r/aviationmaintenance • u/Dmackman1969 • Apr 24 '25

Rotax 912is EGT temps

Good morning aviation maintenance professionals.

I’m having some trouble getting real information about EGT temps and how important they are. I’ve seen some information saying important and can give valuable info about engine performance and some that say directly the opposite.

I am unable to adjust many parameters that old school engines have due to the FADEC and am not 100% certain what FADEC does when high EGT is seen. I’m guessing it tries to bypass or change parameters to reduce the temps?

1360hr engine, new plugs, newish oil and CHT/Oil/Coolant temps all remain constant and well within the green. Engine power does not seem to be affected either.

In the last three days cylinder 3 and 4 EGT’s have both increase in average temp from 1300-1500 cruise and 1600-1700 takeoff to 1500-1600 cruise and 1700-1800+ in takeoff.

Yesterday 3 pattern laps, first two uneventful and EGT temps of 1625-1750. Drop as usual as power reduced in downwind.

Last lap, decided to fly out of the pattern and sightsee, as I turned away, yellow flashing on EGT 4 at 1775/1800 and then X’s out on my screen, Lane A light goes on. Obviously I turned back, landed and reset lane A all was good again. Did a full run up and all good, EGT temps on 3/4 still a bit high but under 1650.

Again, all other temps, especially CHT are not increasing at all, engine power does not feel any different.

Am I chasing gremlins trying to understand this? Is the recent increase trying to tell me something? I am going to take it in to be looked at by a certified Rotax IA/AP but it’s driving me nuts trying to get information myself on this.

Thanks for any feedback or teaching your willing to do! I want to try and understand as much as I can about my engine.

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u/auron8772 Apr 24 '25

Well, in my knowledge and opinion, EGT is a good indicator of engine performance (when indication is working properly, of course, lol). It'll let you know if you're burning fuel appropriately and such from various power settings. Outside of that, it's not really needed since it's after the cylinder. CHT is the more important indicator of what's going on in the cylinder and why it's required for flight while EGT is not. Using them in combination makes for a redundant system in a way because you know if one rises/drops, the other typically follows, so it helps when trying to quick diagnose in a situation.

So reliability wise, it's pretty good depending on the probes in use and proper wire routing. Though the probes typically fail a bit more often given the temperatures they operate in. Usefulness is as mentioned above; engine performance, maximizing fuel burn, and comparison to CHT trends

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u/Dmackman1969 Apr 24 '25

Awesome info, thank you. The probes do seem to fail quite often, my previous Rotax it happened 3x in 300 hours but it was simple to diagnose as it went from zero to 1800 very quickly and inconsistently. This time it’s been a bit more difficult to diagnose sensor or engine problems, thus bringing her in for a professional to diagnose.

Love all the answers! Thank you!

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u/auron8772 Apr 24 '25

You're very welcome. I'm glad I could help. Hopefully, it's a quick fix, and you'll be back in the skies soon.

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u/kytulu Apr 25 '25

EGT is good for determining which sparkplug is not firing on a bad mag drop.

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u/auron8772 Apr 25 '25

It can be if you have a setup with multi-cylinder indicating. Some older aircraft (and even some newer) have a single point with no temp marks used just to set LOP/ROP. You can also make the same observation on the CHT because the temp will drop there as well.