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11-20-2016, 06:12 PM
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Join Date: Feb 2011
Location: USA
Posts: 3,179
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We can eliminate the sticking valve and a leak on the intake tube. Both were checked today and are in great condition. The bore scope also showed the piston head looking good and the intake valve looking new.
I'm installing replacement plugs top and bottom on #2 and we'll watch for any repeat of the anomaly.
While checking the exhaust valve, the mechanics did an interesting thing. It may be common knowledge. But, for those who fear losing a valve into the engine here is the trick:
have a long length of natural fiber rope. With the cylinder in its lowered position, proceed to snake the rope in through the top spark plug hole until the cylinder is completely full. This gives a margin of error against accidentally pushing the valve into the engine. It's still possible to make the mistake but the odds are good the top will give enough resistance under most circumstances. One observation was that the oil is darker / dirtier than I would expect for 18 hours since the last change. This indicates combustion particulates getting past the ring(s).
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11-21-2016, 02:29 AM
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Join Date: Feb 2011
Location: USA
Posts: 3,179
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Quote:
Originally Posted by RV8JD
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I won't say 100.00% it's *not* a sticking valve. However, two A&Ps inspected the valves and performed the wobble test. The results do not indicate a sticking valve.
I need to attempt to reproduce the event. This far, attempts have not been successfull
Last edited by humptybump : 11-21-2016 at 02:35 AM.
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11-21-2016, 03:25 PM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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Quote:
Originally Posted by humptybump
Wouldn't a bad spark plug result in an incomplete burn and thus an elevated EGT - similar to running rich.? Am I mixing up the effects of combustion?
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Not really. one bad plug creates symptoms of retarded ignition timing. These symptoms are reduced CHT and increased EGT, due to the late PCP. The mixture will usually still burn completely, but not as much heat converts to energy and ends up in the exhaust. You can ground a mag in cruise and watch this effect.
Larry
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
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11-21-2016, 03:32 PM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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Quote:
Originally Posted by humptybump
We can eliminate the sticking valve and a leak on the intake tube. Both were checked today and are in great condition. The bore scope also showed the piston head looking good and the intake valve looking new.
I'm installing replacement plugs top and bottom on #2 and we'll watch for any repeat of the anomaly.
While checking the exhaust valve, the mechanics did an interesting thing. It may be common knowledge. But, for those who fear losing a valve into the engine here is the trick:
have a long length of natural fiber rope. With the cylinder in its lowered position, proceed to snake the rope in through the top spark plug hole until the cylinder is completely full. This gives a margin of error against accidentally pushing the valve into the engine. It's still possible to make the mistake but the odds are good the top will give enough resistance under most circumstances. One observation was that the oil is darker / dirtier than I would expect for 18 hours since the last change. This indicates combustion particulates getting past the ring(s).
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How many hours on your engine? Given the new data point about oil darkening prematurely, along with your other symptoms, I would also consider a stuck ring. A stuck ring can reduce your compression, lowering both CHT & EGT. The rings can also unstick themselves, resulting in intermittent symptoms.
Larry
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
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11-21-2016, 06:36 PM
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Join Date: Feb 2011
Location: USA
Posts: 3,179
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Quote:
Originally Posted by lr172
Given the new data point about oil darkening prematurely, along with your other symptoms, I would also consider a stuck ring. A stuck ring can reduce your compression, lowering both CHT & EGT. The rings can also unstick themselves, resulting in intermittent symptoms.
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"Stuck ring?" I assume you meant valve. Bother were checked. No deposit build up and proper amount of "wobble" - e.g. Wiggle the end of the valve and there should be a tiny amount of play.
The borescope did show initial signs the cylinder crosshatch is getting faint. The oil getting dirtier is likely past the ring.
BTW: the oil condition isn't "bad" but it's not as good as I remember 100 or 200 hours ago. The engine is at 500 hours from its major overhaul now so it's not "high time" but it's not new either. Also oil consumption remains low so it's not burning oil.
I'll continue to keep an eye on the oil and continue to see if I can reproduce the #2 CHT+EGT drop.
Good to know they "teach" the rope trick 
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11-21-2016, 07:05 PM
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Join Date: Dec 2014
Location: hilltop lakes' Texas
Posts: 135
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Cht's and egt's
Glen, altitude will really test the insulation of the ignition system in other words the spark will bridge a greater air gap at altitude it's possible that the cylinder in question is jumping spark somewhere from the magneto to the spark plug itself I would check very carefully to see if there are any abrasions or discontinuities in that particular wire, good luck
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11-22-2016, 07:52 AM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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Quote:
Originally Posted by humptybump
"Stuck ring?" I assume you meant valve. Bother were checked. No deposit build up and proper amount of "wobble" - e.g. Wiggle the end of the valve and there should be a tiny amount of play.
The borescope did show initial signs the cylinder crosshatch is getting faint. The oil getting dirtier is likely past the ring.
BTW: the oil condition isn't "bad" but it's not as good as I remember 100 or 200 hours ago. The engine is at 500 hours from its major overhaul now so it's not "high time" but it's not new either. Also oil consumption remains low so it's not burning oil.
I'll continue to keep an eye on the oil and continue to see if I can reproduce the #2 CHT+EGT drop.
Good to know they "teach" the rope trick 
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I meant a stuck ring. I read your posts and see that you have eliminated the valve guide. A stuck ring is a good probability fit with the following symptoms:
sudden Loss of compression (evidenced by tandem reduction in EGT / CHT)
Sudden increase in rate of oil saturating w/deposits (i.e. new, excess blow-by)
Deposits on the valve or seat (causing incomplete seating and therefore compression loss) could also cause the first symptom and would show as intermittent behavior (assuming they clear themselves, which they often do) and explain why your mechanic found no problem with the valve guide. It doesn't account for the second though. That said, it sounds like you may have other issues causing this based upon the borescope.
Remember, oil burning is usually due to issues with the oil control rings and/or cylinder wall surface. Excess blow-by is an issue with your compression rings.
Larry
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
Last edited by lr172 : 11-22-2016 at 08:10 AM.
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11-22-2016, 08:46 AM
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Join Date: Feb 2011
Location: USA
Posts: 3,179
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Interesting. I'd not heard anyone describe a "stuck ring".
We're taking about piston rings - correct?
(noob alert)
How can they be "stuck"? I'm not visualizing the situation?
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11-22-2016, 09:38 AM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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Quote:
Originally Posted by humptybump
Interesting. I'd not heard anyone describe a "stuck ring".
We're taking about piston rings - correct?
(noob alert)
How can they be "stuck"? I'm not visualizing the situation?
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Yes, referring to piston rings. The compression rings float around in piston ring lands, which are cavities cut into the piston to contain the ring. This cavity is sized to provide about .004" clearance between the piston and the ring in the vertical plane. The piston develops carbon deposits on it's side and occassionally the deposits can form between the ring and the land. When enough build up occurs, the rings becomes stuck and is unable to float. The ring provides a seal to the cyl wall via tension (ring is shaped larger than cyl bore and the compression to make it fit (i.e. tension) causes the seal, in addition to the expanding gasses from above.
If the ring sticks, this tension is lost and the gasses from above cannot further force the ring against the cyl wall, resulting in reduced compression, reduced power and increased bloby.
This is a bit of a long shot diagnosis, but it does match the symptoms so wanted you to be thinking about it. You EGT/CHT behavior points to a compression loss and further points to a ring/wall issue due to the increased blowby (you need to better confirm this as it can help include / exclude various failure modes). I suggest you continue to explore all scenarios that could cause intermittent and continued compression loss. It is possible that compression is down a bit and occasionally gets worse. You can lose a lot of compression before you will see that in the CHT/EGT readings. In the early stages, the rings can stick and unstick, causing intermittent behavior.
Larry
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
Last edited by lr172 : 11-22-2016 at 09:47 AM.
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11-22-2016, 11:28 AM
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Join Date: Feb 2011
Location: USA
Posts: 3,179
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Quote:
Originally Posted by lr172
If the ring sticks, this tension is lost and the gasses from above cannot further force the ring against the cyl wall, resulting in reduced compression, reduced power and increased bloby.
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Thanks for the explanation. I understood the behavior of the rings but had not thought they could get "stuck". From your explanation, I can envision a number of resulting issues including the dirty oil I've observed.
Thanks for taking the time to outline the scenario to a "noob" 
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