C

ctennis

Well Known Member
On a flight a few days ago, immediately after takeoff I noticed my #5 CHT being a bit erratic. It would jump up and down 20 or so degrees. There was no apparent other effect - the EGT wasn't jumping around, there was no discernable sound change, and the engine seemed to be making power just fine. My initial thought was: bad sensor.

Then, it started spiking. It climbed way up. I leveled off with intent to return to the airport, and pulled the power back to enough to stay level. The temp kept climbing. Again, no noticeable difference in the corresponding EGT. No roughness, no other indication there was a problem.

The temp came back down. Then it went low. Then back up. Again, no apparent issue. I really thought it was just a bad sensor. I brought the power back in again, and it eventually smoothed out, and was fine the remainder of the ride.

Here's a graph of what the data looked like. The bottom 3 lines are 3 of the CHTS, the top 3 are the corresponding EGTs.

Screenshot 2023-07-26 at 9.39.18 PM.png

We made it to our destination. When we got ready to depart the destination, I observed the same issue again. This time it didn't "spike", but was certainly erratic again, just for the initial portion of the takeoff. Then it was fine.

Screenshot 2023-07-26 at 9.48.16 PM.png

I went back and looked at the previous flight from a few days before the spike issue. Same issue there (I hadn't noticed at the time). Here's a closeup of just 3 CHTs to better see the erraticism.

Screenshot 2023-07-26 at 9.51.05 PM.png

Okay, so it's time for an oil change anyway, so cowling off. I don't see any apparent issue with the CHT sensor, but I suppose it could be faulty. I went ahead and got out the boroscope to check the cylinder, pics attached.

My only main concern is that the spark plugs for this cylinder at darker than the other cylinders. So perhaps some kind of overheating event occurred.

Anything else I should be looking for, or considering here? Maybe there's just a vibration issue at full power that's causing the erraticism, but

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I think if your CHT actually went to 700 degrees F, then there would have been visible damage to the piston. It's most likely a loose connection or possibly a faulty CHT sensor. Check any of the connections on that cylinder's CHT wiring first. If nothing is evident, look at swapping the probe to another cylinder to see if the problem follows the probe.

As a side note, you may want to blow a hair dryer on that cylinder head by the probe to make sure that #5 is in fact #5 on the display. It wouldn't be the first plane to have the wires mixed up, and you want to make sure you're chasing the right thing.
 
Thanks for the thoughts, that's kind of my thoughts too.

No evidence of any metal in the oil filter. I'll do a quick compression check later today on it since the spark plug is out anyway. But I concur, it seems not real.

Still trying to wrap my head around the browning of the spark plugs. I'll put new ones back in before firing the engine up again.



D-Dubya said:
I think if your CHT actually went to 700 degrees F, then there would have been visible damage to the piston. It's most likely a loose connection or possibly a faulty CHT sensor. Check any of the connections on that cylinder's CHT wiring first. If nothing is evident, look at swapping the probe to another cylinder to see if the problem follows the probe.

As a side note, you may want to blow a hair dryer on that cylinder head by the probe to make sure that #5 is in fact #5 on the display. It wouldn't be the first plane to have the wires mixed up, and you want to make sure you're chasing the right thing.
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Connectors

The most common problem with the EGT/CHT indicting issues are the connectors. Replace them first and then swap out the sensor if needed next.
 
For something to help you feel better about it, I've had similar crazy high (500f+ while taxiing) and erratic CHT readings which resolved after reseating the sensor connections.
 
Same here. We had a connector issue and had to disable the aural warning in flight because it went off every few seconds. Temperature changes that fast are impossible, it must be a loose contact of the the connector or the probe itself…
 
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