Approaching a machine with a misfire can sometimes be a daunting task. There are a number of variables that could be causing the misfire which makes it tricky to know where to start. This was certainly the case when we were faced with a small excavator that had vibration and a lack of power which could be associated with a misfire.
Using an oscilloscope during diagnosis, such as an automotive PicoScope, allows you to capture signals referenced to time. This means you can see an electrical signal and how it changes in real time which can be used to determine faults.
When it comes to a misfire, a relative compression test is a must. A relative compression test looks at the electrical load demanded by the starter motor in order to push a piston past its compression stroke. This generates a saw tooth pattern which can be used to see how each compression stroke loads the starter motor. All being well, the pattern will be the same for each cylinder. However, when there is something different which then repeats, we have something to investigate.
By combining additional signals you can build a picture of what is happening within the engine during cranking. Using a current clamp around the starter/battery cable, a pressure transducer in the exhaust and if possible the camshaft sensor for cylinder identification, you gather an overview of the engine’s mechanical health.
The relative compression test above in Blue indicates a lack of current from the starter motor which can be caused by a lack of compression. This pattern repeats indicating we have an issue to look into. By having WPS500X in the exhaust, shown in red, we can also see how this cylinder contributes and changes the exhaust pulse. What we can see is that there is a sharp change in pressure which is happening on the exhaust stroke from the offending cylinder. By including a way to identify the cylinders, such as a camshaft sensor, we could then use the firing order to then determine the cylinder of interest.
With enough evidence in what is a non intrusive test, we are able to confirm further investigation resulting in the removal of the rocker cover to find a broken push rod. To find out more please visit our website www.picoauto.com
