A 2014 Mercedes-Benz CLA220 AMG 2.1 diesel was booked in recently, with the customer complaining about a lack of power. There was also a fairly lengthy story about recent work done to the car.     
    
The vehicle had been booked into another garage locally for the same complaint. This somehow led to the DPF internals being removed and the software modified to delete the DPF system from the vehicle. This was done without the customer being told, and they had not authorised the work. When the vehicle was handed back it was no better, and the customer returned to the garage. It was at this point they learned about what had been done and quite rightly they were not impressed. The car was then taken to another garage which installed a functioning second-hand DPF, corrected the software back to standard and also corrected a few other issues including a seized exhaust flap. However, the car still exhibited a lack of power and clearly didn’t have the AMG get-up-and-go it left the factory with. It was at this point the vehicle was brought to me.

Initial inspection
As can be seen in Fig.1, upon initial inspection we had two fault codes. The first was for the boost pressure deviating from the specified value. According to the sub code of 71, this indicated the actuator was blocked. At this point I wasn’t sure if the turbo had an electronic actuator or used a vacuum/pressure waste gate but this code clearly could cause the issues as explained by the customer. The second fault code indicated a learning value for the injector on cylinder 2 to be at its lower limit. This could also cause a lack of power complaint or limp home to be activated. This meant I had some good diagnostic direction to create a test plan and start doing some checks to locate the problem.  
    
I then drove the vehicle with live data displayed to look at boost data and injector data while also confirming the customer’s complaint at the same time. The vehicle was indeed low on power and felt flat, not what you would expect from a 2.1 litre turbo-charged engine. However, injector information looked okay and the boost levels reached where I expected them to. This included the turbo actuator position changing in relation to engine RPM and demand. The only points to note were that there was only actual boost in data and I could not see what the control unit was looking to see. It also felt too slow for the build in boost to happen and not as instant as it is usually with a modern diesel engine.

Turbo waste gate
Returning to the workshop, I decided to look into how the turbo waste gate was controlled to make sure it was functioning correctly. On this engine the actuator is electrically controlled by the engine control unit, also incorporating a position sensor for feedback to the ECU. The scan tool listed an actuator test moving the actuator arm from 5 and 90% travel. So, I decided my plan would be to test the actuator for correct operation and make sure the turbo control arm moved by the waste gate was free and had full travel. If this is stiff or seized then the actuator will struggle to move it. If all was ok, I would then verify the plausibility of the pressure sensors and also check the injector correction factors to see if an issue could be seen with the injector on cylinder 2, compared to the other three which didn’t log any faults.
    
Carrying out the actuator test, the arm moved freely both from 5 to 90% travel. However, there was not much of a difference in terms of travel between the two positions. With 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 under-boost 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 re-flashed. 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 on cylinder 2. Please refer to Fig.2. However, the actuator blocked fault had not returned. This told me that the software had indeed been altered but with the car still under-boosting, the vehicle still had an issue that I needed to find. I could now proceed knowing the software was standard, and remove that from the list of possible causes.
    
Re-checking everything, the only fault I could find was the amount of travel the turbo actuator made, from 5% to 90%.  I actuated it with a scan tool, but I could not prove whether it was correct or not. Luckily, speaking to a good friend about the problem, they said they had a Mercedes in with the same engine for service. My friend was able to film a good test of the actuator and send me the video to compare. It was at this point I knew this was my problem as the video showed the actuator not only moving further but also much faster. Testing the wiring to the actuator proved it to be good, and checking fitting instructions showed no adaption resets. Checking on multiple scan tools showed no resets, so I was happy to call out the actuator as being faulty. The customer was contacted with prices and was happy to proceed with a new actuator and injector.    
    
Once both components were replaced, I carried out the same test with the scan tool forcing the actuator from 5% to 90%, but to my horror the travel was identical. At this point, I was a little lost as I felt I had covered all bases but was no further forward with fixing the fault. It felt like the unit needed to be learned by the control unit. However, using Autologic, which is Mercedes-based, it did not list a re-set, and double-checking fitting instructions showed no mention of re-setting anything. Afterwards though, I decided to try some aftermarket scan tools for an option and the first two showed nothing. Plugging in Autel and going deep into the menus showed a boost pressure positioner teach in process. Before running the procedure, I decided to plug back in the old turbo actuator, as I am not one to fit parts which are not needed, and if the old unit could be relearned, the new part would not be required. Sadly, the procedure kept failing, so I plugged the new unit in and re-tried and the procedure completed successfully. Attempting the same actuation test as before now showed the actuator arm to match the known good video in terms of speed and travel.

Adjustment range
The old unit for some reason had gone faulty, but had defaulted to a state of not being adapted to the control unit and randomly logged a stuck closed fault. Even though, for the most part it didn’t operate, it worked enough to allow the turbo to create boost, albeit slower than normal.
    
As can be seen in Fig.3, the procedure on Autel lists that when successful, the adjustment range should change within 150ms however when not taught, it would be one second which matched my fault. I just wish I had found this option earlier but was happy to have finally found and rectified the issue. As you can see in Fig.4 and Fig.5, the difference in arm travel is visibly noticeable. Confident the fault was fixed, I then road-tested the car again and the vehicle now drove like a different car, with strong acceleration and boost more like what you would expect from an AMG Mercedes. Upon returning to the workshop and checking for fault codes, I now had no fault codes stored so was happy the vehicle was finally repaired. The customer reported after a few days the car was like a new Mercedes as it drove so well. Once again, like my other articles, this job shows the importance of a solid test plan and system knowledge to find the cause of seemingly insurmountable problems.