Investigation
Amongst all the hi-tech, ultra-fast, multi-computing vehicles of everyday life comes a real gem from yester year that still requires a solid basic understanding of ignition systems and a PicoScope to confirm the ignition cycle in real-time.
The vehicle in question was a 3.8 E-type straight 6 cylinder that I have to say was a stunning example of this model and privilege to work with. Our customers concern was unstable idle speed, reluctance to rev, and horrendous backfiring with hesitation at wide open throttle (WOT [the default position for E-type drivers]).
The vehicle had been fully restored to factory condition a number of years ago and only ever came out for fun driving during the summer months. For almost nine months of the year the vehicle would never turn a wheel and this is a clue to the underlying cause of the symptoms.
Like all vehicles regardless of age a basic inspection was carried out to confirm connections, harness routing, fuel quality and fuel supply were all in place. Given the technology at the time of manufacture, this extended to checking the triple “SU” carburettors for sticking, along with the ignition points contact breaker gap and condition (how often do we say that now?). All the above proved to be fine and so I moved to checking the tuning condition of the engine bearing in mind the symptom of backfiring via the intake and the exhaust.
The ignition timing and dwell angle all proved to be correct and all triple carburettors drawing equal amount of air with very similar main jet seat positions. So on the surface all looks well, until I loaded the ignition system by applying WOT.
Using the timing light to view the timing marks under initial WOT application, the engine became so violently unstable the results of the ignition advance test were virtually useless, and so the only logical step was to view the behaviour of the ignition system using the PicoScope.
Looking at the ignition primary voltage and current at idle all appeared well with adequate ignition coil saturation, equal dwell periods, and even burn times. Then, as the ignition was loaded during WOT the truth was revealed when the ignition primary current increased dramatically at the point of “contacts open”. At this stage the current should have been cut instantly to collapse the magnetic field within the ignition coil and so produce adequate firing voltage within the secondary ignition circuit.
Reviewing the ignition captures using the replay feature of PicoScope Automotive it was easy to confirm that “distributor and rotor shaft wear” were NOT at fault because even though the misfire was clearly evident in our captured trace the dwell periods remained equal. Often at high rpm, wear in the distributor becomes noticeable due to centrifugal force being applied to the shaft which in turn alters the dwell angle, and so misfires develop. See below:
