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Green color in center of exhaust valve

MiserBird

MiserBird

Well Known Member
Doing a borescope inspection on my O320E2A with 188 hour low compression ECI cylinders and found the exhaust valves to be almost too
clean with a green dot in the center of some.

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I have read that green is not good, but the location seems strange to me. I cruise at around 50% power, 325 CHT, 50deg ROP, 5.5 GPH ( by tach
time) 18in hg around 2200 RPM. One PMag 32 deg max on NGK BR8EIX's , one Slick on Tempest UREM37BY's. The plugs were white, with heavier light color deposits in the UREM37BY's than the very clean NGK's.

It has been said that it's difficult to damage a Lycoming at less than 65% power, but I'm beginning to wonder if I need to change any settings.

FWIW, the engine runs perfectly, and will idle down to 600 RPM on the EMag alone, with a Catto prop.

Thanks in advance for any insight, and advice.
 
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exhaust valve stem is sodium filled and acts as a heat sink. Therefore the area with the green dot (center, where the sodium filled cavity ends) will be cooler than the balance of the valve face. Not surprising that this could account for different deposit types or rates. Deposit type and color is often dictated by temperature at the deposited surface. Notice different deposits on exh valve vs intake valve vs cyl crown, etc. All are at different temps.

The green color wouldn't concern me in the slightest. Different fuel additives also account for different colored deposits.

Larry
 
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The bright white color and super clean valves generally means high combustion temps.
If the plugs are bright white instead of the normal tan color I would be looking at low fuel flow.
 
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Thanks for your responses. I'm going to enrich the mixture to around 100 deg ROP and take a look again.

The borescope used is a Vividia VA-400.
 
At such a low power setting I would go to peak on the leanest cylinder. The engine is running too cold! The higher EGT would help to keep from sticking a valve caused by combustion deposits. Lycoming says you can run peak EGT at any power setting below 75%. My personal preference would be below 65%.
 
Why are you running rich of peak. I thought the object would be to get max power from min fuel and to do that you run lean of peak.
 
MiserBird said:
Thanks for your responses. I'm going to enrich the mixture to around 100 deg ROP and take a look again.

The borescope used is a Vividia VA-400.
Click to expand...

100 degrees ROP corresponds to max power, max CHT?s, max stress and strain, max wear and tear, min economy, min longevity. Not a good place to spend much time!
 
Yen said:
Why are you running rich of peak. I thought the object would be to get max power from min fuel and to do that you run lean of peak.
Click to expand...

Max power is ROP. Cannot make power without fuel. As fuel flow is lowered, the engine will start running lean of peak (LOP) producing less power. Less food (fuel), less horsepower.
 
Boyd Birchler said:
At such a low power setting I would go to peak on the leanest cylinder. The engine is running too cold! The higher EGT would help to keep from sticking a valve caused by combustion deposits. Lycoming says you can run peak EGT at any power setting below 75%. My personal preference would be below 65%.
Click to expand...

Valves stick due to coked oil building up in the valve guide. The oil cokes, or oxidizes, due to excess heat.

Larry
 
MiserBird,
Just checking in to see how your engine is running now. I'm seeing the same green center on my engine.
 
Green center on exhaust valves

I'm at 740 hours on the new cylinders, engine is running great. Since my post at 188 hours I've had two stuck valves in flight. I reamed all four exhaust valve guides, switched to non-ethanol auto fuel around 350 hours ago and running much leaner as well. No more problems and haven't seen more green centers on the exhaust valves.
 
Interesting. Did you ever determine the cause of the green center? Do you think the stuck valves were correlated to that, or from how you were running the engine?
 
I think that it was how I was running the engine. 100LL, at low power settings.
Cyl #3 about 100 hours ago:
 

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Very interesting. I usually operate LOP but still have the green color on the center. I'd love to hear if anybody knows what the green is or why it is there.

The symmetry is reassuring, but would love to understand more!
 

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