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Aztec engine near-failure today

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I was doing some training with my local CFI today. After some maneuvers at 3500', we leveled off. I saw the JPI flash and noticed that the CHTs on the left engine were suddenly out of tolerances. Within at most two minutes, one cylinder had stopped running and three others were showing 450°F temps, climbing rapidly.

I tried full rich and reduced power to 20" - no go. The thing that got me was how FAST the temps were still climbing. Further power reduction - temps climbing not as fast, but CHT now about 500°.

I wanted to shut down the engine, but the CFI said, "No, if it's running, keep it, in case it's fuel contamination and affects the the other one, too." Hmmmm.....

Cowl flaps full open, 14" manifold, full rich, 2400 rpm. About 90% power on right engine, which accepts it without complaint. Two cylinders not running now. Temps coming down a little, but still staying at 500°.

Long story short: we lived and landed OK.

This Aztec sat a little while before I got it, and there seems to be some rust in the left gascolator, of which I get a glimpse every time I sump.

Baffling. The only thing I can think that would do that kind of temp climb that does not respond to much of anything is preignition. Going to borescope ASAP - preig only takes a minute or two to melt a piston; whether our low power approach did anything to help us, I don't know. I refueled last night, so both engines were drinking the same brew. Maybe a mag slipped?

Any smart thoughts out there?
 

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