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IO-360 Idle RPM Roughness on Descent

mfleming

mfleming

Well Known Member
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Today while shooting a practice approach, I felt a consistent engine miss when descending and the throttle pulled back. I quickly looked at the engine monitor and #1 and #2 EGTs had fallen way off compared to the other two cylinders. Adding some power cleared it up, pulling the power started the missing. The next flight I did a really thorough run up and all looked good. The flight home didn't see the problem .

I might have pushed the mixture in much higher than normal!! My safety pilot said he heard some popping but it didn't catch my attention

Here's the engine data graph.

IO-360-M1B
1 mag, 1 P-Mag

Any thoughts?
 
@1:08:58?

1. Cam on #1/2 exhaust going bad? - (No, that's kind of a permanent thing)
2. Sticking Valve(s) ... again?
3. The ignition issue that you diagnosed as a sticking valve last year came back?
4. Debris in the fuel system?

If it happens again, turn off one ignition and see where that leaves you.
 
bjdecker said:
@1:08:58?

1. Cam on #1/2 exhaust going bad? - (No, that's kind of a permanent thing)
2. Sticking Valve(s) ... again?
3. The ignition issue that you diagnosed as a sticking valve last year came back?
4. Debris in the fuel system?

If it happens again, turn off one ignition and see where that leaves you.
Click to expand...
One more thing -- where is the RPM coming from? P-Mag, Transducer in the Mag, other?
 
mfleming said:
RPM is coming from the P-Mag

  • Unlikely 2 valves sticking at the same time...
  • Just got the P-Mag back from Hartzell overhaul.
  • Debris...maybe
Click to expand...
Constant Speed Propellor IIRC -- Did you pull the blue knob during the event? The recorded RPM dropped 400.
 
mfleming said:
Today while shooting a practice approach, I felt a consistent engine miss when descending and the throttle pulled back. I quickly looked at the engine monitor and #1 and #2 EGTs had fallen way off compared to the other two cylinders. Adding some power cleared it up, pulling the power started the missing. The next flight I did a really thorough run up and all looked good. The flight home didn't see the problem .

I might have pushed the mixture in much higher than normal!! My safety pilot said he heard some popping but it didn't catch my attention

Here's the engine data graph.

IO-360-M1B
1 mag, 1 P-Mag

Any thoughts?
Click to expand...
More info needed

What power settings were you at? What phase of descent and what do you normally use for settings?

Was your approach stabilized or were you doing something to fix it when this happened?

Adjustable prop?

When did you last look at your spark plugs? How do you normally fly/cruise related to fuel burn and power settings? ROP/LOP?
 
Look at the slight rise in EGTs 1,2,4 at 1:08:47 coincident with an un-commanded RPM drop of ~80; and again at 1:08:50.

* Rise in EGT is caused by P-Mag ignition failing -- advanced spark from P-Mag isn't there -- Fire starts "late" @25° from the mag firing
** Fall in EGT is caused by debris.

That's my vote -- Unless someone decides the "Vapor Lock" is happening (at ~2ATM of fuel pressure with an OAT of 55°F)
 
bjdecker said:
Constant Speed Propellor IIRC -- Did you pull the blue knob during the event? The recorded RPM dropped 400.
Click to expand...
Yes, constant speed prop. Yes, i pulled power so the rpm would be at 2000 for following the GP. So that was me adjusting the power. Noticed the roughness and egt drop and added some more power. Did this a couple times at least
 
RNB said:
More info needed

What power settings were you at? What phase of descent and what do you normally use for settings?

Was your approach stabilized or were you doing something to fix it when this happened?

Adjustable prop?

When did you last look at your spark plugs? How do you normally fly/cruise related to fuel burn and power settings? ROP/LOP?
Click to expand...
Just came out of annual a few hours ago. Stabilized approach, normally run lean of peak but i just flew a missed and was starting a new approach to a different airport and i had leaned just by fuel flow prior to the descent. Then I did my pre landing checklist early and shoved in the mixture while I was pretty high
 
mfleming said:
RPM is coming from the P-Mag

  • Unlikely 2 valves sticking at the same time...
  • Just got the P-Mag back from Hartzell overhaul.
  • Debris...maybe
Click to expand...
Debris floating around somewhere is a decent guess. Not too many things will cause 2 cylinders to drop off in concert. One, sure; four, sure. But two in synch is pretty irregular for an IO. If it was debris, you can be sure it will return. Ignition pretty unlikely with that graph imo,
 
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