want to see a peak of around 180 amps drawn by the starter upon cranking. This way I know the compression of the engine is in the right ball park. As shown in Fig.1, I have put a thin black line across the top of the peaks where you would measure with the scope software and arrows pointing to two individual compression events to show what a good trace looks like. So, for a 4cylinder you would count four and for a 6-cylinder, six peaks etc. You can then add in a reference point to work out which cylinder is 1 or 4 or whatever one has an issue. I will explain more about this later in the article, so read on. Stress test Back to the Citroen; After confirming mechanical integrity in the form of compression, I then decided to make sure we had a good functioning ignition system. As we had a fault code for cylinder 4, I decided to test that first by carrying out a stress test of the ignition coil. Upon pulling the coil from the engine I noticed it was soaked in engine oil and the rubber coating of the coil had been softened by the oil and was damaged and badly mis-shaped. The coils on this vehicle have three wires, which include a power supply, a ground and a control/trigger/turn-on signal. This vehicle uses four individual pencil coils. Upon testing the coil, it was noted there was no spark whatsoever so it appeared to be faulty. To be certain the wiring side was ok, I removed cylinder 3’s ignition coil and plugged it into the connector of cylinder 4. Having four separate coils allows me to swap their positions to prove system integrity quickly. Attempting a coil test again on cylinder 4 showed no spark and swapping the coil back to cylinder 3 and repeating the test showed the coil to fire. This meant we had proven the fault was somewhere in the supply, ground or control of the coil. Using the oscilloscope, I proceeded to test the power, ground and turn-on signal. The power and ground were good; However the turnon signal was missing. Next, I tested the wire from the connector on cylinder 4 coil back to the engine control unit. This proved the wire to be good so the fault had to lay within the engine control unit. The evidence would suggest that the coil was damaged due to the oil leak. The damage to the rubber insulation had allowed the high voltage intended for the spark plug to be instead sent to the control unit causing internal damage. A replacement control unit was sourced, cloned and fitted along with new spark plugs, a new ignition coil and gasket for the cam cover. The vehicle then started and ran without fault. Logging a fault Next was the Dacia Duster which also had a 1.6 petrol engine and was logging a fault for misfires on cylinder 3. Again, it didn’t tell us why, only which cylinder. This is commonly done by the crankshaft position sensor measuring the variations in the speed of the crankshaft. Following the same process as before, I checked compression first which was good followed by ignition which also was good. However, upon testing the fuel side of things I found injector 3 was not working correctly. It still didn’t look correct when looking at its waveform using an oscilloscope. Carrying out some more checks, I found its internal resistance to be wrong compared to the 16 ohms of the other three. As with the coils on the Citroen, when you have multiple components which are the same you can use them for a known good and then use that data to test your component in question. On this injector, the resistance was close to 0 ohms, indicating it was shorted internally. A new injector was fitted, the system retested and we now had a fully functioning engine. Cause for concern Finally, the Nissan Qashqai was looked at which had a 1.2 GDI petrol engine. Unlike the other two vehicles, this uses direct injection. This presents 32 AFTERMARKET APRIL 2023 TECHNICAL www.aftermarketonline.net A trained ear can pick up an issue before any tools need to be used ” Below: Fig 2 Above: Fig 1
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