Odd engine noise

[MAKG1]

I popped into a rental 172 to punch a few holes in the sky today.

The run-up showed some pretty bad fouling on the left mag, so I cleared it. It seemed to clear up pretty good, and the mag test passed. That's normal on this airplane -- I soloed in this aircraft, so I'm quite familiar with it.

But after clearing, it made a rhythmic beating noise, something like a helicopter at a distance. I think it was at half engine RPM. It was quiet at idle, and present at all speeds above 1500 RPM, and sped up with the engine and got slightly louder at high RPM.

This was not a subtle noise, and I hadn't heard it before, so I parked the airplane. I think I may have been hearing a miss, or maybe blow-by or an air leak or mis-time somewhere. It's odd that switching mags didn't affect it (aside from the change in RPM).

Another user had posted a squawk for needing full throttle in the pattern, but the owner had test flown it and called it normal. In retrospect, I don't think it was.

Any ideas? I would have thought a miss would have been present on only one mag, and not the other.

Obviously, I'm not flying that airplane without identifying it. Severity could range from insignificant/normal to catastrophic, depending on what it is.

 

[Cpt_Kirk]

I always go have a talk with the mechanics in person. We can usually pinpoint the problem.. And if not, they can help me decide to down it or not.

 

[Luvrv8]

Sticking valve?

 

[RyanB]

Could be a misfire or sticking valve also ^. Best to have mechanic check it

 

[Dan Thomas]

Sticking valve?

That's my bet. Morning sickness.

What engine is this?

Dan

 

[MAKG1]

Interesting .... the noise is about right for a stuck open exhaust valve, and that would explain why it was the same on both mags.

It's a standard 172N with an O-320-H2AD.

I'm not about to try the usual automotive test. Too close to a spinning prop.

It's not my airplane, but I do expect the owner to call me when he hears about it. He's generally on top of these things. I'll volunteer to visit the mechanic with him.

 

[Luvrv8]

At a shop I work at we have not seen a sticking valve in a while, in the last month we had 4 of them.

 

[Neal Howard]

At a shop I work at we have not seen a sticking valve in a while, in the last month we had 4 of them.

And probably resulting from not leaning enough during ground ops. If you lean enough so that the engine can't run more than about 1100 rpm on the ground then you won't get enough lead deposits to stick valves.

 

[Luvrv8]

Pretty much what we tell the owners as they look at us like we have 3 eyes.

 

[Henning]

I popped into a rental 172 to punch a few holes in the sky today.

The run-up showed some pretty bad fouling on the left mag, so I cleared it. It seemed to clear up pretty good, and the mag test passed. That's normal on this airplane -- I soloed in this aircraft, so I'm quite familiar with it.

But after clearing, it made a rhythmic beating noise, something like a helicopter at a distance. I think it was at half engine RPM. It was quiet at idle, and present at all speeds above 1500 RPM, and sped up with the engine and got slightly louder at high RPM.

This was not a subtle noise, and I hadn't heard it before, so I parked the airplane. I think I may have been hearing a miss, or maybe blow-by or an air leak or mis-time somewhere. It's odd that switching mags didn't affect it (aside from the change in RPM).

Another user had posted a squawk for needing full throttle in the pattern, but the owner had test flown it and called it normal. In retrospect, I don't think it was.

Any ideas? I would have thought a miss would have been present on only one mag, and not the other.

Obviously, I'm not flying that airplane without identifying it. Severity could range from insignificant/normal to catastrophic, depending on what it is.

It really depends on the tone and exactly when it happens and when it goes away. If it is a deep hollow 'tock' that appears to change with engine speed and go away as you add power or are under constant solid load; but reappears as you retard throttle or the engine stabilizes under light load, I would not fly the plane as that is a sign of a bottom end rod problem which will eventually end in catastrophic failure. Maybe not the next flight or even several, but eventually.

Any noise outside of that is harder to make a call as to catastrophic engine or minor accessory. Regardless though, you should not fly a plane that makes noises you don't understand or feel comfortable with. You're paying for this, not getting paid.

 

[MAKG1]

Nope, not rod knock, not piston slap. I know those sounds. And they depend strongly on changes in engine speed, which was not observed.

 

[Henning]

Nope, not rod knock, not piston slap. I know those sounds. And they depend strongly on changes in engine speed, which was not observed.

Yeah, diagnosing an engine noise over the Internet isn't often fruitful. If it makes you paranoid, don't fly it until whatever it is breaks and gets fixed.

 

[MAKG1]

Yeah, diagnosing an engine noise over the Internet isn't often fruitful. If it makes you paranoid, don't fly it until whatever it is breaks and gets fixed.

I may be satisfied if it's identified and benign, but I wasn't planning on flying it prior to that. I scrubbed a flight at run-up, and I do think it was the right decision under the circumstances.

 

[Henning]

I may be satisfied if it's identified and benign, but I wasn't planning on flying it prior to that. I scrubbed a flight at run-up, and I do think it was the right decision under the circumstances.

Yeah, not faulting your call one bit.

 

[N5922S]

That's my bet. Morning sickness.

Dan

Having had an engine with morning sickness once, I can tell you there's nothing subtle about it. It feels like the engine is trying to shake itself loose from the plane.

 

[denverpilot]

At a shop I work at we have not seen a sticking valve in a while, in the last month we had 4 of them.

Makes sense. All the students taxiing around full rich starts in the Springtime when the weather gets nice.