One EGV indicates 500 degrees or more below the others

[dmloftus]

Thanks Rich. Will have it checked out. The interesting thing is it happened intermittently. I would think if I had a failing valve, it would oscillate pretty consistently. Below is a log of the full flight, taxi portion removed. The plot I shared above is an expanded version of one of the troublesome periods, around samples 1000, 2000, 2600, and 5200. Other than those times, the #3 EGT remained fairly steady. As an in-flight experiment, I ran it LOP and ROP several times and noted the differences. The last third of the flight the EGT's were solid again. I'm wondering if I had a temporary deposit on the valve that burned off. I'll be monitoring very closely.

[Chris B]

The interesting thing is it happened intermittently. I would think if I had a failing valve, it would oscillate pretty consistently.

IME, intermittent large variations with no corresponding CHT change are consistent with a failing connector. Though I haven't yet had a dead EGT probe, so that might have similar symptoms as it fails. ¯\_(ツ)_/¯

Try thoroughly cleaning the relevant connector with DeoxIT. Use something like thick fishing line to clean the female end of the connector and a fine brush to clean the male end and see if that temporarily improves the EGT readings. So far that has never permanently fixed the problem, but is a helpful diagnostic.

My understanding of what Mike Busch describes is *much* more rhythmic, driven by exhaust valve rotation.

[dmloftus]

Try thoroughly cleaning the relevant connector with DeoxIT. Use something like thick fishing line to clean the female end of the connector and a fine brush to clean the male end and see if that temporarily improves the EGT readings. So far that has never permanently fixed the problem, but is a helpful diagnostic.

I ran my plane to a shop across the field for a quick borescope. We pulled the top plugs for all four cylinders and each exhaust valve looks perfect, especially #3 where I am getting the intermittent weird EGT oscillations. I was relieved not to see a hotspot. We also removed the EGT sensor from the riser and cleaned off the deposits on the actual thermocouple. Had not seen this post before we pulled and reinstalled it.

BTW, as an unexpected bonus, my starter died as I tried to fire up the plane for the taxi ride home. So my one hour of labor for the borescope turned into a $1400 day ;-) With the new starter, I did a test flight the next day and noticed similar intermittent and erratic 30-50 degree swings. There is no engine roughness, runs very smooth. Which again points to an instrumentation issue.

Chris - when you mention cleaning the connector, are you saying I should remove the yellow wire where it meets the sensor? How do I release that wire? Do I need to remove the hose clamp or do I just pull on the spring at the end of the wire? It didn't seem to be removable, and most thermocouples I have seen have wiring that is permanently attached.

Any other possible causes I should explore, like a partially blocked injector? Seems like the engine would rough rough if that was the cause.

[Rich]

Try thoroughly cleaning the relevant connector with DeoxIT. Use something like thick fishing line to clean the female end of the connector and a fine brush to clean the male end and see if that temporarily improves the EGT readings. So far that has never permanently fixed the problem, but is a helpful diagnostic.

I ran my plane to a shop across the field for a quick borescope. We pulled the top plugs for all four cylinders and each exhaust valve looks perfect, especially #3 where I am getting the intermittent weird EGT oscillations. I was relieved not to see a hotspot. We also removed the EGT sensor from the riser and cleaned off the deposits on the actual thermocouple. Had not seen this post before we pulled and reinstalled it.

BTW, as an unexpected bonus, my starter died as I tried to fire up the plane for the taxi ride home. So my one hour of labor for the borescope turned into a $1400 day ;-) With the new starter, I did a test flight the next day and noticed similar intermittent and erratic 30-50 degree swings. There is no engine roughness, runs very smooth. Which again points to an instrumentation issue.

Chris - when you mention cleaning the connector, are you saying I should remove the yellow wire where it meets the sensor? How do I release that wire? Do I need to remove the hose clamp or do I just pull on the spring at the end of the wire? It didn't seem to be removable, and most thermocouples I have seen have wiring that is permanently attached.

Any other possible causes I should explore, like a partially blocked injector? Seems like the engine would rough rough if that was the cause.

Chris is probably referring to the connector at the firewall end of the lead. These are quick-disconnect types for all the CHT and EGT probes all wadded together at the rear of the engine compartment.

[Chris B]

Chris - when you mention cleaning the connector, are you saying I should remove the yellow wire where it meets the sensor? How do I release that wire? Do I need to remove the hose clamp or do I just pull on the spring at the end of the wire? It didn't seem to be removable, and most thermocouples I have seen have wiring that is permanently attached.

Sorry. I should have been more explicit.

The connector is not at the EGT probe, but in the wiring harness(es) leading from the firewall to under the cylinder heads. The connector is a black automotive-style weather-resistant 2-pin connector. The pins are rectangular (~2mm wide and ~1/2 mm thick).

Any other possible causes I should explore, like a partially blocked injector? Seems like the engine would rough rough if that was the cause.

An injector should result in rising or lowering CHT. Your symptoms really do sound like a flaky EGT connector or EGT probe itself, so I would rule those out before chasing more exotic (and expensive) possibilities.