APRIL 2023 AFTERMARKET 33 Below: Fig 3 www.aftermarketonline.net a further cause for concern as these engines are becoming more and more common for misfires caused by excessive carbon build-up in the inlet ports and on the back of the inlet valves. This is due to the fuel now being injected directly into the cylinder. Previously on manifold injection the fuel would clean the valves on its way into the cylinder. The carbon build-up restricts the path for air to enter the cylinder and this is very important for GDI with its multiple different running modes. However, I will leave the subject of GDI at that as it could make its own article. This vehicle also had a cylinder 3 misfire fault code stored. Upon carrying out my misfire process, the engine could be heard to have an inconsistent cranking speed, indicating a problem. Carrying out a relative compression test showed we did indeed have a problem and no compression was present on a cylinder. To verify it was actually cylinder 3 I connected a second channel on the scope to the ignition coil trigger for cylinder 1. Checking technical data showed the firing order to be 1-3-4-2 so if the missing peak was the next after the firing line for cylinder 1 it would indeed pin down cylinder 3 as the cause of the misfire. As can be seen in Fig.2, the green line indicated when cylinder 1 coil fires and the next peak of compression is missing so it was indeed cylinder 3. The next question was why was there no compression on that cylinder? At this point we had two choices; We either stripped the engine down until we identified the cause or we use a pressure pulse sensor so that we could measure pressure pulses in the inlet, exhaust and crankcase to find where the cylinder pressure was escaping to. This would be much like a cylinder leak-down test only we can do it cranking the engine and without having to set the valves at specific positions and introduce compressed air to find where it is leaking. So, it is much faster. In fact, if we had multiple pulse sensors we could evaluate the entire engine in one cranking capture with an oscilloscope, but I am not so fortunate. Conclusion I went for the latter and connected my one and only pulse sensor to a third channel and one by one captured an inlet, exhaust and crank case pressure waveform against relative compression and the coil trigger for cylinder 1. Then using a cylinder over-lay chart, I could see what each valve should be doing at what time and compare it to my waveforms to find where the problem was. Now to be brutally honest this takes a lot of time and practice to master, and I still find myself needing some assistance. So, after reaching out to a few fellow techs for their opinion we concluded that there was an exhaust valve issue on cylinder 3. Again, this subject could fill its own article so I won’t get into the nitty gritty of how and why but we had an answer where the compression was going. Yes, a cylinder leak-down test would have shown this, but not all running issues are obvious and some require the engine to turn to reveal themselves which a leak-down test will just not show. Now this job so far was still in-house, but if it was a retail job I could tell the customer exactly what the problem was without removing much more than an engine cover or dipstick/pipe to put my pulse sensor in to measure if required. This would allow them to decide if they wanted to continue with the repair or for them to take the vehicle away. This would not be so easy if the cylinder head was removed from the engine. The cylinder head was then removed and one of the exhaust valves was found to be incomplete. Please refer to Fig.3. This backed up the diagnosis made before the engine was even dismantled, proving how accurate and powerful this form of testing can be. All three cars in fact prove how complex the causes of a misfire can be, and how fitting a new coil or spark plugs/leads would not have fixed any of the vehicles in this article. If you carry out logical testing and gather data which drives your next move, you will always find the cause of the fault. Left: Nissan Qashqai Left: DS3
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