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Sticking exhaust valve during flight

SuperCubDriver

Well Known Member
Here is a report with an incident during departure which caught my attention:

Last Friday I was taking off from LOWZ airfield in Austria on Rwy 08 with my RV-7. When I reached a height of 500 ft the engine suddenly was shaking with a significant power loss. I immediately pushed the nose over and turned right knowing I hava to land ASAP. Two options came to mind - a short pattern or land on opposite rwy, chosed the second option as I was in a good position for it. I glanced at the instruments and saw cyl 2 is gone (not physically). Engine was still responsive but with reduced power and out of balance. I radioed I was having trouble and will come in on rwy 26. The approach was not pushed and good on speed and altitude being aware of about 2 kts tailwind on the 2500 ft rwy. I came in slightly high and the plane just didn´t wan´t to descent and become slower, somehow the engine was still producing power and I pulled hard on the throttle - no change.
Now I realized I will not make it and end up in the bushes at the end of the rwy. I made a bad decision but was lucky with it and did go around being aware I could crashland in the golf course ahead. I retracted the flaps to 10 and did a shallow climb with a slight left turn for a teardrop pattern. Then I came in high assuring to make the rwy. Again the airplane didn´t want to descent as usual, there is still the power just like on the first approach. I pulled the mixture and shut the engine down and she descendet normal towards the rwy for a reliefing landing. During rollout I pushed the mixture in again to keep the engine running for a taxi-out. I realized that the RPM was around 1200 while it should be around 650 - so it was really running with power and this explains the unsuccessful approach.

I can tell you that during these two seconds floating over the rwy realizing not to make it and having two bad options left is no fun at all. It is completely different analysing this later on the ground with all the information available.

A question for the engine gurus here:
What could have caused the power increase at idle setting?
My guess is that the sticking exhaust valve in open position acted like an"aggressive cam" with some significant valve overlap so the idle MAP was increased (less vacuum in the intake manifold).

Here are the datas from the Skyview datalog:



J0RCTYZ.png


klrCSNA.png


fAU0bLK.png


VkFZPh2.png



The whole flight lasted four minutes. You can see this in the timelines from 02:30 to 06:30.
Note the RPM decrease at 06:30 when I pulled the mixture. The fuel flow went to zero while the RPM dropped from 1500 to 1100 RPM (windmilling).
 
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Herman, so happy you made it onto the ground safely - that must have been a very exciting landing. Your MAP graph supports your theory of why the engine ran at a higher RPM - something was causing the MAP to be higher.

Any chance the intense vibration was preventing the throttle from closing properly?

You might open a ticket with the SavvyAviation gurus - this is one that will get their interest I'm sure.
 
Wow, so glad it turned out ok!

The high idle is indeed a mystery - perhaps a sticking intake valve could be bringing in more air during that cylinder's exhaust stroke? Meaning at that time, both valves in that cylinder would be open at the same time, allowing exhaust flow to draw air into the intake?
 
A question for the engine gurus here:
What could have caused the power increase at idle setting?
My guess is that the sticking exhaust valve in open position acted like an"aggressive cam" with some significant valve overlap so the idle MAP was increased (less vacuum in the intake manifold).

The whole flight lasted four minutes. You can see this in the timelines from 02:30 to 06:30.
Note the RPM decrease at 06:30 when I pulled the mixture. The fuel flow went to zero while the RPM dropped from 1500 to 1100 RPM (windmilling).

Based upon the FP data, assume you have FI. Fuel flow is based upon airflow through the servo, so having 1 stuck Exh valve should have reduced fuel flow. It is possible that you were too rich and this reduction in flow leaned things up a bit which would increase power a bit if it moved you from excessively rich to best power.

My guess is that you were going too fast and this airspeed is helping to turn the prop. Note that even when at ICO you are still turning 1100 RPM. That is an indication of too much airspeed. These are slippery planes and when an FP prop is in place, pulling the throttle back during a decent is not always enough to make the plane "go down now." Some times you need to lift the nose to bleed enough airspeed to make a more aggressive descent and slips can help. In my 6, if I am doing 100 MPH, it doesn't really want to descend much without increasing speed. Drop the speed to 80 MPH and it is a manhole cover. If you are going too fast and push the nose down, there is no real way to manage things. Pushing the nose down increases the speed and just makes things worse. A descent to landing pretty much has to start at a low enough airspeed to make things work.

Larry
 
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Glad that you have pulled it off.

My guess is that with the valve stuck open, the compression is obviously less and therefore the prop has lost some of it's braking capability. I assume it is a fixed pitch prop?

Secondly I would like to know the engine hours and also if it has done more than 400 hours, why did you not do Lycoming SB-388C?
 
I´m having an ECI fuel injection system, this controls the fuel flow not according airflow but according the position of the throttle plate. Well - it is kind of dependent of airflow in this regard. But the actual fuel flow is controlled mechanically.
I do not agree in my case that the airspeed messed up my approach. My experience of many thousand landing approaches with all speeds possible just doesn´t match. And pulling the mixture on the second approach was just like pulling power to idle.

Note that the exhaust valve was stuck OPEN. That means when this Cyl 2 is on it´s intake stroke the exhaust valve is still open and the Cyl doesn´t need to suck air in as outside pressure is coming via the exhaust port. There might have been even some pressure from the exhaust and so increasing the MAP. As the mixture is rich on idle (when the mixture lever is set to rich) the other cylinders can burn this fuel with the additional air and so produce more power.
 
Glad that you have pulled it off.

My guess is that with the valve stuck open, the compression is obviously less and therefore the prop has lost some of it's braking capability. I assume it is a fixed pitch prop?

Secondly I would like to know the engine hours and also if it has done more than 400 hours, why did you not do Lycoming SB-388C?

Thanks for your input.
It makes sense on the braking capability but as said it was completely different when pulling the mixture.
The engine is a low compression IO-375 with a 200RV constant speed prop.
I did the SB-388C at 1000 hours. The cylinders are having the marks saying it should be done at 1000 hours and not 400 hours.
The event happened at 1248 hours.
 
Hallo Hermann, yes, I'm also happy all went well in the end :)

Was it really a sticking exhaust valve? Have you found physical evidence for this, after the flight that is? Do you have the MP graph as well?

I've had 4 cases of sticking valves in the past few years... 3 of them on the smaller Conti C-90 engine, and one on a O-320. Well, none of them had any increase in RPM recorded, actually a slight decrease at idle on all of them. I didn't have any fancy engine monitoring on those airplanes/engines, but the symptoms were coughing, as in missing beats, and extreme vibrations forcing a massive reduction in power. Then once on the ground the faulty cylinder was identified by being cold to the touch... and the exhaust valve totally stuck in their respective guide, so as too be hammered in for the guide to be reamed. Or the cylinder replaced with a cylinder kit...
 
Herman, well done managing your issue, happy to see a good outcome.

Just a few weeks ago I had an incipient sticking valve that I noticed on taxi out. EGT was about 200 degrees on my #2. Rough idle and would settle down after a two minute warm up. I also noticed that during the rough period the idle fuel flow was nearly 5 gph, vice the normal around 2 with throttle/mixture in normal positions. Once the idle smoothed out the FF returned to normal.

So my FI system was somehow increasing FF while that valve was stuck open. (standard AFP FM150 FI).

The high fuel flow doesn't seem to be on your graphs, but I do believe there is some interaction between cylinders while a valve is stuck open that might be causing unusual FI behavior. Imagine the upset manifold pressures and non normal flow of combustion mixture and exhaust mixing to the other cylinders with the exhaust valve stuck open.

Just a thought that has gong through my head for a while. I would like to learn more about my event and get to the bottom of your issue as well.

Cheers
 
Similar Experience

I had #3 exhaust valve stick open about a month ago leading to an unplanned landing away from home. I was fortunate to be at 8500' when it happened. Violent shaking that reduced with power reduction. EGT dropped to about 200. I had an airport about 8 miles away and made it there, but almost overshot as well. Aggressive slip aimed at the 1000' foot mark of the 4000' runway because I didn't want to come up short or have to go around. In the round out it didn't want to slow down and just kept floating. Stuck it on the mains and began braking with tail still up. Got it stopped by the end of the runway and found it to be idling at 1200 RPM. WTF? Lots of braking needed to taxi to the ramp. I don't understand the high idle with a dead cylinder. There was no compression on 3, so the engine is easier to turn, but I would think that would be offset by that cylinder not making any power?
 
Some more data points:

Dan, yes I found the exhaust valve stuck open. The following day I did the rope thing on that cylinder and tapped the valve through the guide into the cylinder. Reamed the guide and there was this black buildup on the greasy reamer. The valve then went in smooth as silk and the flight home was normal without any issues.

Steves landing sounds exactly like my first try. I did slip and the aircraft kept floating but "my" rwy was only 2500 ft long. I think I would have made it on a 4000 ft rwy. And 1200 RPM idle RPM is just the same what I saw.

I will try to run the engine the next days with one spark plug removed and see if I can reproduce the behavior and report back.
 
...
I will try to run the engine the next days with one spark plug removed and see if I can reproduce the behavior and report back.

Not sure I'd do that - I'm as curious as everyone about what caused this, but this seems pretty risky. Perhaps check with your engine builder to see what they say, or Joe Godfrey of Savvy Aviation.
 
Thanks Steve.

This increase in RPM is interesting, and I wonder if there is any correlation between a stuck exhaust valve, and injection, though I may be totally wrong here...
In the 4 cases I've experienced, I had none of this idle RPM increase, and all engines were carbed... with no connection to the MP, therefore my question(s).
The valve sequence overlapping in the 4 cycle is, AFAIK, happening on all Lycos and Contis...
 
I might have an explanation for this:
On a carbed engine during idle the low MAP is pulling fuel through the idle circuit, so when the MAP is increased due to an open exhaust valve and not with an opening throttle plate I think the vacuum signal pulling the fuel is weaker. On an injected engine the three healthy cylinders are just getting more air and so are burning the excess fuel which in turn is producing more power.
I might be wrong but this makes sense to me.
 
Possible solution:

With the exhaust stuck open, you have the exhaust pressure from the other cylinders working against any fresh air flowing though the dead cylinder. The throttle plate is in the idle position and the three running cylinders make enough low pressure to suck in the same amount of air. Fuel mass depends only on throttle position and is the same too. On the same fuel and airflow three cylinders make the same power as four would make, but without or with reduced pumping losses from the dead cylinder the engine speed goes up.
 
Good point with the reduced pumping losses, this is fact. But on the other hand this cylinder doesn´t produce power at all. And - there is still the increased MAP which I believe is the key to this problem.

Bill, exhaust is 4-2.
#1 and #2 going into one pipe.
 
MAP/RPM reason….maybe…

Herman,
i am glad to hear about the outcome.

My ideas about increased rpm.

With a stuck-open exhaust valve you completely miss a combustion stroke and the necessary work to do it. (I think those are the “pumping losses” you already talked about).
Even so this cylinder ist dead and produces “Zero” power, the inertia of the rest of the engine is big enough to keep it running (proofen!).
I think the inertia of the entire engine, 3 power strokes (75%), makes up for the missing cylinder. In addition to that, the engine has to do 1 combustion stroke less due to the stuck open valve, thus not using any of that inertia energy produced. Furthermore, with a firing order 1-3-2-4, if #3 is on power stroke, #2 would be on combustion stroke. But due to the missing combustion the engine spools up more than normal as there is no counterforce, resulting in higher rpm as #3 has its standard power output, resulting in more inertia (stored gyroscopic energy)………..and we start from the beginning…

This does only come in place in the low-power, low-speed regime, when the power for turning the propeller is (almost) neglectable. As soon as the prop requires serious power to be turned (as when producing thrust), this result is no longer current as the high pitched propeller blade uses all of the inertia (and produced) energy from the engine to produce thrust/overcome drag. Afterall, your engine is missing a cylinder, and therefore power.

The c/s prop is supporting the whole thing while beeing in fine pitch-stop at those rpm, keeping the power required to turn the prop at a minimum.

A FP prop, due to higher pitch, resulting in higher drag (need/use more of this inertia) would probably cover the symptomes above. Even so they are still present, but not as dominant.


My ideas about increased MAP.
As your prop is at fine-pitch-stop, and nowhere near the control range, we are technically speaking about a FP prop (with minimum pitch, probably even smaller than a climb-prop).
But, now MAP and RPM are direct proportional (let’s not talk about windmilling effects now). Higher RPM means higher MAP, period.

And the higher RPMs come from the reasons above.

If you look at your EGTs right after start, there are running steady at 1100 F-ish. I would say steady.
At 03:15 #2 “departs”.
After landing, around 06:45, your engine is running as stated above with EGTs around 1300-1350 F-ish. Same fuelflow as upon initial startup, higher RPM (more air), engine is running leaner than upon initial start, therefore higher EGT on the remaining cylinders.
Higher RPM, higher MAP.

Your engine doesn’t run more efficient or anything in this direction, most probably producing less torque and overall-power-output.
But, In a low speed, steady-state environment, those are probably the results that we see……at least to my understanding.

It still seems a bit weird to me too, but those are my conclusions.

How did the testrun with the removed sparkplug went?

See you around
 
Peter, I see you did some serious thinking!
Note that before takeoff the idling RPM was around 900-1000 and throttle not completely closed. If I close the throttle completely my idle RPM is around 650.
After landing with the stuck open valve the idle RPM was around 1150-1200 but with a completely closed throttle. The additinal MAP must have caused cyl 2 with the open exhaust valve.
I didn´t do the testrun as of today but intend to do the next days.

If anyone can run a simulation then this could help explain things.
 
So here is an update:
Meanwhile I flew 3.5 hours since the event. Today I did a 50 h inspection and oil change so this was a good opportunity trying to replicate the event.
So after a flight today just before shutdown I pulled the throttle all the way out to the mechanical stop, mixture full rich: MAP 13.2 / RPM 630 / EGT around 1050 F.
Then I removed the cowl and the #2 spark plugs to simulate the open exhaust valve, then started the engine again and saw following datas with throttle all the way back again and mixture full rich: MAP 17.0 / RPM 1150 / EGTs around 1000 F except #2. Both tests are done stationary, so no forward speed.

This looks like proof that the engine is in fact producing some power or thrust with idle power and throttle closed with one exhaust valve stuck open. The only difference during this test was cylinder #2 breathing through the spark plug holes instead through the exhaust valve, so this takes out the influence of the exhaust. I think this is a representative test and confirms the original data.
Just one more info in case somebody is wondering: Yes I did ground the removed spark plugs to protect the ignition modules of the Pmags.

Here are the screenshots:

y2Ultjv.png

Throttle fully closed, mixture full rich

zrTTBDn.png

Throtle fully closed, mixture full rich, #2 spark plugs removed
 
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Daida,
I’m sorry but there are some wrong assumptions.
Your first paragraph about the increase of RPM contradicts the law of conservation of energy.
To increase RPM with constant TAS you have to add energy and/or reduce losses.

“Higher RPM means higher MAP, period.”
With a fixed pitch Prop, RPM follows MAP. With everything else being constant, If MAP is increased, pumping losses are reduced and RPM goes up. If additionally the fuel flow goes up, power is increased and RPM goes up higher.
With closed throttle, the pumping losses are highest and the pistons try to suck as much air in as possible. If now RPM is increased, they need more air, but because the airflow is restricted the MAP has to go down. Without any flow restriction, MAP would stay constant, but never rise. You can try this in your aircraft.


SuperCupDriver,
where do you measure MAP?

“But on the other hand this cylinder doesn´t produce power at all. “

This is correct, but this cylinder sucks air from the open exhaust valve. The amount of air that flows through the throttle in idle position, is completely used up by the remaining cylinders. A constant air mass is producing the same power, no matter whether it is used in three or in four cylinders.

“And - there is still the increased MAP which I believe is the key to this problem.”
Do you know the valve timing of the intake and exhaust valves? I don’t know, if this is sufficient to solve the riddle, but it is necessary.
 
Ingo,
MAP is measured at the intake port of #4 cylinder, I assume this doesn´t make a big difference.
I don´t know the valve timing or valve overlap, this data is probably available somewhere. I do have a roller cam with roller tappets.
For me the testrun yesterday proofs that the stuck exhaust valve in open caused a significant power increase, it must have acted like a significant intake leak. Looking at a closed throttle plate showes only a tiny gap for the intake air to go through.
 
collecting sticky valve data

I am collecting data for sticky valve experiences (engine type, fuel, oil, operating data etc.) . If you interested in contributing please send an email to valvedata@protonmail dotcom and I will send you the data fields we are collecting.

Not sure what may come out of the results, but maybe it will be show up something consistent across several hundred instances.....

Glad you didn't have any other issues on the flight!

Thanks,
Paul
 
Alright, some feedback my side… post annual run-up today, engine warm idle 720 RPM. Jumped out of the ship, removed spark plug #1, engine start, idle has now increased to 940 RPM.
Not as big an increase as Hermann‘s test cited in post #22, but nevertheless surprising as I thought it could be an injection, or not, related problem. So the answer is definitely found in some of the posts above :)
 
Alright, some feedback my side… post annual run-up today, engine warm idle 720 RPM. Jumped out of the ship, removed spark plug #1, engine start, idle has now increased to 940 RPM.
Not as big an increase as Hermann‘s test cited in post #22, but nevertheless surprising as I thought it could be an injection, or not, related problem. So the answer is definitely found in some of the posts above :)

Do your test again, and this time, watch the timing with a timing light.

Does not apply if you have 2 mags.
 
With the throttle closed, the engine is being deprived of the airflow that it seeks, hence the low MP (a vacuum). With an exhaust valve stuck open, there is now a new path for air to enter the intake system (when the intake is open simultaneous with the stuck open exhaust). So, the MP increases, resulting in more RPM, albeit with a bit different air/ fuel mixture ratio.
Combine that with less-than-normal compression braking (prop driving the engine), and you will see higher RPM, along with shallower rate of descent when trying to land.
 
Alright, some feedback my side… post annual run-up today, engine warm idle 720 RPM. Jumped out of the ship, removed spark plug #1, engine start, idle has now increased to 940 RPM.
Not as big an increase as Hermann‘s test cited in post #22, but nevertheless surprising as I thought it could be an injection, or not, related problem. So the answer is definitely found in some of the posts above :)

This is to be expected AT IDLE. At idle, the throttle plate creates a huge restriction. Whenever the #1 intake valve is open, it becomes a bypass to this restriction due to the missing plug. With an FI based engine, the result is the same as opening the throttle plate with some leaning but not a huge amount as the servo reacts to the lower MAP. On a carb'ed engine (fully dependent upon airflow through the venturi in the carb) it will get lean and run rough, though is not lean enough to die, should result in an RPM increase.

This effect will become less and less as the throttle is opened and will be barely noticeable at WOT.

I believe there is a pretty minimal overlap on the Lyc cam and therefore, at full power, a stuck open exh valve (very small leak path to the intake s ide) should create a net RPM reduction. At WOT you can't create a significantly lower MAP via a leak and the reduced output of the cyl with the stuck valve should create a net RPM reduction. The effect of the extra MAP, if any, is much less than the reduced power from the low compression cylinder.
 
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