experience, I felt this wasn’t correct as most I had seen before travelled further. However, not having a lot of product knowledge with Mercedes, I took it to be ok but decided to keep it in the back of my mind, just in case everything else tested good and I still could not find the fault. Removing the arm onto the actuator from the turbo vanes itself and moving it by hand showed it to be nice and free, so all seemed fine on the turbo side of things. Why then was it so slow to build boost on the road? Checking the plausibility of the pressure and temperature sensors showed all was well. However, checking injector data showed cylinder 2 to be way off compared to the other three. This indicated a fuel delivery issue or mechanical issue, and the control unit was attempting to alter the fuelling on that cylinder to smoothen out the running of the engine and balance it against the other cylinders. This was why I didn’t notice a running issue when driving the vehicle, as the management system was compensating for it constantly. Smooth running correction For smooth running correction, the engine control unit measures the speed of the crankshaft by the crankshaft position sensor on each firing stroke and in relation to each other. To prove whether it was an injector or mechanical fault, I then carried out a relative compression test with an oscilloscope. If there was a lack of compression on cylinder 2 this would be detected by the crankshaft sensor. This would then induce the control unit to inject more or less fuel on multiple injectors, depending on the engine arrangement and firing configuration in an attempt to make the crankshaft speed change to match the other cylinders. This is because one cylinder has a knock-on effect on the next cylinder in the firing order. However, all was well so the fault must lay in the injectors. I expected this to be a nozzle or delivery problem especially given the fact the engine had done 150,000 miles and looked to still be on the original injectors. At this point, I decided to clear the faults and road test the vehicle again. I still had time left for the initial assessment, so I wanted to see if I could find what happened when the fault code logged for the actuator before being stuck on the ramp. All tested ok and freeze frame data revealed no clues either. Before I contacted the customer, I wanted to try and have a prognosis for both faults the car was logging or a path I wanted to take if more time needed to be authorised. Driving the car again, the same symptoms were present, but this time the boost value was far lower than I expected. However, no faults returned when checking afterwards, which was very surprising as I drove it low on boost for several miles and no underboost faults were stored. With my initial assessment time nearly up, I decided to recommend the injector on cylinder 2 was replaced and the engine control unit software was checked and updated. I suspected some fault codes had been deleted from the vehicle by manipulation of the software. Also, if this had been done there was a possibility that the turbo boost control software had been altered and was also incorrect, and was causing the symptoms being experienced. Speaking to the customer, they were happy to arrange getting the vehicle to a garage with the Mercedes Benz factory scan tool first to flash the software, then afterwards to replace the injector. The vehicle was then taken away and the customer would report back if the software corrected the boost problems. Re-flashed, re-scanned and re-checked A few days later the customer phoned to say the software had been reflashed. Unfortunately, upon leaving the garage and driving home, the engine management light illuminated and the vehicle still had the same lack of power. So, it was brought back for me to take another look with more diagnostic time authorised. On arrival and following re-scanning for fault codes, we now had a P0299 under-boost fault stored alongside the same fault as before for the injector 28 AFTERMARKET MAY 2023 TECHNICAL www.aftermarketonline.net Below: Fig 2 Above: Fig 1
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