May 2019

BY John Batten I f you’re familiar with my train of thought, then you’ll know that I’m all about getting to a first time fix in a timely manner. In our world of ever- changing technology, the timely element can often be a challenge given the number of systems on a vehicle, as well as the number of different manufacturers that most independent repairers have to understand within a working week. This is why I’m always keen to let you know about some test opportunities that open up the door to a quick win without compromising on that first time fix. What is this miracle process and where do we use it? The good news is that this applies to one of the most common faults on a vehicle, the humble misfire, and the miracle tool in question is a pressure transducer. Three points of failure. A misfire - where the crankshaft fails to accelerate as expected – is usually caused by an issue in one of three main areas, namely; an ignition related fault, a fuelling fault or a mechanical fault. We’ve taken a look at spark related faults previously, so the focus of this article is how with the right information, equipment and some basic knowledge, we can peer into the innards of the four-stroke cycle and identify mechanical failures prior to a physical mechanical strip-down. This will mean that you’ll not only decrease your time to diagnose but you’ll know exactly where the problem lies when the time does come for a physical inspection of the failure. The patient in this instance is a 1.4L petrol Fiesta and to say it’s running a little rough would be an underestimation. It’s clearly sick on at least one cylinder and we need to discover why. Testing so far has ruled out fuel and ignition related faults which meant our diagnostic direction was pointing us towards a mechanical issue. We’d also carried out a cylinder balance test and identified that cylinder 4 was at fault, and whilst having some input was definitely that cause of our sick engine. A compression test was completed and all four cylinders showed a max compression of 200psi, this was a little disappointing as I was hoping that we would find cylinder four low and I’d be in for an easy life.Where do we go from here? We’re pretty sure it’s a mechanical fault but what to do next? Under pressure In many diagnostic situations it helps to look at the same data from a different perspective. While a compression test is a common test carried out in workshops up and down the country daily, there are times where it does not reveal all the information available. It’s in situations like this that we need to look at cylinder pressure from a different perspective. Enter the in-cylinder pressure transducer. I’m using the WPS500 from Pico, along with a Picoscope to capture the waveform in Fig 1. The benefit of a pressure transducer over a compression gauge is that the transducer does not have a non-return valve (as in a compression gauge) which means that you’ll see an accurate representation of the pressure in the cylinder throughout each of the cylinders four cycles. It’s a very cool way to see what’s happening in each part of the cycle and reveals so much more information than a standard gauge. Having a basic understanding of the four-stroke cycle as well as an insight into the points at which valve timing occurs is the key to the in-cylinder pressure transducer waveform. If you don’t have one from a known good car then all is not lost, as there is a lot of generic diagnostic information to be gleaned from this. It’s also worth remembering that like ignition diagnosis comparison of a good cylinder to the bad one can get you a long way down the road to a first- time fix. Wondering where do you start with this waveform? The following points act as a good starting point for diagnosis: Peak cylinder pressure Expansion pocket Exhaust pressure Intake depression Exhaust valve opening 18 AFTERMARKET MAY 2019 TECHNICAL www.aftermarketonline.net MECHANICAL DIAGNOSIS THROUGH THE EYES OF A SCOPE John Batten takes a look at how using a scope will save you hours when it comes to diagnosing mechanical misfires Fig 1