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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?

Rick Gostlin
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    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 05:20
  • 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
  • Assuming this is a 2 wire coil. From the wave forms shown it's hard to say the PCM is dead. The current wave form suggests that the PCM is turning the coil on and off. If it wasn't then the wave form would be a flat line. – vini_i Jan 01 '17 at 14:42
  • how's the wiring routed on this engine? does the harness come near the alternator at all? is it a shielded? – Ben Jan 01 '17 at 20:11
  • @vini_i Yes it's a two wire coil. The issue I have with the current waveform is that for any current to show up on the probe, there must be current in the line. If there is current in the line, it can only go one of three places:
    1. Into line capacitance, but there is too much current for that to be plausible
    2. Some unknown wiring short. This is plausible, there could be a broken wire with some path to chassis ground. I have checked as best I can and wiggled the entire harness while watching the scope
    3. Back into the PCM. As in its getting past the driver, therefor a weak or bad driver
     – Rick Gostlin Jan 03 '17 at 23:05
  • @AliChen, Ch1 is the primary voltage. In these screen shots it is attenuated. It usually pulses up to about 300V, but has the same shape. – Rick Gostlin Jan 03 '17 at 23:10
  • Did you resolve this? If yes, could you say what the problem was? If not, (1) is the issue re the faint (dashed) line in trace 'b' in the 2nd image? (2) Trace 'a' in the 1st image looks flat; was that just not connected? (3) Trace 'b' is the secondary current? – George Mar 19 '17 at 11:46

1 Answers1

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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

Bevan
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