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  #1  
Old 01-18-2022, 09:25 PM
YellowJacket RV9 YellowJacket RV9 is offline
 
Join Date: Oct 2012
Location: Clearwater, FL KCLW
Posts: 1,374
Default Erratic RPM reading

Hi all,

Had an odd event today and after looking through the data logs wanted to get some feedback on my troubleshooting process. Was cruising along at 3000' and noticed a very slight roughness that self-corrected within 10 seconds or so. On the EGT trend graph on my engine monitor I noted a corresponding brief increase in EGT on cylinder 4, quickly settling back to baseline after 30ish seconds. That's all I noticed at the time.

In reviewing the logs this evening, it appears that after this event, I have what looks like more noise in my Tach signal. It is normally rock steady, but looks much more erratic from this point on.

My thought is something ignition related, like a plug or wire going bad, that is causing some electrical interference.

For reference, this is an O-320, with a SureFly on the left and Slick on the right. Running aviation plugs on both.

Any other ideas or places to look? Thanks

Chris
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RV-9A - Done(ish) 4/5/16! Flying 4/7/16

Last edited by YellowJacket RV9 : 01-18-2022 at 09:45 PM.
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  #2  
Old 01-19-2022, 06:01 AM
vlittle's Avatar
vlittle vlittle is offline
 
Join Date: Jan 2005
Location: Victoria, Canada
Posts: 2,458
Default

Your instinct is correct. Check the plugs and wires for integrity, and pay attention to the spark plug gaps. Erratic RPM indication that develops from a normally function system is telling you something.

One of the advantages/disadvantages of the way some EFIS systems measure RPM is that they are sensitive to the ignition waveforms. For example, a worn plug with a larger gap will fire at a higher voltage. This shows up in an oscilloscope trace as one pulse being larger than the rest and possibly triggering a (false) RPM reading.

After several years of flying with my IO540, I began to get false RPM indications. Turns out that I had a burn-through on a plug wire due to the proximity of the wires to an exhaust pipe. Saw it on my portable oscilloscope in flight. Replaced and rerouted wires, everything OK.

Also of note, this is more common on six-cylinder engines (more cylinders).

Cheers, Vern
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  #3  
Old 01-19-2022, 09:00 AM
YellowJacket RV9 YellowJacket RV9 is offline
 
Join Date: Oct 2012
Location: Clearwater, FL KCLW
Posts: 1,374
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Quote:
Originally Posted by vlittle View Post
Your instinct is correct. Check the plugs and wires for integrity, and pay attention to the spark plug gaps. Erratic RPM indication that develops from a normally function system is telling you something.

One of the advantages/disadvantages of the way some EFIS systems measure RPM is that they are sensitive to the ignition waveforms. For example, a worn plug with a larger gap will fire at a higher voltage. This shows up in an oscilloscope trace as one pulse being larger than the rest and possibly triggering a (false) RPM reading.

After several years of flying with my IO540, I began to get false RPM indications. Turns out that I had a burn-through on a plug wire due to the proximity of the wires to an exhaust pipe. Saw it on my portable oscilloscope in flight. Replaced and rerouted wires, everything OK.

Also of note, this is more common on six-cylinder engines (more cylinders).

Cheers, Vern
Thanks Vern excellent advice. I found the gap on my #4 mag-driven plug to be pretty wide, especially as compared to all the others. I regapped it and cleaned them all, and the latest data after a ground run looks nice and clean again. Having the ability to view this data sure makes troubleshooting easier.

Chris
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