Under no pressure

Top Technician 2015 winner Andy Gravel finds a DPF regeneration to be more than it seems

Published:  06 August, 2018

Once the news started to spread about my Top Technician win, the phone started to ring with more interesting and challenging jobs, usually ones that have been doing the rounds between other garages without success.
  
 A phone call came from a local parts supplier, a visiting rep was having issues with a DPF. They believed it needed a simple regeneration to get it back on the road and asked if I would be able to do the job. After checking the Blue Print G-Scan, the function for a forced regeneration was available, I believed I would be able to carry it out and booked the job in.

Basic beginnings
After traveling from two hours away, the vehicle arrived. The customer was questioned, ‘Why do you require a DPF regen?’ Being a parts rep within the motor trade, her garage visits were frequent; various attempts had been made to resolve the issue. With conflicting advice being given and quotes between £600 - £1200 to fix the vehicle, the customer was obviously confused and unsure about what to do.
    
The engine management light was on, so the obvious place to start was a scan check for fault codes. The vehicle showed P2002: Particulate Trap Below Threshold.
    
Viewing the live data for the DPF pressure sensor, key on engine off, displayed a 0kpa pressure reading, a good start for a sensor plausibility check. With the engine running and RPM increased, the sensor reported a suspiciously low-pressure reading, not one I would associate with a saturated DPF. I decided to use the Pico Scope to look at the DPF pressure sensor voltage in real time. After confirming the power and ground circuits to be ok at the three wire pressure sensor, the signal wire was checked. Again key on engine off, 750mv was displayed, a sensor plausibility check and again this was good. Starting the vehicle and increasing the revs revealed exactly the opposite to what I had expected, a negative voltage reading. The voltage should increase as the exhaust pressure increases.

What’s wrong?
One area I wanted to check was that the pipes were not connected the wrong way around. I decided to use the Mity Vac to apply pressure to the sensor pipe connected in front of the filter. This showed a positive rise in voltage, further proving good sensor functionality and confirming the pipes to be correctly connected. Connecting the Mity Vac to the pipe after the filter and applying pressure, simulated the negative voltage which was seen when the vehicle RPM was increased, simulating the fault. The sensor pipe in front of the filter must be blocked.
    
I located the steel pipe that is fitted in the exhaust in front of the filter to reveal soot marks, it had been leaking exhaust gasses. On a closer look it was unscrewed from the exhaust while still located in the hole due to the pipe bracket allowing the slight leak of exhaust gasses. Once the pipe was removed it was clear to see the soot had built up and blocked the small hole in the end of the pipe. I unblocked the pipe, checked to make sure the mounting hole on the exhaust was clear and refitted it.
    
Using the Pico Scope again on the signal wire, it now showed a positive rise in voltage when the RPM was increased. The live data now showed a small pressure increase, the filter was not blocked. With all fault codes cleared, an extended road test was carried out, the pressure reading stayed low throughout and no fault codes reoccurred confirming the fix, the vehicle did not require DPF regeneration.

With no parts required to fix the vehicle the repair cost was far lower than the customer expected due to the previous attempts. The vehicle was returned to the customer who was surprised by the
outcome of the repair and relieved by the associated costs.



TT Archives:  Top Technician issue nine 2016 | www.toptechnician.co.uk

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