectors and combustion side.
4. Keep it clean
Once it's sorted, use a fuel conditioner like Fuel Doctor at every fill. Use the right low-ash oil, and give the car a decent highway run every week or two if most of your driving is short trips.
When a clean won't fix it
Chemicals can't fix a cracked filter, a filter packed with ash after years of the wrong oil, or a failed sensor. If the light comes straight back after a clean and a long drive, get it scanned. The live data will show whether the pressure readings make sense. That's a job for a diesel specialist.
Peugeot and Citroën diesels with an additive tank
Some European diesels use a separate DPF additive tank that has to be topped up at service time. If yours is one of them, a low additive level will also stop the DPF regenerating properly. The JLM DPF refill fluid kit is made for that job.
Not sure where to start with your vehicle? Give us a call on (07) 3340 4392 and tell us what it's doing.