I would appreciate any help with diagnosing this problem:

I was flying home last weekend, cruising lean of peak at my normal settings (2350rpm, 25-26" MAP, 42L/hr = 11gph, about 63%-65% power)
Suddenly an alarm attracted my attention. The EGT on number 5 cylinder cylinder had shot up to about 816*C = 1500*F, this is significantly higher than all the other cylinders. The EGT probes have suffered from erroneous readings and due to imperfect connector, before.

I tried leaning the mixture (away from peak) to see if the reading was obviously erroneous / false, and the EGT responded by skyrocketing well past 875*C = 1600*F. This suggested I was still getting meaningful readings. At this point, I noticed the #5 CHT was also dropping sharply (cooling), which also suggested the readings were real.

At this stage, I hadn't developed a theory about the problem. So all I could do was reduce power, then quickly play with the mixture until I fixed the issue by trial and error. Returning the mixture full rich put all the EGTs back into line, and the #5 CHT started to heat up again. I leaned for normal rich of peak cruise and this was fine also.

I tried putting it back lean of peak. There was a slight rough-running and the EGT shot up to the same high temperature again. I repeated this twice more with the same result.

I ran it rich for a while, and thought about theories. I guessed that if the #5 exhaust valve was not seating properly, that would explain the loss of CHT and vibration (#5 cylinder developing lower power than the rest) and high EGT (hot combustion gas escaping the cylinder). It does not explain why a full rich mixture solved the problem, or why the #5 EGT went higher as the fuel mix got leaner.

Nearing my home base, I tried leaning again. This time everything worked perfectly, CHTs all stayed the same, EGTs all moved together. There was no vibration unless I leaned too far and the engine started missing. I fooled around with the settings until I was convinced that the symptoms had disappeared.

I inspected the engine after landing and saw nothing out of the ordinary.

Thoughts?

Maybe a spark plug issue?

I agree. I'd move the spark plugs to a different cylinder and fly for a while. If the problem resurfaces in the cylinder with the suspect plugs, you'll have great confidence that you truly found the problem.

If you have a borescope, a look inside while the plugs are out is another first step you can take at minimal expense.

Maybe something clogged the injector and cleared itself by time you got back?

Going rich had to cool something down.

Don't know what engine you are running.
Some injected or turbo 206s used to vapor lock.

chedrick wrote:Maybe something clogged the injector and cleared itself by time you got back?

That would be my first inclination if you are in fact running an injected engine.

My first inclination is spark plug related as well.
To me a clogged injector would not produce an even EGT temp at full rich and then reproduce the problem at LOP.

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Yes - sorry - per my avatar details, engine is an IO-540.

Thanks for the suggestions. I'll pop the plugs and test them, only a ten minute job.

Sticking exhaust valve
injector problem
fouled plug
Those would be the first things I would check.

I would bet that something is wrong with the ignition system. Anytime I see increasing egts, I think of an ignition problem. It could be anything from the plug to the wire to the mag. If it is isolated to one cylinder, then it is more likely to be a plug or wire. Running lean demands more output from the ignition system than running rich. You could analyze by performing a mag check when lean. If you want to check when on the ground, run it up to desired rpm and lean aggressively before doing check. If you want to check in flight, run at your typical LOP settings and check mags, but be sure to know the procedure to restart if the engine quits because you run the risk of an induction fire if you don't restart properly. Good luck and report back wth findings.

Well, I can't repeat the problem. So I don't think it's related to permanent damage to any component or system.

I tried running on one mag (LOP), and it doesn't cause the EGT to spike. So a failure of a plug, ignition harness, or mag seems unlikely to be the cause.

I am still having a hard time thinking of a transient fault which explains all the symptoms.
So far, I think
1) a sticking exhaust valve, or
2) something clogging the valve seat
are the two scenarios which best explain what happened - but they aren't a perfect fit either, because the don't explain everything, so I am still left wanting more ideas.

Your second theory was my guess. Chunk of debris allowing the valve to leak hot gases past the valve seat and into the header.

Battson:

Was that one mag at a time- or only ONE mag?

I assume one at a time, but...

Do you lean "aggressive" after start and during taxi?

Do you run "Fat" plugs or Thin wire?

As another said, "Running RICH should COOL something." which could slightly
alter "size" and or shape of that "something?" with rich mixture cooling.

Trim

Trimtab wrote:Battson:

Was that one mag at a time- or only ONE mag?

I assume one at a time, but...

Do you lean "aggressive" after start and during taxi?

Do you run "Fat" plugs or Thin wire?

As another said, "Running RICH should COOL something." which could slightly
alter "size" and or shape of that "something?" with rich mixture cooling.

Trim

Tested each mag, one at a time.

Yes I always lean aggressively at soon as the oil pressure comes up, and only go full rich when I need more than 75% power and for landing.

Massive electrode plugs, Tempest I think.

I thought about rich mixture cooling too. The cylinder heated up again after the problem went away, the cylinder was running colder while the EGT was hot. So I wasn't sure if rich mixture cooling explains that, or not? I guess the CHT is only measured at one place in the cylinder, which isn't the plug or the valve.

Nobody has mentioned detonation or pre-ignition which makes me happy...

Now it makes sense to me that it was a plugged injector. If the cylinder cooled down, could have possibly been due to not firing properly because the fuel just running in, not atomzing. This could also cause the increased EGT because the fuel would then want to burn in the exhaust. I'm no engineer, so can't say for sure, but it makes a bit of sense. A though you should have noticed that on the fuel pressure/flow guage...

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A1Skinner wrote:Now it makes sense to me that it was a plugged injector. If the cylinder cooled down, could have possibly been due to not firing properly because the fuel just running in, not atomzing. This could also cause the increased EGT because the fuel would then want to burn in the exhaust. I'm no engineer, so can't say for sure, but it makes a bit of sense. A though you should have noticed that on the fuel pressure/flow guage...

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Don't think so. When running LOP, any reduction of fuel would lead to a decrease in EGT. If isolated to a single cylinder due to a clogged injector, you would see a decrease in that cylinder egt and a rough running engine if it got really lean.

Battson

Gonna take another swing at this.
It is late for me, so hope I do not get lost in the logic- or sources.

Quote:
It does not explain why a full rich mixture solved the problem,
or why the #5 EGT went higher as the fuel mix got leaner.

Believe these are in the ballpark:
1) a sticking exhaust valve, or
2) something clogging the valve seat

In order to by-pass detonation, for now, I am going to jump to the concussion of a KLINKER from your #5 Mass Electrode Plugs taking its time on the way out.
These are most often on the bottom plugs, and not necessarily IN the gap.

Either way IF, the exhaust valve does not close ALL the way, THEN, hot gasses will be exiting the cylinder, around the exhaust valve, for a longer period of time, during the fuel burn as it continues to heat the 'close down river' EGT probe.

Rich mixture runs cooler, that is why some carbs. have an enrichment function for take offs. leaning the mixture raises the power thus temperature as you climb. (Don’t BOP me with no Gami LOP here.)

Apology for the tone and mythaphores.

Link: http://www.bearhawkin.com/FAQ-Engines.html

Lycomings: Eat up exhaust valves, Pushrods bend. Klinkers can turn off all the fire in the cylinders quick.

Kevin Deutscher
Bearhawk #272
Phoenix, AZ

Now for Detonation- Don't worry yet.
This EAA link is worth the read. All about gasses swirling around in your head.

http://www.experimentalaircraft.info/ar ... nation.php


May need some help to make the links work- just did a copy - paste - Its LATE

Chris C