March 2020

BY Frank Massey I ’m mindful of several recent diagnostic topics that focused on cutting edge opportunities such as noise and vibration analysis. It also reminded me of the most important aspects of fault finding; to focus on the symptoms, ask relevant questions and conduct a methodical approach based on systems knowledge, accurate data and a proven process. All of this really boils down to training, experience, and confidence. There are no short cuts, cheap fixes or internet gurus. There are however basic steps that are easily introduced into your workshop procedures. This brings me to the topic in hand. Can we conduct relativity simple tests on common rail diesel systems? Not only can we, but we must! Remember, the foundation rule of fault finding is a simple methodical approach. Don’t expect a magical fix-all in less than 1,000 words. However, I can provide a pathway that will illustrate the area of responsibility and potential investment in time and money. Vital information The first vital step is to listen and ask questions. Owners often have vital information. Remember this is not a recipe for short cuts or silver bullets for your machine gun. Your approach will always depend on the extent of problems. Will it run? are there any mechanical noises? Is there a loss of power? if so when? Is the fault intermittent and how did it start? There is an endless list of questions that will help establish a hidden history. I often find that a physical examination or health check helps understand the way the vehicle has been driven and serviced. This will often expose basic problems especially with charge pressure circuits. Try to explore all non-intrusive tests first. They may not be entirely logical in order of priority, but do provide results in the minimum time period. With experience, you will hone these steps into a razor-sharp intuitive process. Serial investigation Serial investigation is without doubt the correct first step. Do not jump to premature conclusions as serial data often shows symptoms, not cause. For example, a faulty air mass meter will cause EGR calculation error values, incorrect load and boost calculation. This is a common problem with many causes. The volumetric efficiency relies on the intake system, swirl flap control, turbo spooling, and a free-flowing exhaust system. Please note that I keep my thoughts non- specific yet focused on all possible causes. This is a very important reaction in any diagnostic process. Assuming a non-run condition, excluding any serial clues as often there are none, I would always check for the correct rail pressure. This can be done with a DMM. Expect around 1-1.5v with a quick rise time of 0.5-1sec. If it is slow to rise or low, check the priming system including the filter. This should be done with a gauge. Remember pressure, flow and pump current. This will depend on system type so check the schematics carefully. Most systems now prime at 5-6bar. Isolate components A slow rise time may be due to an internal leak or worn components within the high-pressure system. This includes 28 AFTERMARKET MARCH 2020 TECHNICAL www.aftermarketonline.net DIESEL DIAGNOSTICS FOR THE WORKSHOP Frank examines how following tried and tested diagnostic procedures in a consistent way will enable you to successfully find diesel faults BMW EDC 17 fuel rail capped off for pump proof test Fuel weeping past the limit valve