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Diagnosing cracked spark plugs with an engine monitor

Before sending the plane for its 100h spark plugs check, I spent a few minutes looking at the EDM data; having just finished John Deakin's excellent "advanced pilot" web training, I felt confident I would be able to spot anything suspicious, and would give the maintenance shop a heads up. Apart from a potential fouled plug on cylinder 5, there was nothing obvious. The shop came back the next day, having found a cracked plug on cylinder 1. I've now spent another few minutes (hours?) looking at the data, and nothing shows up, either during engine run mag checks or cruise mag checks. What should I be looking for, either on the EDM or any other instrument? The engine wasn't running rough at all, and isn't noticeably smoother now.


That's strange, because a bad spark plug can normally be identified by erratic EGT on that cylinder

I'm sure I saw something about this in one of Mike Busch's webinars. Perhaps number 10, or number 2? Anyway, they're all worth a watch.

EGBJ / Gloucestershire

What plane do you fly, Denopa? Your profile is empty.

I would definitely not expect a cracked plug to show up in EDM data. The only conceivable way it might show up is if the plug is flashing over somewhere where it should not be, but (I used to work in high voltage) to jump a few mm you need ~ 20-30kV even at sea level pressure, never mind the 8-10 bar or whatever in the chamber at ~TDC (assuming fairly blunt electrodes so less opportunity for corona to kick things off).

If this is a Cirrus, they have a long US history of cracked centre insulators on the turbo engines, and (as with the dodgy Champion resistor saga) people have been moving to Tempest en masse.

I have never seen a cracked plug but then I fly an "easy" 250 HP non-turbo engine

A sure way to crack a plug is to drop it and not say anything. It would take a man with balls of solid brass to spill a load of iridum plugs, worth say €1000, during a 50hr check, and tell the customer he did it.

Shoreham EGKA, United Kingdom

When i had a foul lower spark plug on cyl. 3 some weeks ago ( because i typed in 30 waypoints and forgot to lean the engine) it could clearly be seen on the EGT, and of course the EGT went to 0 during runup on that spark plug

Yes, my engine works hard - harder than a Cirrus as it needs to drive two turbos :-)

Those webinars look great, thanks for the link!


Fouled plug is a shorted plug, which will show up (EGT rise and/or slightly fluctuating EGT). Not same as a cracked insulator.

Shoreham EGKA, United Kingdom
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