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Weird EGT wiggles after hot start

wjb

Well Known Member
I've found an interesting EGT oscillation after hot start that I request the VAF brain trust to opine upon ... This shows up after a hot start during taxi. IO-360-M1B, 30 hrs TTSN, running great in all other respects. The engine broke in nicely during phase 1 and is purring along happily.

I noticed a ~10 second oscillation in EGT after a hot start at low power operations (ie, taxiing back to hangar or departure end after a fuel stop). The plot below shows it on #4, but I've also seen it on all cylinders together (#4 is the most common). This clearly doesn't seem to be the burnt exhaust valve oscillation generated by the rotator cap, but may be due to hot fuel in the spider after a heat soak.

Go fly, and this doesn't ever show up again (until a hot start + taxi ...)

What say you, VAF brain trust?

cht-osc-XL.jpg
 
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I suspect some fuel is vaporizing in the fuel lines. It’s basically normally for an injected engine. A lot of heat builds up under the cowl and fuel can start to vaporize. As soon as you push the power up for takeoff the higher flow rate and pressure solves the problem. In other words it’s normal after a hot start. Many 4 bangers won’t idle smoothly after a hot start with high OAT.
G
 
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I suspect some fuel is vaporizing in the fuel lines. It’s basically normally for an injected engine. A lot of heat builds up under the cowl and fuel can start to vaporize. As soon as you push the power up for takeoff the higher flow rate and pressure solves the problem. In other words it’s normal after a hot start. Many 4 bangers won’t idle smoothly after a hot start with high OAT.
G

What he said.

Look at your fuel pressure to. Such low flow rate means boiling fuel.
 
#4 EGT fluctuation is WAY too stable of a rhythm to be boiling fuel. Looks like electrical interference to me (notice how the oscillation pattern follows the general temp pattern of all EGTs). You can even see how EGT #2 is following a similar pattern with much less severe range of oscillations. Also, boiling fuel won't drop the EGT 600-800 degrees in 2 seconds, fully recover in 6 seconds, then do it all over again. This consistently repeats, in perfect harmony, 30 times with a consistent period. Sorry, but boiling fuel doesn't have such perfectly predictable behavior.

I would be looking for spark plug wires running close to and in parallel with EGT lines. I did that on my 10 and had very similar, wildly intermittent EGT swings. Completely abated when I got separation of the wires. Though in my case, I never saw the issue at idle, like you are.

Larry
 
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