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Some valve related engine faults produce a slow periodic variation in EGT.

The oscillation rate is on the order of one cycle every minute or two. This is just too slow to be identified by occasional observation of the temperatures alone. Yet it is very important to discover this phenomenon because it may lead to a catastrophic engine failure. It readily appears in a slow sampled spectrum analysis.

Temperature samples are collected once per second for each cylinder and analyzed each second. The entire sample interval is about 8 minutes. A normal indication will be a flat line with a little noise, while a trouble indication will show as an obvious spectral peak.