Induction leak? Valve issues? Sensor going?

[moonshine]

Just spent some 18.7 hobbs flying around, and I think the O-360-A4M started developing an issue.

This is the normal EGT spread for my carburated engine

After the climb and level-off on this flight, the #3 EGT was reading lower than normal and somewhat jittery - however notice that the reading is cooler than normal. It's my understanding that a burned exhaust valve will give you higher than normal jittery readings

Things appear normal initially with the spread and "jitterness" increasing and the flight continues

Interestingly enough though, the engine behaves as normal or close to it. Develops full power at take-off (judging by the static RPM - fixed pitch) and cruise is fine too - I get the normal rpm/fuel burn (maybe 0.5gph more FF to keep things smooth, but that's very atmospheric conditions dependent) and speed is also where it normally is. So no appreciable power loss. This is the log from the last flight (all logs in chronological order with one or two flights skipped)

Compression "feels fine" turning the prop. No appreciable power loss. No weird vibrations and such.
How do the EGT probes "die"? What's the normal failure more outside of showing absolute zero?
Could this still be a valve? Induction leak? Definitely having it looked at, but your thought are welcome

 

[Skylane81E]

How does it idle?

 

[Capt. Geoffrey Thorpe]

Compression "feels fine" turning the prop. No appreciable power loss. No weird vibrations and such.

Compression is easy to check if you have / borrow the tool.

How do the EGT probes "die"? What's the normal failure more outside of showing absolute zero?

EGT probes are thermocouples. They operate due to the fact that when you heat a junction made of two dissimilar metals a voltage is generated. They don't generate a lot of voltage, and they particularly don't generate very much current. So, if you have some poor connections in the circuit between the probe and the meter (or in the probe itself) you will likely see a lower than expected reading.

 

[mondtster]

Thermocouples can start reading a bit erratic like that when there is a poor connection and vibration present. I'd start by checking all the connections but I wouldn't be surprised to find a bad thermocouple. You might try moving the probes around and checking to see if the problem stays with the cylinder or follows the thermocouple too.

One thing to note is that thermocouples do not always read low when they are failing. Some data acquisition systems will go to full scale when you have an open thermocouple. This likely isn't the case with engine monitors installed in airplanes but it is something to be aware of.

 

[Henning]

It's impossible to tell from just the graphs as several things can do that, however it isn't reminiscent of the graphs I have seen of valve problems.

Where in Florida are you? Fly down to FXE and I'll meet up with you and give it a listen.

 

[comanchepilot]

Sometimes more data is not useful . . . . especially when the data is bad.

Switch the EGT probe to the other cylinder on that side and see if you get the same readings - if the SAME cylinder reads jittery and colder [meaning now the #2 cylinder since the probe is the #2 probe hooked to the #4 cylinder] then its not instrument. If the #2 cylnder now shows the problem you have an instrument issue. . .

 

[flyingmoose]

Yup that is what I would try also if there is no other issues.

 

[Skylane81E]

Heck, I'd just start by unplugging then plugging the probes leads