Many technicians who succeed at fixing more cars faster generally adopt different fault finding techniques. Those who are doing mostly service and mechanical repair don’t need or don’t want to go much beyond basic fault codes and rudimentary testing. If it’s not a simple fault, these folks are happy to refer the customer elsewhere, or to ‘get a guy in’ to do the ‘difficult diagnostic stuff’. I understand that. These people are happy earning money from service and repair, so they don’t have to get involved in this type of work.
An interesting question to consider is what factors would classify a fault as too difficult? What makes it a fault that can’t be done by the service guy? Would it be a symptom and no fault codes? Or perhaps many fault codes and no symptom? Or even a fault code that fitting the fault coded parts wasn’t fixed by replacing those parts? How about your bog-standard misfire diagnosis? Does this fault classify as too difficult? Not at the outset maybe, but what if new injectors (or a new spark plug and coil pack for petrol engines) didn’t solve the issue? What if compression was good on the gauge, so it had to be something electronic? Well folks, hang on to your metaphorical hats, because I’m going to take you down a potential rabbit hole of a diagnostic problem which revolves around a simple engine misfire.
The guiding principles of misfire diagnostics, when I was a young whipper snapper, was to validate basic elements required for combustion, such as compression, ignition (compression or spark), fuel and timing (all these things in the right order). Several tests exist to ‘prove’ each of these things. It’s likely that the equipment present in the workshop, and the relative skill level of the tech applying them, will define
the outcome of the data gathering. Of course, almost every technician can use a compression test gauge (petrol or diesel), but perhaps it takes a special sort to consider the frailties of this test without external guidance or influences.
My very old and worn out saying ‘the 4-rights of data gathering’ – that you must measure the right thing, in the right way, at the right time, with the right tool – comes to bear. When you can’t diagnose the problem, one of the 4-rights ‘rules’ will have been broken. The compression test has a major flaw; there is a compression fault that will pass the compression test but will cause a misfire condition. ‘The 4-rights of data gathering’ were developed to enable technicians to be critical of the results they gathered and to challenge test results which seemed ‘OK’. Over the years, many of the jobs that appeared at Technical Topics HQ had the accompanying technician statements of what couldn’t be wrong, because these ‘things’ had been tested already. Yes, but was the right thing measured, in the right way, at the right time, with the right tool? Invariably, not. There would be something, often very ‘minor,’ that had slipped through the first garage’s diagnostic process.
So, onto our problem. We are faced by a vehicle with a misfire. My favoured technique is to validate cranking effort via the starter motor current draw. This method requires no parts to be removed, so it’s quick and accurate. We simply hook a current clamp to the battery negative cable, measure the battery voltage directly and crank the engine for 10 seconds. The principle is that the mechanical effort of the engine will cause the starter to pull peak current during each TDC event, in the same way your arm must apply more pressure on the socket and bar when you wind the engine over by the front pulley bolt after a timing belt change. We simply look at the current peaks and check for the differences, in exactly the same way that we would look at the needle on the gauge of a traditional compression tester. The current clamp test technique earns you more money than the traditional test, as it saves a whole heap of time and is highly unlikely to cause mechanical breakages to spark plugs or glow plugs.
Figure 1 shows the resultant current waveform from the misfiring engine during cranking. It can be seen that there is an imbalance in the mechanical effort, as the current draw is different on corresponding peaks, there is one peak for each of the cylinders. Now, let’s see if the gathered data passes the 4-rights sense check. What are your initial thoughts on the cause for the imbalance? Please take a second to consider the answer before you read on.
Thought one, which may be derived by the Anchor Thinking principle, which we covered on the recent Big Day Out in Reading, is that the lower current draw cylinders (they would also be shown as lower on a compression gauge) are indicative of our problem.
Thought two, is that the higher compression is our problem cylinder. This thought is counter intuitive, as we are conditioned to consider low is bad – after all, who ever saw a compression that was too high, let alone one that was too high that caused a compression issue?
The judgement you make based on this data is going to guide your next step and your next test. The benefit of adopting novel data gathering techniques is that non-intrusive testing is possible; mechanical strip and fit then occurs as part of the rectification, not as part of the diagnosis. In practical terms, the vehicle can be fully diagnosed whilst remaining in a running condition. This is particularly useful where mobility and workload planning are influencing factors of workshop management.
Back to our thoughts on diagnostic direction, both answers may be correct and this is often the dilemma we face during diagnostics; a definitive diagnosis cannot be drawn based on this limited amount of data. To be definitive, we should challenge this data/result with another test, or at least have comparative or baseline data. Remember that data must have context to become useful information.
Our compression problem is not unique in the fact that our comparison in data is uni-conditional (99.9% of the time is evaluated against a ‘too low’ value). We will progress to the next step in a subsequent case study.
James’ highly recommended Diagnostic Bootcamp training courses will help you fix more cars, faster. See his website for details: www.techtopics.co.uk/training