Valve Guide Wear?

[dtw_rv6]

I am experiencing symptoms consistent with a stuck exhaust valve, so I pulled the rocker cover and disassembled the exhaust valve for some borescope images. It shook so bad when it manifested in flight, I'm grounded until I figure out what this is. I did determine that the problem would slowly clear up at reduced throttle. It was only at WOT when the cylinder cut out.

When I pulled the valve stem out through the spark plug hole, it was very shiny and smooth. It also slid very easily in and out of the guide before I disassembled it. What is concerning to me, is the photos I have of the valve guide. It looks like a severe taper on the combustion side of the bore - I've never seen one before, so I'm not sure if this is normal.

I didn't do a wobble measurement, but I didn't feel anything excessive when I subjectively compared it to cylinder #2.

Superior XPIO-360, 1100hrs TT. Pmag/Lightspeed ignitions. 8.5:1 cylinders.

Is it time to pull the jug?

Don

 

[Dan 57]

consistent with a stuck exhaust valve

and those are? Your description does not really fit with a stuck valve, but who knows.
Do you have an engine monitor, data?
History, has this been a new occurrence or has it happened before?
From your text the shaking happens WOT, and then diminishes when the throttle is reduced, correct?
What lead you to look for a stuck valve in this cylinder in the first place?

Those are honest questions trying to analyse what's happening in your engine. Not sure about picture #2, is that the guide with the borescope going thru? The valve itself looks ok, no seizure marks on the stem, and the seat face and margin look ok considering the engine hours.
Regarding the observed taper on the guide, the proof of abnormality would be given by performing the wobble test, as this measures not only minimum but also maximum lateral play. Yes, I guess comparing it "manually" to one of the other cylinder would be a step in the right direction.

 

[dtw_rv6]

I didn't have any logging on at the time, but I did see the cylinder temps going down drastically during the event. All was normal after a few seconds flying at reduced power. Now when I try to run the engine, I can't even get above 1800rpm without severe shaking.

I have tried a few other things prior to dropping the valve.

No difference in symptoms during a ground run with either ignition on or off.
Removed, inspected and cleaned the lines from the spider to the injectors - nothing found.
Same for the injectors - nothing found.

When I pulled the rocker arm, it was definitely not stuck, and the valve slides in and out very smoothly. I can see why you don't think this sounds like a stuck valve, but I'm running out of ideas.

I'm not sure what to look for on the air side. The intake runners all seem snug - I haven't removed them to check gasket conditions. Maybe this is an induction leak?

This problem has been getting progressively worse throughout 3 or 4 flights last summer. I noticed a brief stumble on a cross country that ended as fast as I had noticed it. The second time on a subsequent flight, it lasted long enough that I diverted to circle an airport for 30 minutes while I checked ignition and power settings. It cleared up very quickly, and I continued the flight home.

This engine has always run very well, so it was easy for me to notice these small changes. Correcting the problem seems to be the hard part

 

[Roadjunkie1]

and those are? Your description does not really fit with a stuck valve, but who knows.
Do you have an engine monitor, data?
History, has this been a new occurrence or has it happened before?
From your text the shaking happens WOT, and then diminishes when the throttle is reduced, correct?
What lead you to look for a stuck valve in this cylinder in the first place?

Those are honest questions trying to analyze what's happening in your engine. Not sure about picture #2, is that the guide with the borescope going thru? The valve itself looks ok, no seizure marks on the stem, and the seat face and margin look ok considering the engine hours.
Regarding the observed taper on the guide, the proof of abnormality would be given by performing the wobble test, as this measures not only minimum but also maximum lateral play. Yes, I guess comparing it "manually" to one of the other cylinder would be a step in the right direction.

All of the above except: the allowable tolerance of the wobble test is 0.013 and 0.035. I am not sure how sensitive your fingers are but would guess they are not that sensitive. When I was doing my wobble test, the loosest was 0.024 and the tightest was 0.017, Manually wiggling the stem: I could feel no difference in the two. You need to do a wobble test before you pull the cylinder. Time to do one anyway on all 4 cylinders. Do your cylinders have the "C" stamp? Coming from Superior they might not. The 400 hour wobble test SB is on Lycoming engines and might not apply to yours. But would still be good information.

AND: this might not be the issue. What DID prompt you to think it was a stuck valve and why that particular cylinder? What do your fuel injection nozzles look like? Could this be FI-related? Many other things to consider before taking the engine apart. IMHO; YMMV

 

[Dan 57]

I'm sure others will chime in, but it sounds like you either have a fuel, or an ignition problem (yep, that was an easy guess ).
Spark plugs, spark plug wires, ignition system, fuel pressure, etc, etc, all and more could be faulty.

I doubt it is an induction leak since the symptoms would be different and last.

 

[dtw_rv6]

I haven't truly ruled anything out yet since I haven't found the problem. Stuck valve wasn't high on my list, but this is something I received from Mike Bush:

From Savvy Aviator:

"Thanks for the update, Don.

The valve pictures all look normal with a symmetric burn pattern. You are correct that sticky valves are most commonly experienced soon after engine start but they can happen at other times. Another possible cause for the drop out is a clogged injector that cleared itself. This may fit the scenario a little better since the #3 dropped when powering up and recovered when power was reduced.

We wouldn't rule out the sticky valve theory, of course. "

 

[elev666]

That sound’s exactly like the symptoms I had on my first run over hauled Lycoming cylinders 3 years ago, it would act up and go away so when I dropped the oil and found all kinds of aluminum shavings in my filter , thinking it was a wrist pin Cap I was set to remove all my cylinders to find the issue , this is what I discovered under my # 2 Cylinder valve cover ,
what I’ve learned from this is I wish I hadn’t powder coated my Valve covers or I would have found this problem sooner , the cover was badly peened from the inside but I was busy overhauling my Carburetor & mags and didn’t notice the obvious, also very happy I didn’t open my wallet to buy alloy valve covers since my oil would have left my engine very quickly !