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How to diagnose a misfire – by David Massey

By autotech-nath on March 29, 2017
I love the word ‘misfire’, it is the most misused expression in the technician’s vocabulary! Simplistically, it falls into three categories: Ignition malfunction; mechanical faults or fuelling errors. There we are – all done, end of topic? Well, not quite. Let me spend the next 700 or so words expanding on why and how. Rather than ‘misfire’, ‘combustion failure’ or ‘anomaly’ is the correct expression and there are several routes to explore to find the underlying cause of incomplete combustion.
Engines now include variable valve timing on all cams and variable lift, much of which depends on hydraulics and/or electronic actuators. Incorrect oil specification or service intervals are prime causes of valve timing errors. Many also have flexible oil pressure control with PCM mapped piston cooling. Direct fuel injection requires precise fuel delivery, this includes induction turbulence control. Intake ducts, flaps and valve gears often suffer
from carbon contamination, a very common cause of transient throttle combustion malfunction. Worn valve guides affect critical pressures in the combustion preparation phase.
Careful interpretation of the scan tool’s live data stream (parameter IDs) paying particular attention to adaptive

correction values (fuel trim), can help provide an indication of a cylinder underperforming. Conducting a real time in-cylinder pressure evaluation, using the Pico WPS, provides a clear picture of what is happening inside the cylinder.
Note my choice in words carefully! As there is no such thing as vacuum, it does not exist! Its pressure differential relative to atmosphere. As the piston moves on the downward strokes, the volume of the cylinder is increased and a change in pressure occurs. This change is referred to as the expansion void. Careful scope examination will expose discrete anomalies in the operation of cams and valve gear. The two voids indicate intake efficiency and valve overlap map timing.
Screen Shot 2017-03-29 at 19.15.38The latest fuel injection systems now use fuelling maps, which incorporate infinite control deviation, stratified and homogenous delivery modes, split injection timing on both intake and compression strokes, as well as sequential quantity, current correction and simple trim deviation.
The injectors must atomise the fuel correctly across a pressure range of 40-200 bar. The inductance values

are a precise requirement if opening and closing timing is to be met. The introduction of port and direct injection
to reduce particulate emissions introduces complex fuel mapping of timing, quantity, delivery method and pressure.
To help diagnose issues, carry out detailed evaluation of fuel trim values, cylinder deviation, ignition timing correction and exhaust gas temperatures. Further evaluation can be carried out by removing the injectors and conducting precise testing on a bench flow tester such as the ASNU kit, which will highlight any problems with spray pattern or flow rate.
So finally, we arrive at the easy bit! Think again, not only must the ignition system generate the correct energy
required for complete combustion, it must deliver it totally. I don’t subscribe to the manufacturers plug life guide as it has no allowance for operating environment deviation, fuel quality or servicing regime. I own a Seat Leon Cupra 280, the official plug replacement interval is what? You can find the answer at the end of the article. Plugs
must now be torqued so the angle of the electrode is correctly aligned with the injector. Ignition control is also

symmetrical   sequentially mapped and corrected; knock monitoring, exit temperatures and lambda data are just some of the variables as well as multiple strike features.
Although serial data offers some insight into ignition performance such as extensive misfire monitoring, it does not provide definitive diagnostic evidence. Four critical elements in order of importance are: 1/ Ignition burn

time, the precise length of the energy cross over. Its affected my many of the previous problems, notwithstanding its passage through the fuel air mixture, expressed as slope and turbulence. 2/ Primary current control, the inductance in primary is vital for correct secondary energy generation. The saturation time ‘dwell’ controls current flow and peak value, but it’s the speed and nature of magnetic field interruption that ensures good induction. 3/ Coil ringing, the  symmetrical pattern at the end of the burn period qualifies the condition of the coil windings, condition as well as good insulation. 4/ Firing line voltage, the net value of energy required to overcome the path between the coil and the spark plug. It’s the physical circuit as well as the plug condition and can be effected by in cylinder environment changes.
PicoScope evaluation using voltage and time, evaluation of spark profile and primary current path assessment – there is simply no other way. It’s vital you know specific system values as they differ across system types.
The conclusion? I guess I should leave this to you, but if you need help, visit our ADS Training Academy in Preston or Autoinform Live in Edinburgh on 18/19 March, email for course details. Frank will also be joining Autotechnician at the Technical Workshop Theatre from June 6 to 8 at Automechanika Birmingham. We hope to see you soon! Oh yes, the answer? Six years.


About Autotechnician
Autotechnician is a magazine published nine times a year, delivering essential information to independent garage owners and technicians in the UK. Delivered both digitally and in print, autotechnician provides readers with technical, training, business advice, product and news, allowing our readers to keep up to date with information they need to run and work within a modern workshop.
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