June 2021

BY Frank Massey F or this month’s topic I am going to revisit a subject which still attracts a lot of interest, but with perhaps a more forensic approach; Common rail diesel and pressure evaluation using Pico scope with WPS pressure sensor. Before we get into a detailed overview of pressure waveform analysis, let’s explore some of the other important tools at our disposal. Firstly, Communication. This is something that is becoming a lost art, especially with social media and online booking. Never overlook the opportunity to question the driver of the problematic vehicle. Note the expression ‘driver’ as often it may be booked in by someone else. Here lies the first problem, opinion, description, expectation, and cost. This is where, in my opinion, the most skilful member of your team is needed. Apart from the obvious technical information, you need how, where and when. You need to establish if they value their car, and if they do not, they will not value you. Next comes with what I call common-sense; observation. Cars cannot speak but they do tell a story. This could be the most valuable part of your forensic skills, looking for evidence of cause. Non-intrusive examination has many forms, including a thorough serial interrogation. Do not be drawn towards DTCs without knowing why. Look for adaption or correction data, deviation of reliable known values, once again note ‘reliable.’ 30 AFTERMARKET JUNE 2021 TECHNICAL www.aftermarketonline.net COMMON RAIL DIESEL AND PRESSURE EVALUATION Is the pressure getting to Frank? It doesn’t seem that way, but he is certainly getting to grips with common rail diesel issues Reservoir All hydraulic systems, fuel, oil and water, require a reservoir of the appropriate substance. Just think about that one for a while, many do not, at their cost. It must travel from one location to another. Obvious, yes, so how are you going to check it? Pressure may be the obvious answer however, flow, and rate of change, (rise and decay time) are just as important. Let me throw a curve ball in here; Do you have one electric supply pump or several? The laws of physics dictate they consume current, so let us explore how observation of current will predict the physical environment of fluid flow. Initial inrush; How much current is required on start-up can suggest a faulty pump, a restriction in flow or incorrect viscosity. The continual current rate proves a similar conclusion; Too much might suggest a restricted filtration system or blockage. Too little may indicate a worn pump, low pressure or lack of fluid. All of this can be checked from a control fuse easily accessible and quickly. We love quick, and accurate. The hidden problem overlooked my many is experienced when relying on serial data alone or even pump current and flow rate when measured blind. Are there cavitation or voids in the flow stream? The void represents a pressure differential, my favourite subject, between supply and demand. This brings us to pressure gauges and the scope.