EGT probe failure mode

[alcladrv]

The EGT on one cylinder is slow to come alive at startup and the reading fluctuates before settling down about 30 minutes into a flight and is generally in line with the other 3 cylinders. I removed all of the probes and wire brushed any corrosion off of both the probe and exhaust pipe. The symptoms follow the probe when swapped to a different cylinder. Are these readings indicative of a failing EGT probe?
Thanks,
Mike

 

[BoydBirchler]

I've had probes that failed doing exactly that. When the reading changes at a faster and erratic rate than it's neighbors, it is likely a probe failure, rather than an engine event. A probe takes time to heat or cool. I've had several failures where the EGT rose and or fell 500 or 600 degrees almost instantaneously, that defies the laws of physics, a probe in reality cannot heat or cool that fast.

 

[Carlos151]

The EGT on one cylinder is slow to come alive at startup and the reading fluctuates before settling down about 30 minutes into a flight and is generally in line with the other 3 cylinders. I removed all of the probes and wire brushed any corrosion off of both the probe and exhaust pipe. The symptoms follow the probe when swapped to a different cylinder. Are these readings indicative of a failing EGT probe?
Thanks,
Mike

Had the same issue you're having with a single Dynon EGT probe about two years ago. Replaced the probe with a new one and all is well. The other three probes are still going strong after 575 hours. Figures since I bought four new ones thinking the others were soon to follow in failure. Inspected the old probe and there really was no discernible difference from a new probe that I could see with the naked eye. Cut the probe just out of curiosity and it showed a nice cross section of the bimetallic construction but nothing obviously wrong. New probe, cheap fix IMHO.

 

[plehrke]

Timely thread as on my last flight I had my EGT probe on one cylinder rise quickly 30-50 degrees above its normal steady leaned valve and then quickly fall back down in line. It did that several times over a 30 minute flight. I swapped it with one of the other probes but not gone flying yet due to weather. No difference in appearance between the two probes.
My probes have had a hard life as I have them just 2” from the exhaust flange. My peak EGTs are in the 1500-1550 F range. JPI guy at OSH said that was OK but may not get much life out of them. that was 17 years and 1100 hours ago. I guess I was due for a probe failure.

 

[Malndi]

The OP probably has a failed EGT probe, but it’s worth mentioning here that an erratic EGT indication can also be caused by the connector. I’ve been lucky enough to have ’repaired’ several erratic EGT probes by simply pulling the connector and pushing it back together, and they’re still working fine several hundred hrs later. There was a particularly temperamental one that could only be fixed by replacing the spade connector with a (yellow) thermocouple connector, and I know others have resolved erratic EGT indications by changing to EI overlap connectors. But touch wood I’ve not yet had to use the spare EGT probes sitting on the shelf.

 

[Bill Boyd]

The OP probably has a failed EGT probe, but it’s worth mentioning here that an erratic EGT indication can also be caused by the connector. I’ve been lucky enough to have ’repaired’ several erratic EGT probes by simply pulling the connector and pushing it back together, and they’re still working fine several hundred hrs later. There was a particularly temperamental one that could only be fixed by replacing the spade connector with a (yellow) thermocouple connector, and I know others have resolved erratic EGT indications by changing to EI overlap connectors. But touch wood I’ve not yet had to use the spare EGT probes sitting on the shelf.

Interesting that it's called an "overlap connector." I've used this type of connector when experiencing trouble with FastOn spade connectors on probes, and lacking any instructions that I could find, have used them as butt connectors, sizing the exposed wire length and the wire insertion so as not to overlap. They have a conductive (brass?) sleeve inside the phenolic barrel.

Now that I know I have been improperly installing them, I will change the installation technique on my Red Cube wires next time I de-cowl. This could explain why it is behaving very intermittently (I doubt it, but it's definitely worth a try).

I would want to make sure that stripping 3/8" of insulation will allow the bare wires to be underneath both set screws at once, as I recall them being almost that far apart on the barrel - not much room for error since you cannot see the bare wire inside the connector to be sure.

 

[RV8JD]

EI OLC-1 and -2 connector instructions:

 

[Bill Boyd]

Appreciate you posting this, Carl. I've always used the two-set-screw types and as I note again reviewing the instructions, there is no provision for wire overlap in the instructions for the OLC-2. In the thermocouple use case, the OLC-2 will introduce two dissimilar metal connections in each splice if the wires are not overlapped, but since they are of opposite polarity and should be at almost exactly the same operating temperature in most applications, there will be no error introduced, same as for crimping Fast-On's on thermocouple leads (or so Bob N. says). Still, I do wonder if not overlapping the wires inside the connector could be the cause of a (mis-attributed) early death of several Red Cubes in my -10 in the past few years.

I'm going to try solder connections on the latest dying Cube to eliminate any possible connection integrity issues and see if it resurrects. Then I might try same with the previous, presumably dead Cube which I removed but kept on a shelf.