What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Early exhaust valve failure? Need Advice

Lan Vinh Do

Well Known Member
Updated

I just did a oil change and clean and gap the sparks plugs. I always use the scope when I remove spark plug. I found something that could look like a beginning of a hot spot on a valve cylinder no 1 ( upper part of the valve on first picture) ?

Engine run fine. Compression is 78. I run on phillips 20-50 and add. cam guard at each oil change since many years. Always update all my flight on savvy analysis and everything is normal.

I need advice on these picture. The last one was taken. in march. 40hre before.
I run 20-40 lean of peak normally at altitude above 6000ft since 2 years.

Is it early sign od exhaust valve failure of just valve that is cleaning up because of the lean of peak operation? What should i do

- Monitor in another 40hre?, do nothing?, immediate action?

Thank You for your advice

PICTURE OF YESTERDAY ( UPPER PART OF THE VALVE ON THE FIRST PICTURE)
LINK TO THE ALBUM. OCTOBER 2020
https://photos.app.goo.gl/kc3jTbMenp1uAF2U6


PICTURE OF MARCH 2020
https://photos.app.goo.gl/qhvhYtPHshb6yhrS8
 
Last edited:
Chart

I look at that chart many time.

That is why i want to know if i need to do something now or wait and reassess in 25-40hre would be fine?
 
I am more concern about the small part that look that it. clear it's deposit and may be a hotter spot. ( upper part on the first picture)
 
Is it early sign od exhaust valve failure of just valve that is cleaning up because of the lean of peak operation? What should i do

Sure others are gonna jump in... my take: yes, it looks like a hot spot on the valve which could indeed lead to failure. From your pictures the valve seat looks to be affected as well.
I would:
1) inspect the valve seat for overheating
2) proceed as per stuck valve as in morning sickness, remove the cover and valve spring
3) check for free movement of the valve stem in its guide, ref. SB388C
4) perform a lap job on said valve/seat
5) reinstall, fly a few hours, recheck
6) if not ok, remove cylinder for overhaul/exchange
 
Is it early sign of exhaust valve failure or just valve that is cleaning up because of the lean of peak operation? What should i do
- Monitor in another 40hre?, do nothing?, immediate action?

Looks like textbook failure, caught early. That sector of the valve was not transferring heat to the valve seat.

What to do? I'd fly five hours and scope it again, because your 78 leakdown says it may have already fixed itself.

If the next inspection looks just a little worse, maybe it can be saved by checking for valve guide sticking, and lapping the valve and seat. The valve guide check is well documented, and some mechanics lap in place while the valve is loose. The conservative approach says pull the cylinder, get the valve under a 10x magnifier, chuck it for a spin with a indicator to ensure the seat is concentric, and really clean out all the grit after lapping.

If gets significantly worse it needs to be replaced. No shortage of opinions, and it's hard to be too conservative.
 
Doing the "wobble test" and inspecting the face of the valve would be better done sooner than later. Although the leak down seems good, this burned area is not likely to heal itself. It already got hot, so it isn't some fleck under the face.

2-cents
 
Fir the education of those if us not familiar with how this works, I see the discoloration on the valve and on the seat but what exactly causes it? Is it crud on the valve seat that prevents the valve from making contact with the seat so that it can’t transfer heat? It is hard to imagine that there us enough slop in the guide that the valve can stand off in one location and still contact elsewhere but that must be happening.

Does this have anything to do with running lean of peak? Or is it simply carbon or lead buildup?

I guess I need to get a scope. Very good detail on these pictures. A bummer to replace or rebuild a jug, but way better than swallowing a valve. Thanks for sharing.
 
Fir the education of those if us not familiar with how this works, I see the discoloration on the valve and on the seat but what exactly causes it? Is it crud on the valve seat that prevents the valve from making contact with the seat so that it can’t transfer heat? It is hard to imagine that there us enough slop in the guide that the valve can stand off in one location and still contact elsewhere but that must be happening.

Does this have anything to do with running lean of peak? Or is it simply carbon or lead buildup?

I guess I need to get a scope. Very good detail on these pictures. A bummer to replace or rebuild a jug, but way better than swallowing a valve. Thanks for sharing.

It probably has nothing to do with LOP operation. An exhaust valve heats and cools through the combustion cycle. It obviously cools when it sits on the seat, transferring some heat to the seat then head. If it does not seat in an area, then that area will increase in temperature to approach the EGT and higher. Remember, only the expanded gas is measured by the EGT, not the peak. Why did it fail/burn/overheat? Well it could be a number of things, debris does happen, but as the engine ages, the seats get a little wear and not perfectly round, the wear on the valve and do the same, so they have rotators to keep the valve moving. The perimeter of the seat varies in temperature as well and affects the shape and local temps of the valve. We want the valve to experience a time averaged equal temperature around the face, rotators insure that. The semi-circular witness mark around that spot is essentially a constant temperature line where the interior got hot and burned off the face deposits. Only a dimensional inspection of the parts can diagnose the true cause, that is not assured.
 
Last edited:
Seat looks questionable near that spot but with good compression that may not be it.

I would run it for a bit and check the position of that spot again relative to the seat. The valves are designed to rotate. If they're stuck in the same position it can create hot spots and the uneven discoloration shown.
 
UPDATE. lapping valve, cleaning guide and found trouble with ignition

HI

Did the maintenance today
I checked to see if I had a stuck valve. The valve was fine. It move and spin freely without any force with the fingers.

But because I was lapping the exhaust valve I took it out off the guide and pass the hone brush. Nothing came out. It was already clean.

I was anxious to put the valve back in the guide. I was said it could be frustrating and time consuming. With a magnet pass throug de valve guide and the mechanical finger through the exhaust… it tool 30 second to put back in the guide.

To lapp the valve, It helped to put the scope inside. It was easy to put the compound on the valve. In fact, the longest part was to clean the valve to remove the compound after.

I also removed my lightspeed plasma III for inspection. Once in a while I had engine stumbling for ¼ second. I foud on the forum some people had the same thing with lightspeed ignition. Mine is 2006. So I called Klaus for advice and he said that he would test it and upgrade it.

When I removed it, I found what I think is arcing sign on the middle bnc connector. So it will be repair and upgrad. I will change all coax , plug cable and klaus is sending me new coil ( he said the type of coil had been changed since 2006)

Thank you everyone and all the info that we can find on this forum

Here is a link to an album with picture and video of some step of the lapping and picture of the arcing on the ignition connector.

https://photos.app.goo.gl/LNgFfWFSN8Csfr4r6
 
Last edited:
nice

good borescope work. the valve cleaned up nicely.

Question: how did you lap the valve in place? did you apply compound with the Qtip and then spin the valve against its seat?
 
Yes q tips trough spark plug hole and exhaust

good borescope work. the valve cleaned up nicely.

Question: how did you lap the valve in place? did you apply compound with the Qtip and then spin the valve against its seat?

Yes. I did a tool like in the video link bellow to grasp the valve and spin it against the seat with compound on it ( small amount at a time. I had to repeat 4 or 5 time. ) Cleaning with fuel on a qtips after was the longest part.

The video bellow was of great help. But view with the scope help to be sure it’s clean. It’s not so easy to see everything through the plug hole with naked eye.

https://youtu.be/4PM8PFwYJmw
 
Back
Top