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EGT Double peak

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Well-Known Member
Mar 16, 2011
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In a previous post called "Cruise Climb", I kinda derailed it (sorry!) on a tangent about EGT double peaking during leaning. So I've started a new post to continue the saga. If you want the preamble, do a search on "Cruise Climb".

The problem we were discussing was the appearance of a double peak in EGT during mixture lean testing.

I think (hoping) I may have found the cause of the unexpected rise in EGT when leaning beyond 40-50F LOP. The other day I was doing some autopilot tests, running about 50F WOTLOP, and after about 1/2-hr I noticed a sudden drop in power and EGTs starting to rise.

Of course the first things that came to mind, based on all the rhetoric about engine damage when LOP, was the possibility of detonation or pre-ignition happening, so I quickly went full rich and headed back to base to think about things. On the way back I convinced myself that neither could be happening - I was ~50F LOP on the richest cylinder, the CHTs were around 325F, EGTs were ~1350F, and FF was 14.4GPH. For my 300hp Lycoming, that means about 72% BHP. So the engine was cool, the power was low, and the EGTs started to rise relatively slowly. Not detonation, as EGT would drop. Not pre-ignition, as EGT would go quickly off-scale. And it was all cylinders that were affected, not just one or two. Paranoia got the best of me initially, but logic prevailed. Phew! So what was it? Felt like a mag dropping out.

For my next flight, I planned to do a more thorough mag check a) on the ground, b) airborne ROP over the airport, c) 50LOP over the airport to see what I could figure out. Lo and behold, the ground mag check failed - one mag was dead. Tested it 3 or 4 times and deader than a doornail. So I'm pretty sure that was the culprit.

So my theory is this: I had a weak 1/2 mag (dual mag) creating a weak spark on one side. During rich operation, the spark from that side was adequate to ignite the rich mixture, so everything appeared normal. As the mixture was leaned beyond 30-40F LOP (ie. during a leaning test), the spark from the weak mag became less and less able to ignite the lean mixture (you need a healthy spark for lean operation), and burn efficiency started to drop. This caused the EGTs to start to rise with continued leaning (due to slower and slower burn) until down to single mag operation, and then the EGTs started to drop with further leaning as expected. Ala double peak!

So I've bitten the bullet and am having my mag overhauled - actually doing an exchange for an overhauled unit. It's one of those crazy dual mags (Lyc, what were you thinking?!) and Teledyne-Cont announced last year that they are no longer making them or any parts for them. So I figure I should strike while the iron is hot and parts are still available - get a unit overhauled to factory-spec. It could just be something as simple as a shorted cap, but it could also be an internal problem with a coil or E-gap setting. Anyhow, I'm not messing around with ignition problems any more - taking the swap-out option and hopefully that will get rid of the problem.

Let you know how I fare...

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