I have an all-cylinder EGT-CHT monitor. My fuel-injected Lycoming usually operates with all cylinders no more than one “bar” apart from one another on the EGT. Lately I am occasionally seeing one cylinder go 50 to 100 degrees higher on EGT. It seems to come and go in cruise, though lately it has become more persistent. I thought that this meant I might have a bad plug, but a plug change didn’t help. If I switch mags in flight while the EGT rise is there, I can tell I have a missing cylinder. Since it is on the left mag when it misses, and the problem is on #1, it is the top plug; but the plug looks normal when pulled (and a new one was tried). My runup seems to have a normal drop on both mags, and no miss. What is going on?
While this could be a magneto problem, it is far more likely to be a plug harness problem. The harness wire to the top plug in #1 cylinder has an intermittent short in it, most likely (but not necessarily) either at the cap end or the plug connector end.
Here is the scenario. There is comparatively low cylinder pressure at idle and at no-load run-up power; it therefore takes less voltage to ionize the air in the plug gap, and fire the spark. That gives you a good idle and a decent run-up. The magneto builds up only the voltage needed to arc the gap, then the spark jumps and the fuel ignites. When the engine is at cruise power, cylinder pressures are far higher. It takes a significantly higher voltage to ionize the plug gap. BTW, this is why professional plug testing machines pressurize the plug before testing it. The magneto will build up spark “pressure” until it reaches a voltage that is sufficient to jump the insulation… and it doesn’t care whether the insulation is the air between the plug electrodes, or the dielectric in the wire jacket, or the phenolic in the mag tower and rotor. Wherever the “weak point” is, the spark will jump.
Of course, the weak point is supposed to be at the plug electrode, but many conditions can cause it to form elsewhere. In many cases, the bend that often forms at the mag caps, and at the plug connector, combined with normal engine maintenance activity, causes a weak spot in the inner dielectric jacket, and an arc burns through. It can look like a tiny black speck on the white liner.
This doesn’t necessarily mandate complete harness replacement, or magneto work. Any shop that does regular engine maintenance should have a high voltage lead tester. You can easily test the suspect harness lead (and all the rest while you are at it). With the lead disconnected from the plug, the tester is connected to the lead’s plug spring and the button pressed. The tester has a fixed gap behind the viewing window. If the lead’s insulation path is intact, the voltage gets high enough to arc the gap in the window, and you see the blue sparks. If there is a weak spot somewhere, the window stays dark. Sometimes you can hear or see the place where the spark is leaking to ground. Sometimes it will help narrow it down (between lead and mag), if you remove the mag cap to isolate the harness. If the problem is there with the cap on, but the lead tests good with the cap off, then the problem may be in the mag tower and rotor. Keep in mind that if a lead fails the first test (still attached to the mag), you have to follow the instructions and move the prop a bit, to ensure that the rotor isn’t aligned with the terminal post during the test.
If you can locate the problem, in some cases you can repair or replace the failed lead, or mag tower or cap, if everything else looks and tests good. Regardless, if the problem isn’t nailed down, it will mean parts-replacement diagnosis. The right equipment and knowledge can save a lot of time and money.