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Engine Vibration and Exhaust Noise at Low Power Settings

Posted: Sat Dec 14, 2019 12:11 am
by Pawarren
Hello, I'm new to the forum.

I'm prepping for a commercial checkride in my 2012 DA40XLS. When performing simulated engine out maneuvers and steep spirals, I'm getting a fairly significant engine vibration and an occasional popping from the exhaust. This happens only with the throttle closed or nearly closed, and adding a few inches of manifold pressure typically eliminates the vibrations. Engine performance is otherwise near perfect with normal cruise and climb performance, fuel consumption, and CHT/EGTs. It is just out of annual with all compressions >70 and no issues other than an exhaust valve seal that needed replacement. Engine tach time is only 400 hours.

I'm going to need to get it into the shop to address this before my checkride, but was wondering if anyone had dealt with this issue or could give me some idea of where to start.

Cheers,
Patrick

Re: Engine Vibration and Exhaust Noise at Low Power Settings

Posted: Sat Dec 14, 2019 12:44 am
by Rich
Questions:
MP/Prop/mixture/airspeed in these cases?
At what RPM does your engine idle with throttle pulled all the way back?
Have you had idle mixture checked?

Re: Engine Vibration and Exhaust Noise at Low Power Settings

Posted: Sat Dec 14, 2019 2:08 am
by Colin
When this happened to me John Deakin said it was an intake manifold leak. Just in case you have your mechanic looking at it and want to point him in a direction.

Re: Engine Vibration and Exhaust Noise at Low Power Settings

Posted: Sat Dec 14, 2019 2:23 am
by Pawarren
It usually occurs between 68 and 85KIAS with MP<6, prop full forward, and mixture rich. I think the engine idles at around 940-960rpm, but I'll go out and confirm that tomorrow. Thanks for the input.

Re: Engine Vibration and Exhaust Noise at Low Power Settings

Posted: Sat Dec 14, 2019 2:27 am
by Rich
I like the idea of an induction leak.

Re: Engine Vibration and Exhaust Noise at Low Power Settings

Posted: Thu Dec 19, 2019 1:48 am
by Karl
Replace the intake manifold gaskets would be my first call.

Re: Engine Vibration and Exhaust Noise at Low Power Settings

Posted: Sat May 23, 2020 1:20 pm
by caseyayers
Sorry to dig this thread back up, but I was wondering if you might have confirmed if this was the issue or not? Shortly after my annual in February I started noticing a similar issue. I figured it might be some sort of vibration in the cockpit at first, but I consistently only notice this grinding/vibrating kind of sound when at idle or near idle on final with mixture and prop full forward. As soon as I get the throttle back north of 10” or 12”, it immediately stops, without any real fade; it’s either “grinding” or not. Wondering if I should get this looked at and if this might be a good place to have my shop start at.

Re: Engine Vibration and Exhaust Noise at Low Power Settings

Posted: Sun May 24, 2020 6:11 pm
by perossichi
check your MP at idle on the ground. Should be around 9" or so. Higher manifold pressure is associated with induction leaks. MP is really a measure of the vacuum created by the suction from the engine as the MP sensor is downstream from the throttle butterfly valve. So low values with throttle closed are signs of a heathy induction system. I've never heard of an MP value less than 6, but I don't have a lot of experience outside my plane.

RPM at idle should be something like 700-800 rpm.

Can you download your engine data? If so, put it up on the savvy aviation engine analysis site. You will get some free analytics. If you want to, you can pay them for a professional analysis.

A partially clogged fuel injection nozzle can cause engine roughness at low values of MP (with mixture rich). This cylinder will not develop full power because it is running too lean. When you open the throttle with mixture rich, then you may get enough fuel flow to smooth this out. If this is true, attempts to run LOP will cause a great deal of roughness.

I think Rich's theory should be the maintained hypothesis until proven otherwise, though.

Re: Engine Vibration and Exhaust Noise at Low Power Settings

Posted: Sun May 24, 2020 9:36 pm
by Pawarren
Turns out it was low ground idle. My mechanic adjusted the idle speed and mixture and the problem seems to have vanished. Every now and then you get lucky with a cheap fix. Cheers.

Re: Engine Vibration and Exhaust Noise at Low Power Settings

Posted: Thu Jul 01, 2021 3:41 pm
by mhoran
I'm now having this problem as well. My plane came out of annual about 10 hours ago and the situation has slowly been getting worse. I checked with my mechanic and he verified that mag timing and injectors were fine at annual. I believe the vibration was happening before the annual as well, so it would seem there must be something else at play.

I've been poring over a recent engine analysis I uploaded to Savvy (https://apps.savvyaviation.com/flights/ ... 4f74a702a6) but haven't seen anything jump out. At 1000 RPM the MP is just below 11. Seems that's not far off from 9 mentioned in this thread that would suggest an induction leak.

At full power in cruise, there's some vibration but it's not too noticeable. However, when on approach to landing or when taxiing around the airport, the engine is quite rough. I noticed last weekend that the standby airspeed was bouncing around by about 10 knots and the backup HSI was unreadable. My co-owner and I are generally very good about leaning both in the air and on the ground and our spark plugs were clean last they were checked.

Magnetos are okay during run-up but not great. There's some slight roughness on both. But EGT comes up on both mags for all cylinders. The drop is about 150 RPM on both, a bit more on left. I had the idle mixture adjusted a few weeks ago but that didn't help either.

You'll see in the engine analysis above that cylinder 1 runs a bit hotter than the others. I actually have to push fuel flow up to over 10 GPH at 8000 feet to get the temperature down to 404 degrees. This has been a problem for as long as we've had the plane, so I don't think it's directly related to the roughness problem but perhaps it is.

Any thoughts would be appreciated.