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I had this repeatable phenomena that the engine RPM fluctuating while I put the throttle at idle or close to idle when aircraft is on the ground. The engine sounded brief roughness and choking at the time of low RPM fluctuation. When the prop knob moved forward with higher RPM, the roughness disappears. I remember it was mentioned that the roughness at lower RPM could lead to loss of propeller and other serious problem so I talked to the shop and have the aircraft looked at and figure out if there’s any serious issue with it. The mechanic at local shop doesn’t have much experience with the Diamonds, although he clean the fuel injector and spark plug (not sure if he cleaned all of them tho) and the problem still exist.
My questions is 1. Have you had any similar experience with your DA40 ? 2 . Is this a indication of a serious problem?
3. Should I send it to a Diamond certified shop to have it check out?
Many thanks.
I’ll upload the video later for better explanation.


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If the idle is set too low this could happen. My static idle, which I go to right before engine shutdown, is about 640 RPM, with a partially leaned mixture.

There is also the matter of setting the idle mixture properly. And mag timing. Keep in mind that mags have a much weaker spark at low RPM than at normal operating RPM. This is actually all standard Lycoming stuff.

Bryan,

Odds are pretty good the solution is just a simple adjustment. Most likely the Idle speed is just set too low, or timing or idle mixture - as Rich noted. All of these are easy fixes, and none of them are Diamond-specific. When you pull the throttle all the way back, what RPM values does your tach show? 700's? 600's? 500's? Do you have a data file you could upload from a recent flight?

Here’s the link to the video
Corrections: The throttle is set around 1000 RPM in the video.

@Rich @Rick Thanks for your input. It makes me feel a little relieved.
Will relay the info to local shop.




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To summarize what I think I saw in your video:
- the engine was running smoothly at about 1000 RPM (1000 - 1050)
- without any change in settings, the RPM drops to about 850 for a second or two
- it then returns back to about 1000 RPM again without any change in settings

That is a little different then what I thought you first posted - I thought this was happening when you set the throttle to idle, so your video cleared that up.

What happens to your EGT values when you have this hiccup? I couldn't quite read them in your video. Do they all move in the same direction?

Also, what does happen when you pull the throttle all the way back to the idle position and leave it there? Does your engine run smoothly at some RPM around 600-700, or does it hunt around continuously, or drop to even lower values?

And if you keep a data card in the top MFD slot and you have a recent data file with this hiccup on it, that would give some of the engine gurus here on the forum something else to look at!

Rick wrote: ↑Wed Aug 05, 2020 5:42 pm To summarize what I think I saw in your video:
- the engine was running smoothly at about 1000 RPM (1000 - 1050)
- without any change in settings, the RPM drops to about 850 for a second or two
- it then returns back to about 1000 RPM again without any change in settings

That is a little different then what I thought you first posted - I thought this was happening when you set the throttle to idle, so your video cleared that up.

What happens to your EGT values when you have this hiccup? I couldn't quite read them in your video. Do they all move in the same direction?

Also, what does happen when you pull the throttle all the way back to the idle position and leave it there? Does your engine run smoothly at some RPM around 600-700, or does it hunt around continuously, or drop to even lower values?

And if you keep a data card in the top MFD slot and you have a recent data file with this hiccup on it, that would give some of the engine gurus here on the forum something else to look at!

Thanks for the feedback. I recently had a Garmin Navigation and data base update after the event. Not sure if I could still pull some data from the SD card. I'll take a look.
I think according to my CFI, he mentioned that "it only happens at this particular RPM setting".
I'll see if I can put it out today and do another test run and have the video uploaded.
Thanks.

@Rick
I review the video a few times, seems like the EGT reading went up during the RPM drop and come back down when RPM picks back up. Seems to me that they are moving the same direction.

The video won't play for me. But an EGT rise on a cylinder accompanying the RPM drop would indicate a plug misfiring. An EGT rise on all cylinders suggests a mag problem or p-lead intermittent short.

To me - its possible this is totally normal, and maybe due to vapor lock.

Its summer, outside air temp is high and your oil temp is in the 150s, cylinder temps look above 300.

Living in NYC with frequent trump TFRs, Ill often startup with a smooth engine idle, but 30-40 minutes of waiting for clearance at the end of the runway in 90 degree outside air temps, my engine temps heat up the engine does not run happy.

Ive actually resorted to fast taxis down the runway to cool things down, along with vapor lock procedure.

I believe the vapor lock procedure is to run up to 1400RPM, lean mixture, boost pump on, get flow and pressure in the fuel lines and it should smooth out.

With Rick's guide, I was able to upload the flight log file to the system.
There are two of those files, Flight ID 12290 and 12291
Advices and comments are welcomed !