Lycoming O360 A3A odd sound

Indiana_Pilot

Indiana_Pilot

Line Up and Wait
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Connersville, IN
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Flying "B"
I have been noticing a different sound lately in my Cherokee 180 with the O360 A3A. I had my mechanic look at it and he seems to think there is nothing wrong.

I hear what almost sounds like detonation (pinging) but it almost sounds like a rattle when the engine is right around 2450 rpm (or it could be when it's under load when the RPM is reduced to 2450 rpm) I hear it slightly when I start my take off roll at full throttle and then if I climb aggressively.

If I lean the mixture to LOP in the climb it goes away and comes back when I go full rich. I never hear it at level flight at 75% power at either LOP or full rich no matter what. I may hear it if I climb aggressively and full rich...

I only hear it when I am on both mags not one or the other.. (I am guessing due to the RPM change)

It runs great and I was told that I wouldn't even be able to hear the detonation (pinging) if it was doing it anyway... Starts fine, CHT temps are good, power is good, ect..

I/we have checked everything we can think of that could be lose and move around and all is fine..

Any ideas that I can pass to my A/P ?
 
How about loose stuff behind the panel? Mounting brackets etc. I had a jingling sound in the Cherokee that drove me nuts. Only in the climb at full throttle. Turned out to be my key ring in the ignition when it got at just the right angle....
 
If it's a problem, it will make itself known eventually. Did your mechanic hear it in flight for himself?
 
Check the alternator thru bolts and its mount for cracks etc on the front of the engine. It is resonant at about 80 Hz which is close to the second harmonic of 2450 rpm - which is a very strong harmonic in a 4 cyl engine. I'm surprised Lycoming gets by with it.
 
he heard it on the ground but not in flight.. So since it actually sounds like detonation can anyone tell me if I would actually be able to hear it?


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You know what I will check my key ring and will feel like a total idiot if that's what it is... lol that almost seems like it could be the sound too


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Firefighter_324 said:
he heard it on the ground but not in flight.. So since it actually sounds like detonation can anyone tell me if I would actually be able to hear it?


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Yes, you can hear detonation in most planes. It's extremely rare you will get it in a plane engine.
 
Firefighter_324 said:
You know what I will check my key ring and will feel like a total idiot if that's what it is... lol that almost seems like it could be the sound too


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There's always room for another seat at the table...I'll save ya a chair.
:D
 
charlie said:
Piston slap. It goes away running LOP or on one mag because the flame pattern therefore the piston loading changes. A good noise cancelling headset will help.
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A guy from Zephyrhills, named "Charlie" - that's an opinion I will credit! :yes:

So, in the spirit of learning, what is "piston slap" anyway? I have often heard of it, never had the guts to admit I did not know what it was. Now's the time.
 
The side of the piston "slapping" the cylinder wall. Common in diesels before the engine gets warmed up, it's a distinct rattling sound. You'd know it if you heard it.
 
another favorite thing to check is your rubber root seal along the wing and fuselage. I've had sections of that come loose and vibrate.

At various flight angles and rpms it can cause noises and rattling.....
 
I am going to go out to the airport again tonight and look it over again.. Another way I can describe it is that is sounds like a fast but faint tambourine..

I hope it's just my keys.. I still feel dumb about not thinking about that.. :rofl:

So going back to Piston slap... That can't be good right? Something that I should be thinking about having the top end pulled? I have about 1200 hours on it. It did have the top end removed (Cam shaft AD) and new bearings put in about 6 or 7 years ago..
 
If it sounds like a tambourine (jingling) it's not piston slap, that's more a 'thwack' sound. Tambourine is more like detonation.
 
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That is what I was thinking.. so what would cause it to detonate loud enough that I can hear it? If it was way to rich.. then I wouldn't see the CHT go very high right?

Let me ask you this.. suppose the mixture was out of adjusted and actually did produce a over rich situation, could that cause detonation? Also I can acquire a good power increase if I lean before take off and the detonation is not there but the CHT comes close to peak. I even see this with a negative Density Altitude. Seems like it may be running richer than it should be.. is that possible? Would that be a mixture adjustment or carb ?
 
I doubt you'll hear pre-ignition or detonation....from an aircraft engine.
 
Firefighter_324 said:
That is what I was thinking.. so what would cause it to detonate loud enough that I can hear it? If it was way to rich.. then I wouldn't see the CHT go very high right?

Let me ask you this.. suppose the mixture was out of adjusted and actually did produce a over rich situation, could that cause detonation? Also I can acquire a good power increase if I lean before take off and the detonation is not there but the CHT comes close to peak. I even see this with a negative Density Altitude. Seems like it may be running richer than it should be.. is that possible? Would that be a mixture adjustment or carb ?
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If it's way too rich, you won't have detonation, detonation only occurs near peak. More likely if you have a rich running engine, you are running into a pre ignition event on some hot carbon, it will have the same basic sound. My suspicion is you have some carbon loaded valves.

Peek in the cylinders with a bore scope, and have the timing checked, including the e-gap.
 
Checkout_my_Six said:
another favorite thing to check is your rubber root seal along the wing and fuselage. I've had sections of that come loose and vibrate.

At various flight angles and rpms it can cause noises and rattling.....
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This MUCH more likely than any other of the above.:yes:
 
Firefighter_324 said:
Anyway to clear that out? Could it be from over leaning ?


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Run hard and lean and you will burn it away. GM has a product called X-66 Carbon Remover that is one of the three awesome products in a can that actually work, however getting it into an aircraft engine is a bit tricky since you suck it into a running engine. If you add Barkeeper's Friend (oxalyc acid crystals) to it, it's a "ring job in a can".
 
I did but fine wires in awhile back but only recently developed it..


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Ok thanks.. I will try the hard and lean.. I always run Lean of peak and 75% power maybe I'm still a touch to rich.. I use the CHT gage and not the stumble then back off method


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When ANR headsets were introduced, these types of posts became much more common. lol

:)
 
I give the OP credit, he's better than most at being able to describe a sound.
 
The EGT (just 1 probe) seems to be normal in the climb.. Of course if I lean it goes to peak.. As an example since there are no number markings on the EGT.. My peak is usually around exactly half on the gage.. In climb at full rich rattle or no rattle it stays about 1/4..


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You're not LOP in a carbed 0-360. Maybe 1 or 2 cylinders are LOP.
 
Henning said:
If it sounds like a tambourine (jingling) it's not piston slap, that's more a 'thwack' sound. Tambourine is more like detonation.
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Nah, piston slap is more like a "pocka pocka pocka".
 
Seat belt hanging out the door can make a hellofa racket.
 
Could be a wing coming off....
 
Henning said:
Loosing the left wing would suck, but if it's the right you could always hold the door open with your foot to equalize the drag some.
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and losing a wing would be even worse....:yikes::goofy:
 
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I finally found some time to take my 180 up again.. It was definitely not the keys and i was able to find every other rattle vibration noise in the cabin.

It was most certainly coming from the engine or at least ahead of the firewall. I took someones suggestion to run it hard and very lean. So I did that for my hop to the airport in the next town.. (maybe 10 minutes).

I let the EGT peak higher than normal. I actually just leaned until stumble then in until smooth. I usually go about 2 turns in or so following the 50 degree marks on the EGT...

Well it went away and no matter how hard I try to make it happen again at full rich, the noise hasn't came back.. I didn't hear it on my hard climb out at 2450 rpm either on the way home..

So what the heck ?? I will be even more convinced when I fly again today but it actually ran very smooth after that.. Carbon build-up or what ??

Does this mean I am not leaning well enough? I am always afraid of overheating I guess.
 
Correct, your standard operating procedure is leaving you with big carbon deposits. You ever hear the old Bill Cosby Cobra sketch where the mechanic tell him, "You gotta burn the gunk out"? That's what you did with the hard, lean, run, you burned the carbon deposits off that were glowing and causing pre ignition at the right pressure/mixture combinations.
 
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