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Fluctuating EGT on one cylinder

Lycoming IO-390 with AFP fuel injection.

Yesterday I noticed the #3 EGT started to fluctuate quickly while the others remained stable. The fluctuation range is RPM dependent: at around 1000 rpm it may fluctuate about +/- 100 degrees. At 2600 rpm it can fluctuate +/- 600 degrees but more typically 300 degrees. These fluctuations happen within a second or two and are constantly rising and falling. The engine remains smooth with no changes in RPM and no misses detected.

I have the same problem on the same cylinder on my IO-320 with Silver Hawk FI. I can trace the behavior back to a specific point in time on a flight back in August. I am certain that the readings are incorrect and that I've got a probe or wiring issue. I re-seated the probe before my last flight but the problem persists. Please make sure to update us when you have the issue resolved.
 
I fixed my issue (#3 CHT) by securing the connectors a little better. They were tied to my engine mount on either side of the connection but had a little wiggle room at the connections themselves. I wrapped the connectors to the mount with a bit of friction tape and a zip tie. Readings are stable now.
 
Reading this post I have a couple of EGT questions...

First is more of a "Gee Wizz" and that is how much of a EGT spread is reasonable at WOT on takeoff between the hottest cylinder EGT and the coolest cylinder EGT?

On my engine on takeoff four of the cylinders are running together in a "herd" and they are all within 10 F EGT of each other, then cylinder #1 consistently runs about 40 F cooler than the herd (not a big deal) but then cylinder #6 runs 100 F cooler than the herd so it is definitely an outlier.

So is it normal to have one cylinder run much cooler than the others?

When I do the GAMI LOP cylinder #6 will peak with the rest of the herd so I cannot add more fuel otherwise my LOP spread would increase. I thought that since the engine was recently rebuilt about 25 hours ago perhaps the piston rings are still seating so its compression might be lower than the rest however I did an automotive style compression check and cylinder #6 was right there at the same 125 PSI as the others.

The next question is...What Causes A Single Cylinder EGT To Spike???

If you look at the attached PDF print out of my latest takeoff you will see that cylinder #2 EGT line rises in formation with the other cylinders and then it and cylinder #6 will level off at about 100 F lower EGT than the herd... and then all of a sudden...while still at a constant MAP the cylinder #2 EGT line will spike up and join the herd and then its EGT line will just run in formation with the herd for the rest of the flight. So why the spike?

It has done this for a number of flights and I had thought it was an ignition problem. Before this last flight I have replaced both spark plugs and checked the resistance in the spark plug wires and the fuel nozzle restrictor was inspected and is clean so I do not understand what would cause the spike.

I can see that at low power settings a certain cylinder might run cooler than the rest and then when the throttle and fuel flow is increased that that one cylinder might run with the warmer cylinders but I do not understand what will cause a cylinder EGT line to spike at a constant MAP.

On the last flight I did the GAMI LOP test and cylinder #2 and #4 have peaked first so I increased the fuel nozzle restriction on both of those cylinders one step bigger.

Waiting for parts...again...and then I will report back after the next flight.
 

Attachments

  • EGT.pdf
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If you can tell when the spike is occurring in flight, do a mag check. I have had a similar problem and found that while the mag (ignition) check passes on the ground, it may fail miserably in flight. The engine should just run a bit weaker in either mag position, but still smooth. If it stumbles on one mag, the mag (or electronic ignition) or a plug wire may be misbehaving. An ignition check in flight is a good thing to do periodically anyway, even though it feels a bit intimidating.
 
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