Finding the root cause of: a DPF inefficiency fault code

Jeremy Piepmeier, lead mobile technician at Cox Automotive Mobility Fleet Services, has seen more than his share of diesel engine issues. Recently, the veteran technician based in the Atlanta area took time to relate a few of the more challenging repairs he’s had to make and the solutions he found.

A medium-duty truck experiencing repeated self-regenerations was found to have a DPF (diesel particulate filter) inefficiency fault code. The initial diagnosis was for a clogged diesel particulate filter, which was replaced, but the problem continued. The engine would run normally for three to four weeks before the passive regenerations started and the DPF inefficiency code would reappear.

“Every time, we would check the DPF tubes for clogging, the delta p sensor for DOC face plugging, the injectors for overfueling, and the DPF doser for proper fuel pressure and operation,” Piepmeier said. “Eventually, we determined that the DPF doser, or 7th injector, was plugging up just enough for the DPF inefficiency to occur intermittently. That was an issue we hadn't encountered before.”

Another issue Piepmeier remembered was from a truck that was derating and showing an SCR fault code. The shop cleared the code and completed a forced regeneration, but the same derating issue kept returning.

“We checked live data while the truck was running and determined it was dosing DEF when it wasn’t needed,” Piepmeier recalled. “After further investigation, we found that a wire from the ECM was overlaid with another wire, so essentially, the dosing valve was straight wired and was activating all the time. That caused the truck to overdose the DEF.”

A third, and what he called “a rather tricky issue,” that Piepmeier ran into a number of years ago was on a diesel engine that would show a check engine light at idle, but as soon as the throttle was depressed, the light would go off. Additionally, while at idle the engine showed an idle validation fault, which would also disappear at higher RPM.

“It turned out there was an electrical problem in the wiring harness from the ECM to the throttle pedal in that model truck,” Piepmeier related. “The wire was not chafed or worn through but was close to a bolt molded in the engine block. When we pulled the harness off there was a black mark in one piece of wire. The issue was that electricity was arcing through the insulation and it was grounding out at idle, causing the problem.”

Finding solutions

For diagnostics, Piepmeier prefers the Autel MaxiSys MS909CV tool, which provides online repair information for identifying faults and researching issues. But having the right tools is only part of the effective approach that he applies.

“You always have to go back to the root cause,” Piepmeier said. “You have to determine what’s actually causing the problem.”

For example, an engine that is experiencing a code for frequent regenerations might have an issue with the EGR system, the turbocharger, or an injector. In that case, asking why the engine is self-regenerating gets to the cause. Even if it is soot buildup in the DPF, that could be due to the injectors overfueling, the EGR not operating correctly, or the turbocharger failing to actuate properly so there’s not enough air in the intake system. It’s also possible that lower combustion temperatures are causing more soot downstream in the exhaust.