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Exhaust Valve sticking

rwayne

Well Known Member
Read Mike Busch's excellent article on exhaust valve sticking in July AOPA with the recommendation to keep CHTs between 350 to 400F especially Lycomings. When I fly lean of peak the CHTs drop closer to 300-325F. (That's at about 20F LOP.) I had understood that lower CHTs meant less stress on the engine. Do I need to now restrict cooling airflow to raise CHTs or should I avoid LOP?

Wayne
RV10; IO-540-D4A5
 
I have disagreed with many things Mike Busch has written but his position on sticky valves take the cake. I have unstuck many valves on hot-running engines (that right there doesn't fit his narrative) and found its not temperature related but rather due cleanliness of the oil. Dirty, dark oil = sticky valves. When one cleans a valve guide is its black, gooey matter. I generally change oil on-condition. I don't use an arbtrarily number of 25 or 50 hours but when the oil starts getting dark I or my customers change the oil. Sometimes dark oil can be 25-30 hours, sometimes its 50-60 hours. Depends on the engine, how its operated, the condition of the rings, good practices of LOP operation if injected, etc.

My airplanes generally run in the 325 range (where I like it) and I since I adopted the practice of changing the oil on-condition years ago I have had no stuck valves.

I also find running Decalin beneficial in keeping oil cleaner longer.
 
I have disagreed with many things Mike Busch has written but his position on sticky valves take the cake.

+1

Valves stick when clearance is reduced. This reduction comes from build up of carbon / coked (i.e. oxidized) oil. Most oil starts to oxidize around 260* and happens faster at higher temps. Coked oil generally occurs when the exh valve cannot shed enough heat (or insufficient oil flow) and raises the temp of the oil film on the stem/guide beyond it's oxidation point. A recommendation to run higher CHTs makes absolutely no sense. Makes sense for lead deposits, where higher temps help to reduce deposits, but not for oxidized oil / carbon deposits.

I am intrigued by Bob's observations. I would not have speculated that dirtier oil would oxidize more agressively. Maybe it is more about carbon particles coming out of suspension and coalescing, due to the heat in that area. Or possibly the dirty oil has a debris load that has exceeded the capacity of the AD additives and will no longer maintain all of it in suspension.

Larry
 
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+1
Maybe it is more about carbon particles coming out of suspension and coalescing, due to the heat in that area. Or possibly the dirty oil has a debris load that has exceeded the capacity of the AD additives and will no longer maintain all of it in suspension.
Larry

That is exactly what I think is happening. Clean oil = no valve guide deposits.

Think about how arbitrary a 25 or even a 50 hour number is, or even a calendar number. Makes no sense.

When the oil gets dark brown or black, change it!
 
I agree with Rocket Bob's practices. I have also found that marvel mystery oil, both added to the oil and the fuel, helps.

Vic
 
Don't tell me you haven't thought of this solution

You need to sell that aircraft and build a 14. Plain and simple.

:D
 
I read Mike Busch's article and it disturbed me because my IO-360 Angle Valve engine typically runs with 260--290 CHTs. Mike's claim is that the lead scavenging is less effective.

Funny though, we used to run auto engines with fuel with about the same lead content (not the spec, the actual content) and CHT around 190F. I used to overhaul Volvo B-18 and B-20 engines and the degree of lead deposit throughout the engine could be correlated with oil type and change interval. Paraffin-based oils seemed to be less able to hold lead in suspension than Asphalt-based oils. But I never ever had a sticky valve on a auto engine.

I do run TCP in my IO-360, although not religiously. I have been planning a cowl mod that will increase my CHTs, primarily motivated by aerodynamics, not specifically to raise CHTs. In the mean time, I am happy to read opinions here that counter what Mike Busch said. Hope you guys are right.

Seems like with lower CHT, there is more heat flux off the valve through the guides into the head, so the valve stem should be cooler, and have less coking/oxidizing of the oil.
 
I read Mike Busch's article and it disturbed me because my IO-360 Angle Valve engine typically runs with 260--290 CHTs. Mike's claim is that the lead scavenging is less effective.

I do agree that lead scavaging is improved with higher temps. However, I believe it is the gas temp and not necessarily the cylinder temps that make the difference. My research indicated that >800-1000* is the range where lead bromide will remain in aerosol state and not precipitate out as a deposit. Seems to me that EGT is a much better guide for eliminating lead deposition.

It was my understanding that lead deposits are really only an issue below 1000 RPMs, where EGTs are low enough to be nearing the critical temp.

As Bob mentioned, the build up in the guides is jet black (i.e. Carbon). Same as can be seen in the oil filter or screen, as chunks, in engines that burn a lot of oil. Lead deposits are grey. It is clear that valve sticking is NOT due to lead deposits.

Larry
 
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As Bob mentioned, the build up in the guides is jet black (i.e. Carbon). Same as can be seen in the oil filter or screen, as chunks, in engines that burn a lot of oil. Lead deposits are grey. It is clear that valve sticking is NOT due to lead deposits.

Mike Busch’s webinar indicates that laboratory analysis of valve guide deposits indicate it is composed of lead, carbon, bromine and oxygen. Have any of the posters above had a lab analysis done that refutes that? I don’t think black color alone, while different than what is seen in exhaust pipes, is enough to totally dismiss his claim.

Erich
 
Rocket Bob nailed it again! Agree 100% on the MMO. I keep several gallons in my hangar all the time. 4 oz in each full tank and a quart in the crank case at every oil change.
 
Question

Does anyone add MMO with new fresh oil and fly around with 10 - 15% MMO as per instructions on the bottle or only at oil change?
 
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