I haven't posted in a while but thought this might be of interest. I fly an RV-10 Aerosport Power IO-540 with a slick mag and a Lightspeed Plasma III ignition. Everything has worked flawlessly for 1300 hours until a recent flight. I didn't notice the behavior when it first started but on a subsequent flight I was descending into my airport, lean of peak but still plenty of throttle, 8500', when the Advanced Flight Systems EFIS gave me a visual and audible alert that the #6 EGT (blue line on the plot) was higher than the others. I downloaded the data and saw this:
It clearly shows the #6 EGT (blue) departing from the others. The only reason I noticed the slight departure was that it finally got into the range I had set for a yellow alert - at about the 25 minute mark on the above plot. On reviewing the data, the departure only occurred when I was lean and at high power settings, over 20". I then looked at previous flights and found this from a few days earlier.
You can clearly see the #6 EGT departure in cruise (LOP and WOT), as well as a corresponding drop in CHT. However, the EGT#6 didn't depart a lot (80-90 deg F) and never reached the temp (I think 1440) I had set for a visual and audible alert. I decided to do some diagnostic stuff, including several GAMI mixture sweeps and an inflight LOP mag check. Here is that flight:
All the mixture sweeps were normal, my GAMI spread is excellent at 0.1-0.2 gph, but when I did the inflight LOP mag check at 13,500' the #6 EGT dropped out immediately and the engine ran very rough. You can see this where the blue EGT trace spikes downward at about 56 and 59 minutes. The #6 EGT and CHT are also behaving differently than the others throughout much of the flight.
I returned to base and checked the #6 plug (fine-wire Tempest). It looked fine. The spark plug leads all looked fine. The plugs resistances measured normal. I replaced the fine-wire plug in #6 with a used massive-electrode plug I had, just in case, and flew again. The LOP mag check at 9500' again showed the #6 EGT dropping off, but much more slowly than it had at 13,500'. I Repeated it at 6500' and everything ran fine.
The mag was due for a 500-hour, actually overdue, so I pulled it and found this:
There was clear corrosion on the spring in the magneto cap for the #6 lead, and corrosion in the socket on the mag.
Takeaways from this are:
1. Without an engine monitor it would have taken MUCH longer to even notice this problem, much less to diagnose which cylinder was causing it.
2. The Lightspeed ignition covers a multitude of sins in the magneto, which is a good thing...unless I need the magneto. Also, the magneto is not contributing nothing, as some people think.
3. The engine monitor, and paying attention to it, allowed me to quickly isolate the problem to a specific cylinder and plug, then find the problem.
4. The absolute value of EGT means very little, so I've considered removing the EGT alerts that I have set in my Advanced EFIS. However, they are useful for just this reason, to get an audible and visual alert when "one of these is not like the other."
5. The engine appeared to be running fine, smooth, but this was a problem that would not get any better, and might have caused some damage had I not caught it when I did. The corrosion in the lead socket was weakening the spark energy for that cylinder to the point that, under the conditions when the most spark energy was required, it didn't get enough to light the fire. Most importantly, if the Lightspeed had quit working and I needed the magneto it would have been unpleasant.
6. I should be in the habit of downloading engine data every oil change or so, even if everything appears normal, to look for things like this.
7. I should also do high altitude, WOT, LOP mag checks on a more regular basis.
Hope this is of some use to somebody.