We’ve had 14 years to figure it out, yet diesel particulate filter maintenance remains a nightmare for many fleets. Why do DPFs elicit such gnashing of teeth and pounding of fists? Because they are mission-critical maintenance items that provide no reliable guidelines on when service needs to be performed. Proper cleaning intervals depend on so many factors that it’s nearly impossible to chart and calculate repeatable service intervals across a fleet, especially a diverse fleet.

Fleets are just beginning to accept that they have to get out in front of the DPF rather than waiting for the lights to come on. Nobody likes to over-maintain equipment, but with DPFs, the more proactive you are, the less heartburn they cause in the long run.

DPFs are literally sealed trash cans. You can’t see when they’re getting close to full, so the inclination is to keep pumping junk into them until they won’t take any more. By then, you already have a problem.

Increased aftertreatment regeneration is often seen as the solution, but regens can actually compound the problem. They are intended to oxidize, or burn off, the soot particles left behind by fuel that escapes unburned during the combustion process — those particles that used to spew from the exhaust stack as black smoke.

But soot isn’t the only substance trapped by a DPF.

Ash is the big culprit. It’s residue from oil that slips past the piston rings into the combustion chamber. Some of the oil is burned during combustion, producing soot — but the metallic lubricant additives are incombustible and remain trapped in the DPF as ash.

“When we tear down a DPF, we find that up to 90% of the material in there is ash, not soot,” says Jason Gerig, commercial on- and off-highway sector manager for Chevron Lubricants, Americas. “A lot of customers think what’s plugging their DPF is ash from the fuel combustion cycle, when it’s actually ash from the lubricant additives.”

If that ash is not manually removed regularly from the tiny channels within the filter core, it packs in tighter and tighter. Eventually it turns into a concrete-like substance that will, over time, significantly reduce the carrying capacity of the DPF, shortening the backpressure-induced regen intervals. And the high-temperature regens can actually accelerate this “sintering” of the ash, compounding the problem.

“The DPF pressure differential sensor doesn’t know the difference between ash and soot. It just knows that the backpressure is higher, and that triggers a regen,” says Jeremy Anderson, national sales manager for DPF cleaning machine producer FSX. “Because the ash is taking up space in the DPF, it can’t inventory as much soot before the pressure differential sensors go off and start another regen. When the time between regens starts to shorten, you know it’s time to service the DPF.”

Servicing vs. Replacing DPFs

There’s some confusion surrounding the need for hands-on maintenance of the DPF. Doing a regen isn’t the same as de-ashing the DPF with a cleaning procedure. And that’s not the same as ditching the DPF and exchanging it for a new or reconditioned unit.

OEMs have been saying for years that a DPF should last 400,000 miles, but they aren’t always clear about the maintenance that’s required to get that far. The folks who service, clean, or de-ash DPFs (whatever you want to call the process) say it should be serviced annually, possibly more frequently in applications with high idle time and low exhaust temperatures.

“When the OEMs talk about high-mile DPF service intervals, they are using the best-case scenario: trucks that run heavily loaded in high-mile, on-highway applications where passive regeneration can deal with soot loads in the DPF,” says Steve Hoke, president of aftertreatment service center Diesel Emissions Service.

However, high rates of passive regeneration do little to solve the buildup of ash, which accumulates as oil is consumed.

“All the unburned blow-by oil, fuel droplets, and stuff that isn’t atomized in the cylinder goes out the exhaust,” Hoke says. “Pre-2007, it was no big deal. All that stuff just disappeared. But now it doesn’t.” In an ideal world, he explains, it goes through the oxidation catalyst, which oxidizes the soot as it goes through, and all the DPF sees is clean, light, fluffy ash.

“But in the real world, the efficiency of the DOC drops over time because it’s poisoned by coolant and oil leaks, etc. An older DOC might be no more than 50% to 70% efficient in its capacity for oxidizing soot, so you have more unburned particulate or unoxidized soot going into the face of the DPF.”

So even in the best-case scenario, as the truck ages, there’s a good chance that ash, not soot, is mucking up your DPF — and all the regens in the world won’t make it go away.

Removing the DPF from the truck and having it cleaned is about the only way to deal with ash. Lots of shops and most dealers provide the service, and some large fleets buy their own cleaning machines. It’s usually an overnight job, but depending on the volume at the shop, it can stretch to a couple of days.

A popular alternative when uptime is critical is to buy a replacement new or reconditioned DPF.

“Replacing the original with a reman can get you back on the road in a few hours,” says Anderson. However, he notes, “a reman DPF will cost you about three times as much as a cleaning, and you won’t know anything about the history of the reman filter, other than it’s covered by some warranty.”

DPF Cleaning Methods

Cleaning a DPF usually involves removing the filter from the truck and using a high-pressure, high-volume “air knife” to loosen material from the tiny channels within the DPF. Next, the filter is flow-tested to check the level of restriction still present. Several individual channels are measured with a length of wire to see if any material remained behind. A high degree of remaining material may require the DPF to be “baked,” exposed to very high temperatures for up to eight hours, to further reduce the residue to a finer, looser material that can be blown out.

Daimler Trucks North America, however, does not endorse the so-called “bake and blow” cleaning method, saying it does not clean the DPF as thoroughly as a liquid wash.

“We designed our DPF to be liquid cleaned,” says Len Copeland, Detroit product marketing manager at DTNA. “We recommend that our customers run their DPFs through our reman program. The DPFs are washed and serviced, and the customer can just swap out the old DPF for a clean one and they are on their way with minimum downtime.”

Detroit’s system is set up to throw an ash cleaning code after the truck has burned a certain amount of fuel, based on the application. When the ash accumulator is reset, it assumes that the customer has installed a reman filter and resets the cleaning interval to the prescribed number.

Diesel Emissions Service is offering a new process, saying it can do an aqueous wash of the DPF using a surfactant to remove sintered ash during the time the truck is down for an A-service preventive maintenance service, with an oil change, etc. DES has begun selling the machines to its core customers and fleets will soon be able to acquire them for use in their own shops.

“When done according to our recommendations, this process will produce the same results as an overnight ‘bake-and-blow’ cleaning, but in about two hours,” Hoke says. “The service is available in our shops now, but you’ll pay more for it than you would a traditional cleaning. The obvious advantage is the fast turnaround time.”

The process doesn’t cut corners, and it’s not a shortcut to the next cleaning interval, Hoke says. “Our new aqueous process is subject to all the same standards as a bake-and-blow cleaning. It goes by the same flow numbers, the same weighing process. It passes or fails based on the same specs.”

Upstream Failures

Anything that goes through the combustion chamber is going to wind up the DPF eventually. Oil, coolant, even dust from the air intake can foul the filter substrate. Coolant and oil are among the worst, because they coat the face of the DPF and lower its efficiency. Excess fuel from a failed injector can raise soot levels dramatically, but there are no longer any visible symptoms. Any upstream failure should be a red flag to check the DPF too, Hoke says.

It’s hard to track drivers adding make-up coolant or oil to the engine, but if they are doing so, it suggests the fluids are going somewhere — possibly into the DPF. Drivers should be told to advise maintenance when they are topping off fluids so consumption can be tracked.

“The worst case would be a turbo failure,” says Anderson. “That will torpedo an entire aftertreatment system very quickly.”

EGR failures are also responsible for a number of DPF issues, but cleaning and servicing the EGR system can be expensive and time-consuming. According to Hoke, just removing the EGR coolers from one brand of engine is a nine-hour job. Add nine more hours for reassembly and the time it takes for cleaning, and it’s easy to see why fleets avoid it.

Hoke offers a service where the EGR system can be cleaned while the truck in in the shop for regular service without removing the coolers and valves.

“The process is completely self-contained,” he says. “After removing the aftertreatment system, you hook the machine up to the air intake system and run a few liters of cleaning and flushing solution through the engine while it’s running. It takes about 30 to 45 minutes and costs about 500 bucks parts and labor.”

Rethinking Diesel Particulate Filter Maintenance Schedules

The DPF’s ability to passively oxidize soot is highly dependent on exhaust temperature. Exhaust from lightly loaded engines or trucks that idle a lot isn’t hot enough to efficiently oxidize the soot. Cold-weather operations can further reduce the efficiency of the aftertreatment system.

“Each fleet is going to have their own sweet spot, so to speak, upon which the preventative maintenance schedule should be based,” says Anderson. “It should not be based on a pre-determined timetable, but based on the routes, the trucks, the engines, the loads, and the drivers. All those variables play a part in determining the optimum cleaning intervals.”

Fleets have spent years streamlining their preventive maintenance processes, but because aftertreatment maintenance can’t be based strictly on a one-size-fits-most mileage recommendation, fleets have to learn to restructure the PM if they want to reduce DPF headaches. 

Anderson strongly recommends setting up alerts in a fleet’s maintenance management system to flag when the regens are becoming more frequent. Barring that, get drivers to report regen events so they can be tracked and regen histories built around the data.

Darry Stuart, an independent “limited-time executive” who provides maintenance and operational expertise for dozens of fleets, says the 2.5-hour PM is a thing of the past.

Among other things, DPF canisters should be inspected regularly for possible damage, cracking, and breakage. He also suggests inspecting and cleaning the sensors and their exposed contacts to ensure they have good electrical connections and are not gummed up by contaminants that could cause false fault codes.

“We’re just now beginning to accept that we have to do things differently from what we have been doing all our lives in order to maintain the aftertreatment system,” he says.