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2012 subaru forrester constant misfire at idle. Waveform taken with the COP probe.Ignition spikes can also be seen at the MAP and MAF. All four coils is identical. Can all four coils be faulty or can a bad ground cause this.
Not sure what all that noise is before the firing line on the ignition waveform, but if all four coils have that same characteristic, then I'd suggest it's not part of the problem.
If it's a dead misfire at idle, then I'd back up and do a power balance test to see which cylinder is weak.
Constant misfire at idle that smooths out at higher RPM would make me look for a valve sealing issue. As for the wave form it is weird as it is on the left of the firing point it isn't anything inside the cylinder.
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Last edit: 7 years 8 months ago by Andy.MacFadyen.
Had a relative compression done and results came back 100% 100% 84% and 94%, so there fore if an intake valve is leaking engine takes in more air which explains the lean condition.
I was focusing on the noise before the voltage spike and missed the misfire, maybe this weekend ill check the other oils for a lean condition , if its the intake valve thats leaking. Two coils should be showing lean and two should be ok.
Exhaust valves give more compression problems than inlets, the heat and hot burning gasses especially if the mixture is weak however relative compression is more a pointer to carry out further test than a definitive diagnisis. Istr a year or so back there was a Subaru with dead miss at idle that went away at higher RPM that turnrd out to be a trigger wheel crank sensor issue.
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
Regarding the hash just before the spike: this ignition system utilizes current control. On a good waveform, you would see the current ramp up as a result of the current building up. Once the current reaches a predetermined peak, the driver holds the current at a fixed level which is typically identified in the voltage waveform by the flat step that occurs before the spike. To investigate this more, you might want to capture an amperage waveform to see if the amperage is a clean signal ramping up and then flat topping when the current control is activated. Note that the voltage can look bad but the amperage might still be good. Either way, driver does have an issue but might not be the issue you are looking for.
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