RPM fluctuations

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Robodog
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RPM fluctuations

Post by Robodog »

This intermittent problem seems to happen mostly (but not ALWAYS) when starting a warm or hot engine. After starting and setting the RPM at about 1100 RPM, the RPM momentarily drops to about 880 and then immediately climbs back up to 1100. When left alone, it will continue this cycling approximately every four seconds. It will do the same thing at other idle settings and recently the engine has quit with the throttle pulled all the way back to low idle. (That may simply be a matter of readjusting the low idle, which seems to be a bit on the low side anyway).

This fluctuation problem does not appear at higher RPMs and nor does it appear during runups and usually disappears completely after a runup.

As to the vapor lock question, leaning seems to help a bit. It does not have to be a high ambient temperature for the problem to appear. So for example at 75F, we can just as easily have the problem as at 85F. Engine runs beautifully in flight at all RPM settings.

Anybody else have similar experiences?

THX.
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Steve
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Re: RPM fluctuations

Post by Steve »

Roger:

Check your idle mixture setting on the RSA fuel injector. Too lean of an idle mixture can result in surging (at idle speeds only). Also, check to see if you get the appropriate RPM increase when shutting off the engine with the mixture control.

Steve

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Rich
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Re: RPM fluctuations

Post by Rich »

Theory from left field: One of the mags is not always producing enough spark at low RPMs to fire its plugs. But is OK at operating RPM. If so, a partially shorted coil.
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astaib
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Re: RPM fluctuations

Post by astaib »

It happens to me only after starting hot (after refueling).
It last some minutes only.

I thought it was because of the starting procedure on warm engine (full throttle, full lean / when fire immediately full Rich and throttle idle).

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Don
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Re: RPM fluctuations

Post by Don »

Have you tried turning the aux fuel pump on when this is occurring to see what happens?
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Rich
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Re: RPM fluctuations

Post by Rich »

Monitoring fuel pressure at these events is worth it. Last week - first time ever - after a hot start in which the engine started normally and was idling fine, I got a low fuel-pressure light. The pressure wasn't zero, but about 12 psi. I turned on boost pump.The Pressure went back to ~24 psi and the light went out. The engine then sort of sputtered for about 1 second and things went back to normal. I turned off the boost pump and pressure remained fine. No further issues. Pretty classic instance of vapor in the line somewhere.
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Lance Murray
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Re: RPM fluctuations

Post by Lance Murray »

Intake air leak?
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AndrewM
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Re: RPM fluctuations

Post by AndrewM »

Roger, I am on my second DA40 and this has occurred on both planes, on long cross-country legs where you stop to take on fuel and then get going again right away, hence a HOT start situation and also most notably in the summer, when ambient temps are usually higher.

The AFM does go into this a little and describes this can occur after extended ground operations / high ambient temps. It outlines arbitrary changes in RPM and fuel flow and slow reaction of the engine to throttle, and the AFM also describes that the engine may not run in idle (sounds similar to your case???). I have not experienced that, but usually I do not do a full run-up and check idle operations, when just doing a quick fuel stop.

The stated remedy is to run the engine 1-2 mins at 1800-2000 RPM, however oil and cylinder temps must remain within limits, and then confirm idle smooth running. It also suggests immediately prior to take off, to set mixture for take off and then full throttle for 10 seconds prior to taking off.
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Re: RPM fluctuations

Post by Robodog »

Thanks all for this input. Recently the plugs were cleaned and re-gapped. Problem stopped. But...more recently, it is starting to recur in a little less dramatic fashion but still...

Will suggest some of these diagnostics. Thx.
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Re: RPM fluctuations

Post by Robodog »

This problem seems to have finally been put to bed. The plugs were replaced. The coil also had a problem and it was replaced. Finally, the left mag (a slick mag) points were replaced.
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