The Truth About Diesel Particulate Filter (DPF) Problems | Auto Expert John Cadogan

The truth is: DPF failure is usually a symptom: not the disease. If all the dealer does it treat the symptom, he won't cure the disease, and you'll be back with the same problem, again and again.
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In theory a DPF needs no maintenance - it’s just a steel chamber up near the engine full of Cordierite or silicon carbide designed to trap microscopic soot, and then periodically the computer turns the chamber into a furnace by injecting extra fuel, thus burning the particles into a less harmful state.
But DPF blockage is a symptom, not a problem. Like a headache is a symptom, and drinking 14 bottles of tequila last night is the problem. Replacing the DPF usually doesn’t cure the problem - in fact, it could mean you’ll be back for another replacement in a few weeks-to-months if they don’t actually cure the problem.
The problem could be as simple as: you didn’t go for regular highway drives you were supposed to. City driving is generally inadequate to allow a filter-regenerating burn.
But many people who experience DPF problems are actually doing more than enough requisite highway driving - so it’s often not that. It could be a problem with the inlet air plumbing - lots of DPFs fail this way. See, a modern diesel is turbocharged, and that means the inlet air plumbing is pressurised between the turbo and the engine inlet. And there’s a lot of plumbing: hoses connect the turbo outlet to the intercooler inlet and then there’s a dirty big plastic elephant trunk thing in the case of Subaru - so there’s a lot of potential for inlet air leaks. The problem here is: the mass air flow sensor. The MAF sensor measures the mass of air going into the engine, in real time. That’s important because the computer uses that information to determine how much fuel to inject.
Only two things go into your engine - air and fuel. And the ratio of the two needs to be bang-on. Measuring the mass of air makes sense because it self-corrects for pressure and density and the generally squishy properties of air. The mass is all that matters.
But if there’s a leak in the inlet air, downstream of the MAF sensor, because a hose gets a tiny crack in it over time, some of that air that the MAF sensor measures is not actually going into the engine.
And that means the engine will be over-fuelling, continuously, under boost, and that’s going to produce a whole bunch of additional carbon that will be like opening the Book of Revelation inside your DPF. That’s bad. Four tiny horsemen in there, wreaking carbon-based, end times havoc.
Induction leaks are real killers of DPFs - so if you are buying a used diesel car or SUV, it could be an entirely prudent idea to replace all the induction plumbing hoses as a pre-emptive strike against early DPF failure. Especially on the wrong side of 50-60,000km.
Over-fuelling is sometimes also caused by leaky fuel injectors - so I’d be getting a diesel specialist involved in the proposed purchase of any used diesel with a DPF - and not handing over the cash until there’s a clean bill of health from an expert.
It’s also very bad to use the wrong engine oil. DPFs require a special low-ash oil - and there’s more than one grade of that, so make sure you get the oil selection exactly right for your car. Another way to kill your DPF with oil is via a leaky turbo oil seal. So I would absolutely not be letting those service intervals slide.
Sticky EGR valves are another DPF killer - pumping excess crankcase vapour into the exhaust. Faulty glow-plugs - also DPF-deadly - because they cause the engine to run too rich on start-up.
And then there’s the differential pressure sensor across the DPF - all that does is measure the pressure drop across the filter. That tells the control computer there’s enough soot built up to trigger a regeneration.
But if the plumbing to that pressure sensor gets clogged, the computer presumes no regeneration is necessary - even if the filter is in fact choking to death.
I hope you can see that this is a complex system that, at times, to me, seems almost doomed to fail. And the failure is hardly ever intrinsically due to some deficiency in the DPF itself. In general, just replacing the DPF will lead to just replacing the DPF again - in a few weeks to months. And nobody wants that - even though plenty of people do it. Often the dealership will be ill-equipped to solve your DPF problem. They are generally fairly crap at diagnosis
Were I you, on the dreaded DPF merry-go-round - I would look for an independent diesel specialist with runs on the board with DPF diagnosis. It’s not rocket science ‘down there’ - but it is a bit complex for your average dealership Muppets who think taking orders from a scan tool is the same thing as being a mechanic.