I have a missfire on cylinder 3. I have swapped the coil from cylinder 6 and 3, but the missfire remains on cylinder 3.

I have also done the following: - replaced all spark plugs. - checked the wiring harness resistance all the way back to the controller.

I have access to a good scope and current probe.

This is the current waveform at idle of a good cylinder. The trace has 15 seconds of persistence, to show consistency in the waveform. Good cylinder current waveform at idle

This is the current and voltage waveform of the bad cylinder. The trace also has 15 seconds of persistence. Bad Cylinder current and voltage at idle

The voltage waveform in the above shot is attenuated, but shows the noise involved.

Notes: - no DTC is set with the miss-fires - no miss-fire felt at idle (above shots are taken at idle). It is worst at about 1600 RPM in 6th gear. - This is a 2010 mazda CX9, it has dual spark ignition below 1500 rpm. - The Fuel injector on this cylinder was bad and replaced. - Intake manifold gasket was replaced. - intake was de-carboned.

Am I looking at a bad engine controller?

  • 1
    This could be a failed driver circuit. These are usually driven by relativity simple setup. The usual failure is breakdown of a MOSFET or similar transistor. It can be helpful to see the voltage signal where a failure of the transistor pull all the way ground intermittently is the most common failure. – Fred Wilson Dec 31 '16 at 5:20
  • How many wires go to the coils? – vini_i Dec 31 '16 at 12:22
  • can you capture the coil trigger signal as well? – Ben Dec 31 '16 at 22:16
  • @vini_i IIRC it's a 2 wire deal – Ben Dec 31 '16 at 22:19
  • What is on Ch1? – Ale..chenski Dec 31 '16 at 22:40

Waste Spark systems would have 2 cylinders giving trouble if it's anything upstream of the coil, as the one coil does both cylinders. Did you swap the dropper tubes and lead over too? Sounds like a dead dropper tube to me.

Some coils, normally 3-wire ones, have drivers in them as opposed to in the ECU. It may be as simple as swaping coils around and seeing what happens on the scope

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