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When leaning the fuel mixture on a C172 using the lean assist feature on the G1000, the hottest cylinder's EGT will be displayed, along with a reading showing the difference between the current temperature and the peak temperature recorded.

Whether rich or lean of peak, the delta should be negative, indicating a temperature colder than the peak. In practice, however, this number can be positive. What does this mean? Isn't a temperature "hotter than peak" simply a new peak temperature? Does it mean a different cylinder has surpassed the original cylinder's peak? (I am allowing the G1000 to automatically select the hottest / displayed cylinder.)

The instructions for leaning in the POH say to find peak, then enrich the mixture to 50 degrees below peak. How do I incorporate positive delta peak readings into this process?

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I believe I have found the answer to my question in the G1000 manual (previously I had looked at the POH for the C172 NavIII).

The ASSIST Softkey aids in the leaning process by identifying the peak of the first cylinder whose temperature falls. This cylinder’s number below the EGT bar graph is highlighted in light blue as the selected cylinder. If the temperature of the peaked cylinder exceeds the peak value, the peak value is not updated. Monitoring of the cylinder continues until the ASSIST Softkey is pressed again which disables lean assist, and removes the peak block from the bar graph and the temperature deviation from peak (ΔPEAK)

(emphasis mine)

This suggests that as soon as one cylinder's temperature drops, the temperature it had reached right before dropping is considered the peak. Even if it should get hotter that that temperature as the mixture is further leaned, the original peak remains the point of reference for the ΔPEAK indicator.

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  • $\begingroup$ This most likely is the correct answer, however, later version of G1000 handbooks have omitted the paragraph you cited, for whatever reason. $\endgroup$
    – Jpe61
    Nov 7, 2021 at 20:48
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    $\begingroup$ Thanks for posting the answer you found. BTW I found it now in the NAV III (I only checked the latest version), and it has the same info if you want to add that link or the main download link. Also @Jpe61 $\endgroup$
    – user14897
    Nov 7, 2021 at 21:04
  • $\begingroup$ Thanks for chiming in @ymb1 I'll delete my answer. I'm pretty sure I've seen what it was based on somewhere, but since I cand find it anymore, well, might just be the early stages of dementia then 🤔 $\endgroup$
    – Jpe61
    Nov 7, 2021 at 21:17

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