proper preventive maintenance (PM) schedule for
the aft ertreatment system is vital to the effi cient
operation of the vehicle.
When addressing what’s thought to be a fi lter
issue, “everybody does one of two things: they
either point at the fi lter and say, ‘that fi lter’s bad,’
or they do a manual regen to try to get the truck
back up and running and send it down the road,
and they don’t address the issue that’s going on
with the truck,” Griffi th advises.
A regeneration is completed on the DPF when
the fi lter has reached a certain saturation level,
monitored by the engine control unit (ECU).
Th ere are three types of regeneration that can be
done: a passive regeneration automatically completed
by the vehicle, an active regeneration manually
initiated by the driver, or a forced regeneration
done by a technician in the shop using a scan tool.
Regardless of the type of regeneration, there are two
requirements for proper regeneration of the system:
adequate air volume and proper temperature.
When in the shop, a scan tool can be used to
confi rm air measurements and proper temperature
for diagnosing issues with the aft ertreatment
system and DPF.
Air volume aids in pushing the particulate
matter through the DPF; a hot enough temperature
allows the system to burn the soot into ash.
Griffi th notes that air intake issues, such as a
leak, can cause a decrease in air volume further
down the system. A high-pressure smoke machine
can aid in diagnosing an air leak, he says.
“Th e number one thing that we fi nd that causes
premature issues with engine aft ertreatment
servicing can be pointed back to an intake or
exhaust leak,” Griffi th says. “Or, a leak in the
system with clamps and gaskets.”
“It used to be, you could fi nd an exhaust leak
really easily because you had a whole bunch
of black soot around wherever it was that was
leaking,” Griffi th adds. “Th at’s not so much the
case anymore. Making air visible using a smoke
machine to make sure that your clamps and
gaskets seat well is really important. If you get
air in the system that hasn’t been heated through
the DOC or a loss of back pressure due to the leak,
you’re going to have problems.”
When it comes to high enough temperatures,
Griffi th says some fl eets may see a drop in system
temperature if they do not properly re-insulate the
exhaust pipe aft er service.
Conversely, the system can also run too hot,
particularly when there may be a leak upstream,
according to Denso’s Ramirez.
If more oxygen is accessible to the system
through these leaks, it “can result in an overly rich
mixture reaching the aft ertreatment equipment,”
Ramirez says.
“Th is causes higher-than-normal heat, which
can lead to destroying the PM coating and melting
the substrate,” he adds.
Temperature readings, measured with exhaust
temperature sensors, are a key indicator in diagnosing
an issue within the aft ertreatment system.
“We don’t want to have too big of a temperature
drop across the face of the DPF,” Griffi th says. “As
far as diagnostics go, DOC outlet and DPF inlet
sensors are the fi rst place to check to ensure a
proper regen is taking place.”
“A lot of times we can start diagnosing by looking
28 Fleet Maintenance | November/December 2018
» In addition to reducing fuel consumption,
an air heater also removes the DPF from
the warm-up process, helping to prolong
its life. By not idling the main power unit,
a driver doesn’t need to worry about the
DPF not achieving the proper temperature
to actively regenerate and clean itself.
Photo courtesy of Webasto » A coolant heater, such as the Webasto
at the temperature sensors,” he explains. “On
a DOC, you have to have a temperature of 600
degrees F to get the ramp up and the catalyst
eff ect, to get that exhaust really hot. We’ll use
the front temperature sensor to make sure that
we’re getting that temperature that we need to
make the ramp up happen, then aft er the DOC on
the inlet of the DPF, we look at that temperature
and make sure we’re getting enough heat — somewhere
around 1,000 degrees F on the inlet side of
the DPF — and then we look at the outlet side of
the DPF to make sure we’ve got some temperature
coming through there.”
Also as it relates to sensors, additional sensors
used in the aft ertreatment system include the
pressure diff erential sensor, used to measure the
current capacity of the DPF, and the NOx sensors.
“You have one NOx sensor at the turbo outlet,
and you have one at the tailpipe outlet of the SCR,”
explains Cummins’ Lightner. “What those are
doing is measuring how much oxygen is in the
exhaust fl ow, as well as how much NOx is in the
exhaust fl ow, and it determines how much DEF
needs to be injected to eliminate the NOx level at
the tailpipe.”
“Th e number one sensors that we’re seeing people
having problems with are the NOx sensors,” Griffi th
adds. “Th ere’s one on the inlet side and one on the
outlet side, and there’s been some talk in the industry
about having problems with the NOx sensors
on the outlet side being caused by water that falls
in the stack if it’s got a vertical exhaust … If you
get a lot of water in the exhaust — of course, water
and electricity don’t really work together very well.”
Thermo Top C, warms the coolant helping
to bring the engine up to operating
temperature. According to Webasto officials,
pre-heating the engine can help reduce the
number of service intervals for the DPF.
Proper DPF management
and maintenance
As part of a regular PM program, DPF management
can be handled through four options: fi lter
exchange programs, cleaning the fi lters in-house,
cleaning with a service provider or replacing with
new fi lters.
DPF exchange programs are designed to allow
fl eets to send used fi lter cores to be remanufactured.
Th is can help alleviate cost, but Emission &
Cooling Solutions’ Griffi th suggests understanding
the details of the exchange program to be aware of
the history of each remanufactured fi lter.
“Th e primary benefi t of the exchange program is
uptime, assuming the exchange unit is available,”
adds Denso’s Ramirez. “Th e risk is the unknown
history of the core, which could contain multiple
cleaning events.”
“When you get a reman fi lter, there’s no way to
know what happened to that fi lter along the way,”
says Griffi th. “You don’t know how many miles are
on it, you don’t know any of that.” Griffi th suggests
confi rming the cores sent in are the same cores sent
back, and that is done through a service provider
like Emission & Cooling Solutions. “Fleets can tie
it back to a unit, and they know if there was an EGR
cooler that leaked on that truck for example. Or,
if you’re having trouble with a specifi c truck, you
can track that fi lter through your fl eet.”
Heating
options to
reduce service
requirements
on the DPF