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Rough running engine (sticky exhaust valve)

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Peter wrote:

Currently, I am grounded with a rough running engine and could be grounded for months, with potentially the engine having to be removed and shipped to the US for a stripdown – because I have no suitable maintenance facilities at my base.

There is nobody in the UK who can do that? Very bad indeed.

Isn’t it still under warranty? Wasn’t that long you got that engine?

LSZH, Switzerland



Yesterday I noticed a very lumpy idle. Obviously, cancelled the flight.

On the ground run (not flying it like this) I now notice #2 cylinder being clearly abnormal. The EGT is not much out of line until you drop the revs below 1000 and then it more or less dies.

At 2000rpm the EGT is OK but the CHT is as always well below the others. And no amount of leaning at 2000rpm helps.

The issue is same on both or either mag, which pretty well rules out the ignition.

That leaves a sticky valve, fuel, or air.

I noticed that aggressively leaning at say 900rpm brings the depressed #2 EGT upwards very noticeably, until it is in line with the others, while the others remain fairly constant. This suggests #2 is too rich rather than too lean.

Any guesses as to the cause? Next debug session will be Saturday.

Administrator
Shoreham EGKA, United Kingdom

Clogged injector?

EBST

Does the issue go away after the engine is run at medium power for a while? That may point out to morning sickness (exh vlv sticking open).
Sticking valves, if bad enough, can also stay stuck after running the engine. You can test for that by feeling cyl compressions with the engine stopped, rotating by hand. The leak on #2 would be obvious by comparing with the other cylinders. If the leak is not there then the exh and inlet valves are closing properly and, as long as the issue stays at all times with a cold and hot engine before and after some running, then very likely that is not the problem.I believe we have a ‘morning sickness expert’ in the forum…was it @achimba ?

A blocked air inlet on one fuel injector, although rare, might cause those symptoms. Easy to check by swapping injectors. You can do that for ground checks even if you have GAMI’s. It’s a 15 mins job.Just need a deep 1/2" socket wrench.

Bad mag timing can also cause roughness on either and both mags, but then the EGT/CHT symptoms would be on multiple cylinders, not one.

Antonio
LESB, Spain

Does the issue go away after the engine is run at medium power for a while?

Not at all, after multiple 2000rpm runs, at full rich, peak EGT, mag checks, etc. Nothing changes the issue.

A blocked air inlet on one fuel injector, although rare, might cause those symptoms.

That seems likely, given that leaning the engine at low rpm, when the #2 EGT is showing as zero bars on the EDM, raises the #2 EGT to a more normal level.

Taking a previous flight, #2 CHT is lower than the others but by much less than the 50F showing in the video. This is a graph of the CHTs during a normal warmup

What could cause just one cylinder to be running rich?

Is it possible to block the injector air inlet?

Administrator
Shoreham EGKA, United Kingdom

While you’re checking and if you can’t find anything more obvious, broken valve springs have been known to occur on Lycomings.

Presumably there would be no compression in that cylinder, if a valve spring was broken?

Administrator
Shoreham EGKA, United Kingdom

Maybe that’s a silly remark but did you swap the probes? Also, the JPI probes are connected to the harness using screwed-on connectors and I’ve seen some get loose over time.

Peter wrote:

Presumably there would be no compression in that cylinder, if a valve spring was broken?

The symptoms are weird, two springs per valve.

I don’t know if it’d be useful but would thermal imaging the motor whilst running help pick anything anomalous regions compared to the others.

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