RPM fluctuation on idle and engine choking

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yl472401
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Re: RPM fluctuation on idle and engine choking

Post by yl472401 »

briankelly327 wrote:
Wed Aug 05, 2020 8:01 pm
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.

I'll try the trick and see if there's any improvement.
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Re: RPM fluctuation on idle and engine choking

Post by DaveS1900 »

Just to add extra info: I had fluctuations and found there was an existing AD on my fuel control unit. Have your mechanic check to see if your unit requires the AD and if it has been done.
Dave S.
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Re: RPM fluctuation on idle and engine choking

Post by Thomas »

It seems normal to me with the hot engine (190 OIT and 300 CHT). How does it behave when started first time of the day, mine is absolutely stable when cold?
These phenomena I see since I have the DA40 (11 years), and it was the same with the previous, Maule MX7 with IO540.
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Re: RPM fluctuation on idle and engine choking

Post by Rich »

Looking at the new information, this doesn't seem to warrant any real concern. Keeping in mind I have a VM1000 EIS, rather than the G1000, I note that today, after landing, with the baseline mostly just as you show (300-ish CHT's 1180-1200 EGT, RPM 1040) mine does hold RPM OK. The one difference I will note that with engine parameters as you show, the mixture setting I use for ground ops (roughly halfway back) yields fuel flow of 2.5 GPH, not 1.5.
2002 DA40: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision
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Re: RPM fluctuation on idle and engine choking

Post by yl472401 »

DaveS1900 wrote:Just to add extra info: I had fluctuations and found there was an existing AD on my fuel control unit. Have your mechanic check to see if your unit requires the AD and if it has been done.
Dave S.
I looked through the PDF copy and wasn’t able to find such entry. But I purchased this aircraft from a flight school seems like they were pretty on top of everything. I’ll look at the original file and see if I can find it.
Thanks for the input !!


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Re: RPM fluctuation on idle and engine choking

Post by Donut »

I had a very similar rpm fluctuation occurring to what I saw in the video about 2 months ago. When at 1000 rpm on the ground I would see the rpm drop down to below 900 every few seconds. I brought the plane to the shop and it turned out mag timing was off. $180 and 2 hours later all was fixed and I was in the air again.
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Re: RPM fluctuation on idle and engine choking

Post by yl472401 »

Thanks for the info. I did a little search and think that's for airplane model before 2006. The Diamond mechanic also mentioned that AD was for earlier model :D
DaveS1900 wrote:
Fri Aug 07, 2020 1:50 pm
Just to add extra info: I had fluctuations and found there was an existing AD on my fuel control unit. Have your mechanic check to see if your unit requires the AD and if it has been done.
Dave S.
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Re: RPM fluctuation on idle and engine choking

Post by yl472401 »

Thanks for the heads-up. Wish I could get his info earlier. Right now they are swapping out the fuel mixture control unit and will get it back by the end of this week.
I'll mention this to my mechanic and see if he can find something.

Donut wrote:
Tue Aug 18, 2020 7:35 pm
I had a very similar rpm fluctuation occurring to what I saw in the video about 2 months ago. When at 1000 rpm on the ground I would see the rpm drop down to below 900 every few seconds. I brought the plane to the shop and it turned out mag timing was off. $180 and 2 hours later all was fixed and I was in the air again.
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Re: RPM fluctuation on idle and engine choking

Post by yl472401 »

Aircraft came back from the shop with the repairs listed below:
replace engine air filter, replaced fuel mixture spring, replaced the fuel servo and adjusted the idle mixture and speed.
I did notice the difference when the throttle is set at middle and at idle. The speed at idle could be increased and fuel flow shows ~1.9.
A slight variation of the RPM but no more dramatic dropping of RPM observed. Hope it is fixed...
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Re: RPM fluctuation on idle and engine choking

Post by Roxi5m9 »

Rich wrote:
Wed Aug 05, 2020 6:09 pm
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.
That’s not necessarily true from my experience, Rich. This last spring we noticed some intermittent engine roughness. After a lot of troubleshooting and doing an inflight mag test we figured out the number 2 cylinder on the right mag was intermittently misfiring. We replaced the plug, no help, replaced the harness, no help, realized we had 900 hours on the mags, replaced them, fixed it. Basically, somewhere in the core of the mag it was wore out. From what I was told, usually a mag will impact all cylinders but not always.
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