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Engine stumbling - analysis of engine data

On my way home from Brac I made a fuel stop at Poitiers (LFBI), and after departure, during the climb, after having reduced to climb setting, the engine stumbled. At first I was not quite certain whether it was the engine that had stumbled or turbulence, but shorthly thereafter it happened again, stronger, and that time I was looking out and saw the shaking. I increased the RPM, turned the electrical fuel pump back on and proceeded to turn back to the airport where I glided to a landing on the opposite runway. Not that the engine was not developing power, but I wanted to reach the runway in case it would quit.

The next day, after having spent part of the morning speaking to mechanics, I did a full power run up during which I could not detect any problem and therefore decided to continue to Pontoise.

Before putting the airplane back in the hangar I downloaded the flight data from the G1000, and here is a snapshot of the data with my annotations:

The first time the engine stumbles there is a dip in cyl 4 EGT, and maybe even in RPM. Then, 40 s later, there is another, bigger dip in EGT on the same cylinder before the EGT raises quite rapidly compared to the other cylinders, before it goes back to normal.

After I got home the aircraft was in for a 100 hrs and I recently learned that they had to replace cyl #4 because of a “burned exhaust valve”.

Do you think a burned exhaust valve is consistent with the engine stumbling and the above graph? Otherwise what would be your diagnostic?

Last Edited by Aviathor at 19 Oct 20:27

The data is really interesting. Maybe a bad spark plug or other ignition issue? Just a wild guess. Then you could spectulate whether an ignition issue could over time cause a burned #4 exhaust valve. Maybe somebody else has a better theory.

If a valve was sticking that would certainly make the engine do funny stuff. @achimha (not sure if he still reads EuroGA) wrote at some length on his own experiences of that. The pre-“C” cylinders had the valve sticking problem. The chrome valve guides more or less solved it, c. year 2000.

But if a valve was just burned the engine should still run, even if the valve was leaking quite badly.

A burned valve can be caused by past mismanagement (e.g. flying at peak EGT at 100% power for a while) or may be due to defective manufacture (bad valve seat machining).

It could also be the ignition system…

Shoreham EGKA, United Kingdom

I wonder if there was dirt in the injector, such that fuel flow was restricted, thus EGT went down. Dirt freed, but then more dirt clogged up injector, thus EGT went down again. Then injector partially unclogged which led to a leaned mixture, so EGT went up. Next day dirt was resolved, hence no repro.

What engine was it ?

PetitCessnaVoyageur wrote:

What engine was it ?



The strange thing with the burnt valve hypothesis, is that the next morning, you have been able to perform the flight normally. Unless it is related to an unconfy and unstable position of the valve at the moment the engine stumbled ? Which would have then taken a better place ? Don’t understand it.

Do you know if the cylinder are C-marked, which would allow us to forget the sticking valve ?

What do you think about the clogged injector option ?

Last Edited by PetitCessnaVoyageur at 20 Oct 09:36

I have had engine stumbles last months on a rented SR22 TN. A lean of peak mag check showed cylinder #2 EGT go up on one magneto (right I think). The owner and his mech told us to observe it and not give it to a mechanic right away.

It then reoccurred irregularly, but only above 12’000 ft. Plane went into 100hrs check afterwards but I don’t know if or what they found. I’ll have to ask.

Is your problem maybe dependent on altitude as well?

I think I would be looking for first dirt in the injectors and second water in the fuel, you may have picked up some dirty fuel.

Hm , I don’t think it’s a clogged nozzle, EGT would rise unless your LOP. It happened in climb, so I guess he was ROP. And water? Why should it affect only one cylinder? I neither can imagine a valve to stick nor to burn like that. In both cases EGT would drop to zero and won’t come back. Intermittent failures might be caused by an ignition problem. However, one would expect the EGT to rise with one plug not firing, not to drop!

Did I get it right, #4 is already replaced now? Anyway, I would do a lean sweep and an inflight mag check. If the problem still persists, this may give some clues.

EDFM (Mannheim), Germany
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