Engine Vibration and Exhaust Noise at Low Power Settings

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Pawarren
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Engine Vibration and Exhaust Noise at Low Power Settings

Post 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
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Rich
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Re: Engine Vibration and Exhaust Noise at Low Power Settings

Post 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?
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Colin
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Re: Engine Vibration and Exhaust Noise at Low Power Settings

Post 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.
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Re: Engine Vibration and Exhaust Noise at Low Power Settings

Post 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.
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Re: Engine Vibration and Exhaust Noise at Low Power Settings

Post by Rich »

I like the idea of an induction leak.
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Re: Engine Vibration and Exhaust Noise at Low Power Settings

Post by Karl »

Replace the intake manifold gaskets would be my first call.
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Re: Engine Vibration and Exhaust Noise at Low Power Settings

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

Post 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.
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Re: Engine Vibration and Exhaust Noise at Low Power Settings

Post 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.
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