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IO-540 at 2500rpm with EFII - drops cylinder

It turns out that either the intake or exhaust hydraulic lifter bodies was causing the problem. I don't know fully what was happening, but apparently at 2,500 rpm there was some resonance in the spring and the hydraulic unit was either not deflating or not inflating or over inflating, causing a valve to not fully close.

I would not be comfortable thinking the hydraulic unit is the root problem, as it cannot extend enough to hold a valve off its seat without a momentary reduction in opposing valve spring force. The fact that (1) the problem is evident within a single narrow RPM range, and (2) varying the hydraulic units changes the symptom does suggest spring resonance of some nature, which could potentially provide that reduction. The literature suggests valve springs should have a design target natural frequency of 15x the cyclic frequency in order to ensure the cyclic or one of its multiples does not overly excite the spring.

Here's the thing. The first natural frequency of the spring can be expressed as...

F = 0.5 * sqrt ( spring rate / spring mass )

...which tells us reduced spring rate results in lowered natural frequency.

I couldn't walk away from this one without pulling the spring(s) for a rate check. I suspect a soft spring.

Which one? An exhaust valve held off the seat generally kills just that cylinder. An intake valve held off the seat tends to affect all the cylinders. So the lead suspect is exhaust, but heck, may as well check both.
 
I would not be comfortable thinking the hydraulic unit is the root problem, as it cannot extend enough to hold a valve off its seat without a momentary reduction in opposing valve spring force. .

+1

certainly not saying it isn't possible, but it is beyond strange that a lifter could overfill itself at 2475 rpm and then return to normal operation at 2525, with 100% repeatability. I am admittedly uneducated on harmonics, but this seems odd to me. I can see how a defective plunger assy would allow too much oil to over extend a lifter, but just don't see how it can only do so in such a small operating range.

I hope for you sake this is the issue, but can't seem to resolve it in my head. I also am inclined to believe that some strange harmonic in the springs is causing a momentary valve float over an extended lifter. Even a tight valve guide is more plausible. I just can't wrap my head around an overextended lifter body happening at 2475 and magically retracting at 2525, repeating consistently on cue; hot or cold.

Definitely an education for all on this one.

Larry
 
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???

First off, I would have lost any related bet.

I didn't read the root cause response quite the same as DanH or Larry; assuming I correctly interpreted their writings. Trying to conceptualize the description given, I thought the spring constant/resonance (combined effect of the both check valve spring and entrapped fluid) of the lifter proper was the suspected culprit. The resulting lifter "geometry" was affecting the kinematics of the valve.

If the lifter does reach an undampened resonance, can it "float"?
 
I don't see how this can be anything other than a problem with the fuel MAP. What else is so precise as to fail at exactly 2500 rpm consistently? Certainly not valve springs or injectors. If there was a wider spread - maybe, but to be that precise I just don't see mechanical problems.
 
If the lifter does reach an undampened resonance, can it "float"?

The compressive stiffness of oil would be very high yes? I doubt the hydraulic unit can resonate at the comparatively low frequencies here.

"Float" is generally assumed to mean the valve is no longer following the motion dictated by the trailing flank of the cam lobe. It is valve spring force which keeps the valve in contact with the rest of the valve train when the cam lobe rotates past full lift, and the valve's direction of motion is reversed. In the classic view of valve float, a weak spring generally limits max RPM, or the steepness of the cam lobe profile.

Here I'm suggesting spring surge, one with waves resonating up and down its coil. The spring always has some wave action, but resonance can conceivably drive it to high levels. If valve spring pressure momentarily drops below oil pressure x hydraulic unit piston area while the valve is off the seat, the hydraulic unit will extend, and the valve cannot return to the seat later in the cam rotation. The cylinder dies for lack of compression. Different RPM, spring not resonant, cylinder normal.

A quick look on BoobTube finds a few videos of valve spring vibration. These are normal wave action, not resonant.

https://youtu.be/50F-haEs3pg

https://youtu.be/yfmb-tCo2yA
 
Hmmmm

DanH, Someone made a comment that the inner spring had "broken"' in the videos.

I wonder if this might be related to our subject? John
 
DanH, Someone made a comment that the inner spring had "broken"' in the videos. I wonder if this might be related to our subject? John

First video, I see a chip fly out at around :31, and the outer spring fails about 1:40.

I think Jesse would have noticed a broken spring.
 
DanH, Someone made a comment that the inner spring had "broken"' in the videos.

I wonder if this might be related to our subject? John

A month ago I noticed a compression of 50 on #2. During investigation, I pulled the rocker cover. To my surprise, the inner intake spring had gone from one piece to 4 pieces of steel. A couple of taps with a brass hammer on the valve stem loosened the deposits that had, in theory, been able to develop on the valve or seat due to reduced spring pressure or undesired harmonics and compression came back to 75.

Only symptom was #2 was slow to warm up and replaced spring didn't resolve that. No other noticeable symptoms (still trying to figure out the root cause). I am pretty certain that a broken outter spring would have created valve float. The inner spring is really more of a damper, from what I understand.

Larry
 
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Same issue sort of

I have a Super 8 with an O-540 and had a similar issue. Climbing, 25/25, passing 9500 for 11,500 the number-1 cylinder EGT started to vary up and down and then went cold with the CHT following, but the engine sounded smooth the whole time. I chocked it up to not leaning correctly/enough in the climb or perhaps a sensor issue. It went away as I leveled off and pulled the RPM to 2300. Two subsequent flights and it has not repeated but both were low and I didnt spend much time at 2500RPM.
What struck me is that for both of us it involved an O/IO540, the number-1 cylinder, and a Dynon Skyview. Not too rare, but rare enough to catch my attention. If this comes back or if mine does, lets talk.
 
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We did carefully inspect the springs and they both appear intact and the same.

I can’t imagine it’s a Dynon thing. Ours had noticeable roughness. It felt almost as if we were firing on only 5 cylinders, which we were. It appears to have something to do with oil temperature. First takeoff of the day, if the taxi is short, we would sometimes get a momentary “burp” just once, then everything acted like it should. The times we didn’t get it was with a longer taxi and/or runup. Before replacing the lifter bodies, I could get it any time I tried, and it would continue as long as I wanted it to. Once, before we figured out the 2500rpm thing, it did it for 23 straight minutes. The EGT dropped from 1300 to 400.

On a side note, this would have been a nightmare to try to troubleshoot without a 6-cylinder engine monitor.
 
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