Engine Vibration and Exhaust Noise at Low Power Settings

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perossichi
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Re: Engine Vibration and Exhaust Noise at Low Power Settings

Post by perossichi »

I think what we commonly identify as "engine roughness"results from an uneven production of power across the four cylinders. That is, one cylinder is producing much less power than the others. If you accept this as the fundamental cause of engine roughness, then anything that affects all cylinders equally cannot be the source of roughness. Technically, it could be two of the four cylinders but I think the roughness produced by more than one cylinder not producing full power on a 4 cylinder engine would be horrific.

I then would move to the 3 things necessary for combustion: 1. fuel 2. air 3. spark.

Fuel
What about the fuel system could cause lower flow to one cylinder? - a clogged injector nozzle is one good candidate. Fuel pumps and other parts of the fuel system can't be the cause since they affect all cylinders.

Air
could be an induction leak that affects specific cylinders but can't be bad air filter, throttle valve etc.

Spark
Very unlikely to be a bad spark plug for two reasons: 1. there are two plugs in each cylinder- both would have to be bad 2. If there was a only one bad plug, this would show up on the engine monitor if you switched to one mag -- i.e. the engine would run very rough on one mag but run smoothly one both mags or the other mag. The bad plug could easily be identified by checking EGT and CHT during an inflight mag check. The bad plug would have higher EGT and lower CHT.

Mags can't possibly be the source of the problem since they affect all cylinders. The wiring harness could be the source of the problem but again the leads to both plugs on one cylinder would have to be bad.

The reported high CHTs on cylinder 1 are exactly the problem I had on my 2002 and were completely eliminated by installing the "arizona" baffle. I rarely get over 380 CHT (even in climb) while running LOP. Could the cooling problem be related to the roughness problem? It would have to be that the fuel injector nozzle for cyl 1 is partially clogged -- resulting in that cylinder running much leaner than the others which could also cause roughness as well as higher CHT. An induction leak to cylinder 1 would cause cylinder 1 to run very rich -- which would lower CHT but could cause roughness. So my money would be that the cooling problem and the roughness problems are not related.
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Rich
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Re: Engine Vibration and Exhaust Noise at Low Power Settings

Post by Rich »

perossichi wrote: Sun Jul 11, 2021 5:26 pm Spark
Very unlikely to be a bad spark plug for two reasons: 1. there are two plugs in each cylinder- both would have to be bad 2. If there was a only one bad plug, this would show up on the engine monitor if you switched to one mag -- i.e. the engine would run very rough on one mag but run smoothly one both mags or the other mag. The bad plug could easily be identified by checking EGT and CHT during an inflight mag check. The bad plug would have higher EGT and lower CHT.
Anyone who's ever had a fouled plug is aware a single fouled/bad plug will do it. The reason is that the plugs are individually placed suboptimally for proper ignition. Note the significant drop in RPM when running on one mag during the mag check. Also, if you do a mag check in flight and run on one mag for an extended period you'll notice significant rise in all EGTs. I saw this when troubleshooting my Surefly installation when running on the non SF mag.

Matt, when you say "low power settings", is it RPM dependent? A prop imbalance could be a driver at a particular loading/RPM if it's at a harmonic point for the propeller/engine/mount subsystem.
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Re: Engine Vibration and Exhaust Noise at Low Power Settings

Post by mhoran »

I think I am experiencing multiple issues, all which I've reported in this thread so that's probably throwing off the scent -- but I do appreciate all the input as there's some good stuff for us to dig in to when I bring the plane back in on Tuesday.

First, the original vibration issue. I've noticed the vibes between idle and 1500 RPM. Above that point, I haven't noticed abnormal vibration. Given the steps I've taken so far, I believe the remaining vibes are likely a prop balance issue, which may have been with us for some time but I didn't notice / care about it until we started having the roughness as well. This is most noticeable on approach to landing. At takeoff, climb or cruise, it's not noticeable, given the high RPM.

Second, the engine roughness. We replaced the mags on Friday, as they had 666 hours since last inspection. That seems to have helped with the left mag drop as well as left mag roughness during mag check. However, the right mag is still running rough both on the ground and in the air. Interestingly, switching to the left mag in the air does *not* resolve the vibration issue, again leading me to believe that the bad vibes could just be a prop balance issue. We also found two leads on the wiring hardness to be bad.

Third is the high CHT on cylinder #1. Given we have a 2007 I believe we have the Arizona baffle, and comparing with photos of the original vs. Arizona this seems to be the case. I believe this might just be a bad probe or perhaps an airflow issue at this point. I'll check that out on Tuesday.

Given the above, I do suspect the plugs are responsible for right mag roughness. I looked through the logs and I believe two plugs have 1200 hours on them. Given the issues with the wiring harness leads I think that could be an issue as well. Cylinder #3 has been slow to rise during mag check so there could be something there. We also found oil on the terminal side of the top plug on cylinder #3, which baffled both my mechanic and I.

Edit: oh, and we don't suspect the fuel system for the roughness at this point, since the engine runs fine on the left mag since it was replaced. Also of note: another mechanic had played with the idle mixture which caused significant RPM fluctuation on the ground and roughness on both mags. We dialed that back one click towards rich which resolved that issue.
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Re: Engine Vibration and Exhaust Noise at Low Power Settings

Post by perossichi »

If you only have roughness on the right mag and you suspect a bad plug, just look at the egt and cht values. Typically, a fouled plug would show higher EGT and lower CHT. If possible, do a inflight mag check that is LOP for best diagnostic value. Similarly. a plug with high resistance will not fire as hot a spark and this will also show up as higher EGT and lower CHT for that cylinder. The reason is that with a poor spark the combustion will proceed more slowly and not all of the fuel will be burned.

When I was diagnosing high CHT in cylinder 1, I swapped the CHT probes between cyl 1 and cyl 3 to make sure that the probe wasn't the problem -- it wasn't.

Two other cooling possibilities: 1. poor baffle seals (remove the top cowling and look at the black rubbing marks from the baffle seals-- you can see a gap there -- look for the kinks in the black rubber seals. 2. Missing or mis-positioned intercylinder baffle between cyl 1 and cyl 3 (this a metal baffle held in place between the cylinders by a rod. It blocks air from flowing between the cylinders and forces cooling air to flow thru the cylinder head fins. )

Note: when you do a LOP mag check either on ground or in the air, you look simply for all EGT rising -- the RPM drop is not useful. In fact, Lycoming has a service bulletin on this very thing. Note: EGTs are only meaningful in a relative sense -- there is no "right" level of EGT and it is not necessary that EGTs be the same for all cylinders. Don't tell that to the average A&P -- they will roll their eyes and starting swearing at Mike Busch!

On plugs, when I bought my plane it had all Champion fine wire plugs. I tested the resistance of all plugs and found they were all absurdly high. I threw them all out and put in tempest fine wire about 500 hours ago. No problems since then. Fine wire should easily go over 1000 hours without replacement, although they are stupid expensive. It is my understanding that Champion has "fixed" this production problem but if you look at the Tempest plug it is much better made and sturdy.

best of luck. Troubleshooting can be both very frustrating and fun.

p
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Re: Engine Vibration and Exhaust Noise at Low Power Settings

Post by mhoran »

The troubleshooting continues. We're all pretty much stumped at this point. After flying the plane up to Take Flight yesterday, we spent five hours swapping plugs, testing the ignition harness, and swapping the right mag again. The issue persists. I ended up leaving the plane overnight so they can take another stab at it today. Unfortunately it's looking like I'll be scrubbing my plans to fly to the Bahamas on Friday. Was hoping for a simple fix, but the plugs have all been cleaned and moved around, and the mags don't look to be the issue. The only thing we haven't swapped is the ignition harness, but it tested just fine.

The mechanic who initially replaced my mags suggested a compression check for cylinder #3, which is the only one presenting anomalies during run-up. It shows lower EGT by about 50-100 than the rest of the cylinders. Primarily this is on the right mag, though it also happens slightly on the left. He said that when he had similar symptoms, he found low compression and failing piston rings on a cylinder.

I didn't have the chance to perform another in-flight mag check yesterday as it was IFR and I didn't want to risk losing the engine at 4000 feet with an overcast layer below me.
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Re: Engine Vibration and Exhaust Noise at Low Power Settings

Post by Steve »

Matt:

How did your mechanic test the harnesses? Simple DMM ohmmeter or a dedicated spark harness tester? Compression test is easy, so that should definitely be part of the diagnostic workup. I can't remember if you have cleaned the injectors, but that is easy to do as well.

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Re: Engine Vibration and Exhaust Noise at Low Power Settings

Post by mhoran »

They used a dedicated spark harness tester. I suggested a compression check to them this morning. The injectors were cleaned ~11 hours at annual but I'll mention that as well.
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Re: Engine Vibration and Exhaust Noise at Low Power Settings

Post by perossichi »

I don’t know if this will help. I find a lower EGT on cylinder 3, right mag during both run up and inflight mag checks. This is very strange as cyl 3, right mag is the top plug. Typically it is the bottom plugs that run a lower EGT. However, the engine runs very smooth on both mags. At the annual, the plugs are rotated and I still get this. I’m stumped.

You may be aware that it has been demonstrated that even cylinders with very low compression develop full power. This was done by intentionally filing down the rings to get extremely low compression. But the engine developed full power.

Finally, poor compression on cyl 3 can’t possibly be consistent with engine roughness on the right mag and NOT the left. Let’s say you had a failing exhaust valve or ring problems in cyl 3. If this really was causing cyl 3 to develop less power, then this would happen on both mags— that is, you would get roughness on both not just on the right.

I’m a little surprised that the A&P didn’t understand this point. You might want to point this out and have a discussion.

So let’s keep score—

You have eliminated
Mags
Plugs
Partially clogged nozzles
Harness
as the source of the roughness problem.

Presumably the engine runs smoothly on both mags. Are you getting full takeoff power? If so, I wouldn’t worry.

On the vibration at low rpm, I did get a dynamic prop balance after doing the power flo install and it made a huge difference and only cost $150. However, my perception was that the prop balance reduced vibration more at cruise settings than at low power. But I don’t think that has to always be the case.

I’m working on getting a glider rating and after flying without vibration and noise my DA40 seemed horribly loud and vibrating like hell.

Best of luck with your frustrating situation.

P
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Re: Engine Vibration and Exhaust Noise at Low Power Settings

Post by perossichi »

Also, some experts like Mike Busch report that the only time they get clogs in nozzles are after they are cleaned as the fuel system is opened. They advise doing a GAMI lean check before each annual and ONLY clean the nozzle identified as running prematurely lean. If GAMI check is good, then don’t clean the nozzles, he advises.

I’ve been doing that for 4 years now. Our engines have a very small GAMI spread so this seems like a great test for us.

Again, a partially clogged nozzle could be the cause of roughness but in that case the engine would run rough on both, l, r mags and you report only roughness on the right mag. A real head scratcher.
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Re: Engine Vibration and Exhaust Noise at Low Power Settings

Post by mhoran »

Well I just can't seem to catch a break. The shop spent two more days troubleshooting but didn't find anything. They tried a new ignition harness but that didn't help. They tested the spider and it was fine. In the end they suggested since everything was within limits it was probably fine. I flew the plane back to my home base and found that it ran well enough for now and decided to proceed with my planned trip to the Bahamas.

Unfortunately this morning as I was taxiing to the runway I found that my alternator was no longer working. I had the shop replace the brushes and apparently that was a mistake.
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