EGT went high on one cylinder today...exhaust valve?

tawood

tawood

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Tim
Hi all,
Flying along today in my pa28 (o320) at 2500 feet for 10 minutes or so...I approached Saginaw bay, and decided to climb to 4500 for open water safety reasons. Since I only lean the carb above 3000, I hadn't been paying much attention to my egt gauges until I finished my climb. When I looked down at it (before leaning): holy smokes, the number 3 cylinder egt is 250+ degrees hotter than the rest!
I turned around and headed for home, and it ran hotter the whole way. I tried running on each mag just to see if it changed things, but it only made it hotter (all hotter with #3 still way hotter). I've never seen this much variance, and usually it's number 1 cylinder that's slightly hotter (25-75) than my coldest EGT. I also notice it seemed to be burning more fuel on my fuel monitor... maybe 1 - 1.5 gph more.
So, what am I looking at...bad exhaust valve?
 
I'm no A&P so my opinion doesn't mean much, but I'm going to say bad sensor. If the exhaust valve was stuck open or closed I'd expect low EGT, not substantially hotter. If the intake valve was stuck, say partially open, it could run that cylinder leaner, but you'd also notice a loss in power.

Also the increased fuel burn seems it would drive EGT down as well.
 
PaulMKE said:
I'm no A&P so my opinion doesn't mean much, but I'm going to say bad sensor. If the exhaust valve was stuck open or closed I'd expect low EGT, not substantially hotter. If the intake valve was stuck, say partially open, it could run that cylinder leaner, but you'd also notice a loss in power.

Also the increased fuel burn seems it would drive EGT down as well.
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That was my initial thinking too, until I grabbed the manual that came with the gauge. They say a malfunctioning gauge/sensor will read low, not high.
 
PaulMKE said:
I'm no A&P so my opinion doesn't mean much, but I'm going to say bad sensor. If the exhaust valve was stuck open or closed I'd expect low EGT, not substantially hotter. If the intake valve was stuck, say partially open, it could run that cylinder leaner, but you'd also notice a loss in power.

Also the increased fuel burn seems it would drive EGT down as well.
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If the intake valve was stuck open the flame would most likely spread to the intake and kill the engine


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Better safe than sorry. Have your mechanic check it out with his compressionometer.
 
Stewartb said:
First suspicion? Intake leak.
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Same suspicion here (but I am no mechanic so take this with a brick of salt).
An intake leak should be even more prominent at lower MP (higher pressure gradient between intake and ambient pressure) so it should be easily reproducible on the ground with partial power so no need for a test flight.

Let'sgoflying! said:
Swap sensors, see if the problem follows the sensor?
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I'd recommend that too.
First rule of diagnosing a bad reading is to determine whether the equipment is causing the bad reading. :)
 
Usually with an induction leak you'll find blue fuel stains on the bottom of the intake runners.
 
Not an A&P, but I'll +1 the intake leak likelihood. All (I)O-320 and -360 models are prone to intake leaks, though they're usually minor enough that it won't be noticed quite like that. In my case, it was the swaged-in intake stubs that were cracked and leaking. I guess I don't know if there was a huge rise in exhaust temps, since I only have the stock single EGT probe. I will note that there wasn't rough running before or after the repair and the EGT peak that settled out wasn't too different after the repair.

Gah, but that was an ugly repair. It takes a rented tool to fix and I understand there are only three across the country available for rent. It had leaked before and it had been sealed with RTV or something, probably because the owner was too cheap to fix it for real. The tool ended up not arriving and my aircraft was down for a week extra during the 100-hour it was caught at. Quoting my own words from the plane's website: "The #3 and #4 induction tubes were leaking a little. $550 got two new tubes, 3 hours of labor, and rental of the special tool to install them." What I don't mention is the week I waited for the stupid $8000 tool.
 
kgruber said:
Better safe than sorry. Have your mechanic check it out with his compressionometer.
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+1. I just read an article in "Aviation Consumer" where an owner had written in about his C-210. He had a similar problem, but apparently kept flying. The cylinder seized. It turned out to be a broken ring.
 
What was your oil temp and pressure looking like?

Anything smell funny?

Was the engine and cowl area hot on the ground, any signs of high temp under the cowl?

250 OVER your OTHER cylinders...in a climb, I'd think you would notice that unless it was just a wacked sensor.
 
James331 said:
What was your oil temp and pressure looking like?
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Both normal.

James331 said:
Anything smell funny?
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No.

James331 said:
Was the engine and cowl area hot on the ground, any signs of high temp under the cowl?
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No.

James331 said:
250 OVER your OTHER cylinders...in a climb, I'd think you would notice that unless it was just a wacked sensor.
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If you mean "notice" on the gauge...I admit (ashamedly) that up to this event the EGT was not a part of my normal scan before leaning...my EGT gauge sits under that dash and back a bit...I have to tilt my head to the side to see it, and before this happened I would only start looking at it after I started thinking about leaning, or once I leaned the engine. It could have been showing high temps and I didn't notice it, although I rarely fly below 3000 feet, so I am always leaning on every flight. It hasn't happened before that I am aware of(at least above 3000 feet). I did notice on just this flight that the fuel was flowing higher, and that gave me the initial, "hmmm, something's-not-right feeling", before noticing the high temp.
 
I'll roll with intake leak leaning the mixture on that cylinder or exhaust valve leak. If it were an IO-360 it could also be a partially plugged injector.
 
tawood said:
That was my initial thinking too, until I grabbed the manual that came with the gauge. They say a malfunctioning gauge/sensor will read low, not high.
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That's bogus... one failure mode of a TC is to become like a thermopile, two junctions in series... reads high.

Paul
 
My mechanic states that it was bad magnetos....I'm not convinced, but I'll keep everyone posted of the progress.
 
Nine years ago I had a plug that would stop firing at high temps. When I leaned to peak (around 1450°F, typically), it would quit firing. At full power you won't feel one plug dropping out, but the EGT on that cylinder will shoot up. In my case the EGT on that cylinder went up about 100°F to 1550°F -- not quite the 250° increase you were seeing. An in-flight mag check revealed this quickly: when the mag driving the good plug in that cylinder was shut off that whole cylinder quit -- bad mag check. This was a tricky problem because the plug worked great on the ground, and even at cruise at best power settings (100°F ROP ~1450°F). The point being that a single EGT going high could be a bad plug or a fouled plug. It seemed counterintuitive to me at first, but with only one plug, the burn is not complete when the exhaust valve opens and some of the fire travels out through the exhaust port and heats up the probe.
 
Ignition related, sounds entirely plausable. A worn bushing can cause a retarded timing issue. Retarded timing will cause high EGT, and higher fuel burn.
But now that I think about it, that should manifest it's self on all cyls. Not just one.
Then again, depending on the wear pattern, I suppose it could well, retard one, and advance another.

Also, incomplete fuel burn due to bad plug, (as mentioned above) or mabe a bad plug wire.
 
I promise to update this thread when my mechanic is done. The plane is also getting annual-ed, a heated pitot added, a new TC, and a new radio, so it will be at least another week (and a few "aviation monetary units" paid too).
 
Well, the plane has been at the mechanic for over a month, but still not flyable. My mechanic sent my mags out to be rebuilt, but found they were too worn, and I need new ones instead. The new mags should be in this week, and hopefully I can test fly this weekend. Although I trust my mechanic, I'm not confident that the new mags will fix the high EGT problem.
 
Is that (mag change) the only thing that will be different?
It is hard to give credit or blame if you change multiple things at the same time.
 
Let'sgoflying! said:
Is that (mag change) the only thing that will be different?
It is hard to give credit or blame if you change multiple things at the same time.
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Basically yes. It's also getting its annual, so it will have normal maintenance things done. But I can't see how any normal maintenance would be the cure for a high egt.
 
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