[FORANE]

On my last flight I noticed what seemed like an occasional slight misfire. The engine monitor appeared normal except for cylinder #2 running higher than typical EGT. The EGT on that cylinder would drift up a few times during the flight. I could reliably cause it to come back down by slightly enriching the mixture. I was running lean of peak at 9000 MSL. I was tempted to do a mag check but chose not to due to solid IMC with low layers.

So, what would cause high EGT on one cylinder? CHT on that cylinder remained normal. I considered it may be due to running lean. Maybe an induction leak? Maybe a clogged injector? Maybe a failing spark plug?

I pulled the wires off the Lightspeed coils and found this on the number 2 cylinder coil pin.


The other coil also exhibited some erosion on the number 3 pin.

I'm guessing this started as corrosion, led to increase in resistance, heat and finally erosion of the pin. Has anyone seen this before on a coil? Should I be using dielectric grease on the boot connections?

 

[hgerhardt]

Check the cables for proper resistance, and check while bending the cable on both ends. And check the coil for proper resistance. I've had a dual LS Plasma 2+ system for 18 years now, and the only time I've seen discoloration is when the cable had high/infinite resistance. I've never used dielectric grease.

And then replace your coil mounting bolts with the proper length, and installed in the proper direction (heads on top, nuts with washers below).

 

[bjdecker]

Check the cables for proper resistance, and check while bending the cable on both ends. And check the coil for proper resistance. I've had a dual LS Plasma 2+ system for 18 years now, and the only time I've seen discoloration is when the cable had high/infinite resistance. I've never used dielectric grease.

And then replace your coil mounting bolts with the proper length, and installed in the proper direction (heads on top, nuts with washers below).

…and proper nuts (read: MS21042)

 

[FORANE]

Check the cables for proper resistance, and check while bending the cable on both ends. And check the coil for proper resistance. I've had a dual LS Plasma 2+ system for 18 years now, and the only time I've seen discoloration is when the cable had high/infinite resistance. I've never used dielectric grease.

And then replace your coil mounting bolts with the proper length, and installed in the proper direction (heads on top, nuts with washers below).

Yeah, I checked resistance today. 8,000 ohms across both coils and the proper 50 ohms per foot on all cables was present.

Good catch on the attachment hardware. Ha, I didn't even notice that. Wasn't my install...

 

[blaplante]

I wonder if the connection in the boot wasn't fully seated on the coil... causing arcing ...

 

[FORANE]

I wonder if the connection in the boot wasn't fully seated on the coil... causing arcing ...

Didn't appear to be the case, but I suppose that would do it.

 

[lr172]

When running on one plug, egt will rise around 150*. Your terminal shows signs of arcing, so no surprise that you are getting occasional egt spikes. I heves seen this with my msd wires, but at the spark plug side . Used pliers to squeeze down the terminal a bit and problem disappeared.

 

[AlexPeterson]

It is very difficult to get the boots (and hence the internal clip) fully seated unless you trim a little bit from the boot for those coils.

 

[olyolson]

On my last flight I noticed what seemed like an occasional slight misfire. The engine monitor appeared normal except for cylinder #2 running higher than typical EGT. The EGT on that cylinder would drift up a few times during the flight. I could reliably cause it to come back down by slightly enriching the mixture. I was running lean of peak at 9000 MSL. I was tempted to do a mag check but chose not to due to solid IMC with low layers.

So, what would cause high EGT on one cylinder? CHT on that cylinder remained normal. I considered it may be due to running lean. Maybe an induction leak? Maybe a clogged injector? Maybe a failing spark plug?

I pulled the wires off the Lightspeed coils and found this on the number 2 cylinder coil pin.
View attachment 119125

The other coil also exhibited some erosion on the number 3 pin.
View attachment 119126
I'm guessing this started as corrosion, led to increase in resistance, heat and finally erosion of the pin. Has anyone seen this before on a coil? Should I be using dielectric grease on the boot connections?

When you clean the corrosion off the pins, reconnect the plug wires and go fly please report back if that fixed your issue.

 

[FORANE]

When you clean the corrosion off the pins, reconnect the plug wires and go fly please report back if that fixed your issue.

Cleaned the pins and flew last night. I have new coils and plan to change them to new but last nights flight home was on the old coils. This is a screenshot of my CHT/EGT in flight prior to cleaning. The number 2 cylinder always ran a little higher CHT and EGT than the rest but not this high. Also note that this was the numbers on its baseline and a few times the EGT wandered higher from there. When it wandered higher a slight richening would make it come right back down to baseline.


Below is a screenshot of the flight last night after cleaning the coil pins.


We'll see if it changes at all with the new coils. If not I may investigate my injectors to see if there's a way to make #2 more in line with the other 3 cylinders.

 

[bjdecker]

Assuming no other parameter changed (Density Altitude, OAT, Pressure, fuel flow), I'd say that cleaning the terminals had the desired effect!

Just out of curiosity, did you measure the secondary coil resistance on the new coils? 8K ohms seems a bit high (from memory), I thought these were 6K...

...and...

How much air is getting to your #2 cylinder - do you have a temperature riser installed? Pictures?

 

[FORANE]

Assuming no other parameter changed (Density Altitude, OAT, Pressure, fuel flow), I'd say that cleaning the terminals had the desired effect!

1st flight was 9,000 IFR at 8.3 GPH. 2nd flight was 10,500 VFR at 7.3 GPH. Both LOP. That 2nd screenshot is typical of where it has run for the year I've owned the plane. So yes, I'm convinced the corroded terminal on #2 coil was responsible for the elevated EGT. Prior to this incident I was unaware a corroded spark plug connection would manifest in elevated EGT.

Just out of curiosity, did you measure the secondary coil resistance on the new coils? 8K ohms seems a bit high (from memory), I thought these were 6K...

...and...

How much air is getting to your #2 cylinder - do you have a temperature riser installed? Pictures?

Just measured the brand new coils. One was 8.2k ohms; the other was 8.4k ohms. I think Lightspeed says 8k ohms is expected on good coils. My badly corroded one measured 8k ohms and the other measured 8.1k ohms.

Should be plenty of air. My plane isn't an RV. It lived at Reno Stead for over a decade and was setup to race. The guy I bought it from said it was fast but would overheat unless it was going very fast. So he made numerous cooling improvements increasing airflow and slowing it down in the process, but it doesn't run hot anymore.