Diagnostics by definition

An easy fix is not the same as an easy diagnosis... but what exactly does that term mean?

Published:  08 January, 2014

By James Dillon

During the booking in process it became apparent that this job may not be straightforward after all. The vehicle owner had a common tale of woe, having been around to several other places before, who had all done a 'diagnostics'. I am beginning to wonder if diagnostics is THE most over used and abused word in the motor trade. What exactly is a 'diagnostics'? It seems to be a very ambiguous term. So I was wondering, thinking like a non-technical car owner, what would I think a 'diagnostic' is? Is it something a technician does? Does it relate to a piece of equipment? Is it a process or a procedure? Does it require special skills or training? Is it the same, or at least related to, a 'plug-in'? In truth, there is no common view on what 'diagnostics' is. The great irony of this situation is that, I bet you a pound to a penny, there is a market price for one in your local area.

Interestingly, a dictionary definition of a diagnosis is: (a) A critical analysis of the nature of something, and (b) The conclusion reached by such analysis.

Figure 1: This injector was barely moving

The bottom line is that the art of diagnosis involves a significant element of data gathering. You know the sort of thing I mean, this evidence can include customer questioning, visual inspection, road testing, fault codes, scan tool serial data stream analysis, multimeter test and measurement, fuel pressure, vacuum, leak off, back pressure, oscilloscopes, gas analysis, smoke machines and so on.

During the relevant data gathering, critical analysis should occur. Then, and only then, it might be possible to reach a conclusion. As a business or a technician selling diagnostics, you are being paid to gather data, perform a careful, critical analysis and reach a conclusion or make an appraisal based on the gathered data. Then, and only then are you in a position to recommend corrective action and what steps should be taken to correct the symptom.

So, back to the faulty vehicle case in point. The vehicle had plugs, coils, new MAF and O2 sensor fitted to date. Did any of this new stuff improve the situation? Not in the eyes of the customer. The car, in her words, was exactly the same. The symptom had developed out of the blue, no other problems were present. The car was 'juddering' when sitting at idle and it lacked power when driven, all of which are classic misfire symptoms. A quick visual inspection showed nothing strange or out of the ordinary, no air leaks, no rubbed through wires, loose or bad connections.

The lady brought the diagnostic print out from her previous garage visit with her. It showed P0300 and P0340, random or undetermined misfire, and misfire cylinder 4. The MIL was on and the fault codes stored within the engine computer were the same - no real surprise. My next step was to determine if the misfire was due to a lack of fuel, a lack of a spark, a loss of compression or one of these things occurring in the wrong order. The gas analyser is a great tool to gather data on the nature of a misfire event. Lack of fuel gives a low hydrocarbon (HC) reading and a high oxygen (O2) and a lack of compression or a sparking issue gives a high HC and high O2. The gas reading indicated a low HC and high O2. Low HC combined with high O2 indicated that no combustion had taken place (otherwise the O2 would have been 'used up' in the process).

Therefore, critical evaluation of the data led me to conclude that there was a lack of fuel. Only one cylinder was misfiring, this was confirmed by gathering data via a cylinder balance test. Cylinders 1, 2, and 3 were all contributing significantly to the 750 rpm idle speed (each cylinder caused the idle speed to drop between 200 and 300 rpm when disabled). I could have proved compressions were good by performing a relative cranking compression test but felt that I already had enough conclusive data.

Figure 2: Comparison with a neighbouring good cylinder

What was immediately obvious was the presence of any injector command voltage, however there was very low current flow. The lack of current meant no (or poor) operation. If you can imagine, the injector was the equivalent to a bulb with a poor connection. Rather than the bulb barely 'glowing', this injector was barely moving, resulting in a lack of fuel flow. Figure 2 compares the same waveform taken from a neighbouring good cylinder.

Now my task was to determine the root cause for poor current flow. It's likely to be caused by a high resistance, which will have affected either the power or the ground element of circuit. It is possible to determine which by further analysing the waveform. In this case, I zoomed in on the grounding part of the waveform and analysed the data. I could see a large voltage drop. This meant that the vehicle had a wiring or an ECU fault. In this case, the most obvious places to check were connectors and multi-plugs. En-route to the ECU there was an engine loom multi-plug which showed signs of water ingress, once this was cleaned and repositioned back in its fixing clip, the misfire disappeared.

This case highlights that electrical fault diagnosis is geared towards spending a large amount of time testing and analysing and a short amount of time 'fixing'. Whereas on typical mechanical faults, a short amount of time is spent diagnosing and the bulk of the time is spent fixing. Perhaps this completely opposite way of working is affecting non-specialist diagnostic technicians approach to doing 'diagnostics'. Perhaps a kind way of putting it is that garages who don't specialise in diagnostics are trying to 'diagnose' the fault too quickly? I suppose this, allied with the fact that they may not have the necessary tools to gather all of the relevant data and could lack the required skills, experience and training to execute careful analytical evaluation.


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