Troubleshooting help needed

I was flying in cruise at 9500' for about 20 minutes, lean of peak, about 60% power and all egt's started rising and the engine got rough. When I went rich of peak everything seemed fine. I finished the flight, put the plane away and came back a few days later to see if I could reproduce the problem.

Start up was fine but during runup, when I did the mag check, #1 and #2 cylinders showed dramatic cooling off of EGTs (cold cylinders) on each individual mag check but when in the BOTH position everything was fine. Just to make it clear, Both position=everything good, Left mag=#1 and #2 are cold/not working and Right Mag= #1 and #2 cold/not working.

Fuel Injected, Dual PMags.

I took apart the key switch which is off/left/right/both NO start position and I could see no evidence of arcing or fouling. It looked pristine. I did a continuity check on the ground wire at the switch and it shows that when touching the other side of the DVM to anywhere on the airframe it shows continuity (beeps). I checked each individual ground on the wiring block at the back of the PMag that goes to the engine case and they appear solid.

I'm having a hard time accepting a simulataneous dual mag failure and one that takes out only 2 cylinders but the same two cylinders no matter which mag is isolated.

Any help with troubleshooting procedures would be very welcome. I haven't called Brad at emagair yet because I'm assuming he's swamped at Oshkosh. Thanks!

Lean running cylinders can cut out during a mag check, In my case it was a leaking induction tube. Just a little air will do it.

Bad spark plugs

It sounds like one bad spark plug in Cyl 1 and Cyl 2.
Swap top and bottom plugs of Cyl 1 and do a mag check.
One mag should check good the other bad.
That will isolate which plugs to replace.

I would pull the spark plugs out and ground them to the block and do a spark test to confirm for sure to see if they do spark on Left, Right or Both. This should determine for sure if the PMAGS are working, although it will not be a good measure of strong spark or weak spark. Perhaps you can also swap the spark plugs with other cylinder to see if any thing moves or the trouble will remain with the Cylinder 1&2.

I don't think I would need to remind you to pull all spark plugs so there is no chance of the engine firing, while you do a manual test and turning the prop.

When troubleshooting this problem things to know are, pmags use a lost spark design, only two coils are used to fire four plugs. So, 1&2 share a coil and 3&4 share a coil. You could have a bad plug or shorting plug wire on one side of the lost spark and this will effect the other plug on that circuit. You could also have a problem with the coils, there have been some failures. Also, unlikely, the pmags are not triggering the number 1&2 coil. Also, unlikely, there is a connection between the number 1 and 2 induction system having a problem at the same time. If the mags prove to be working properly checking compression and the cam would be a logical step. The cam lobes are shared between opposing cylinders. Good luck

update...

A friend discovered a spark plug wire in contact with another spark plug wire. I had been careful to keep them away from each other and anything else that could cause them to short to anything other than the plug gap, but over time things rotated and they touched. I went looking for any telltale signs of arcing and sure enough on the top #2 cylinder wire I could see a small, well-defined black spot that looked pitted.

I went through and separated, insulated and secured it and other wires that were not far enough away from problems and with high hopes did the run up again.

The engine started great, mag check on the right was perfect, but the left mag still had #1 and #2 cylinders cold. It was suggested that the shorting wire could have fried the coil feeding those 2 cylinders.

A call to EMagAir was actually answered even though I thought it would be hopeless due to Oshkosh and they verified that the coil was probably toasted by the short. The good news is that the coils are apparently field replaceable, so they're sending a new coil, some new ignition leads and 4 terminations.

The repair will have to wait until I'm back in town after my music festival in Sun Valley Idaho (great classical and pops concerts with some of the best players in the country, all FREE admission) but at least I'll be there with optimism that I will be able to get Stella flying again quickly, when I get back.

jjconstant said:

A friend discovered a spark plug wire in contact with another spark plug wire. I had been careful to keep them away from each other and anything else that could cause them to short to anything other than the plug gap, but over time things rotated and they touched. I went looking for any telltale signs of arcing and sure enough on the top #2 cylinder wire I could see a small, well-defined black spot that looked pitted.

I went through and separated, insulated and secured it and other wires that were not far enough away from problems and with high hopes did the run up again.

The engine started great, mag check on the right was perfect, but the left mag still had #1 and #2 cylinders cold. It was suggested that the shorting wire could have fried the coil feeding those 2 cylinders.

A call to EMagAir was actually answered even though I thought it would be hopeless due to Oshkosh and they verified that the coil was probably toasted by the short. The good news is that the coils are apparently field replaceable, so they're sending a new coil, some new ignition leads and 4 terminations.

The repair will have to wait until I'm back in town after my music festival in Sun Valley Idaho (great classical and pops concerts with some of the best players in the country, all FREE admission) but at least I'll be there with optimism that I will be able to get Stella flying again quickly, when I get back.

Click to expand...

Were the wires simply crossing or touching each other that caused them to arch? I know we are suppose to keep them separate but I didn't think they would cause such drastic problem if they do come in contact with one another.

They were in physical contact with each other. They are not supposed to touch each other, or anything else that is metal such as engine, engine mount, airframe or other wires or adel clamps and such. Anything that can conduct electricity could cause them to arc through the insulation. I wish the insulation was thicker or more effective so this wasn't as big a concern.

How old are the wires? I suspect they are due for replacement.

Jeremy, how much for the replacement coil?