What would cause my temperature probe to fail?

[FPK1]

G1000 NXi system, Lycoming IO-390 engine, airplane is a Tecnam P2010.

Cold engine start was normal, upon looking at the OIL temperature it started bouncing around the temperature range and after a few minutes later the dreaded red X.

Upon reviewing video from my flight the day prior, when I did the ground check, immediately upon returning key to both the OIL temperature gauge failed. I didn't notice it at the time so when I started the engine the next day it was not working properly.

I replaced the probe and everything has been fine; and at the end of a flight last Saturday I did the ground check and watched the OIL temperature gauge, all seemed normal. However, today, when I started the engine, shortly after engine start the OIL temperature gauge bounced all around... I did a run up and it never stabilized, and I never got the red X. But clearly something is wrong again. This new probe only has 7 hours on it.

The probe is the right Garmin probe 494-70004-00 and is ungrounded. Mechanic stated grounding check should have nothing to do with it. Perhaps there's some sort of other issue that an avionics expert should look at.

I let the plane sit a little while and running it off batteries, the OIL temperature probe seems normal now. I will start it again tomorrow and see what happens.

By the way, all CHTs are normal.

Any thoughts?

 

[Ryan Klems]

I am not familiar with the G1000 setup in a Tecnam, but am very familiar with the setup in Cessnas. If it is similar, it uses a simple resistive probe wired to the GEA71. There is a single wire from the GEA71 that goes to the probe, which grounds through the engine, however there is a second wire from the GEA71 that grounds to the engine for the ground reference. We recently had a CAP 182T with unusual oil temp readings that was eventually tracked down to that ground reference wire. On the 182T it's grounded at the left magneto to the same location the P-lead shield is grounded. The wire had mostly fractured in the crimp at the ring terminal and was making poor contact. I would suggest you get the wiring diagrams for your installation and see if it's wired similarly. Also check the ground between the airframe and the engine block itself. Poor connection there could also cause that reference to float.

edit: You mention "engine temperature" so I'm not entirely sure my interpretation of "oil temperature" is the correct one ... if it's some other temperature (EGT? CHT?) there are other possibilities

 

[FPK1]

Thanks to the help from @Ryan Klems mechanic discovered a bad connection where the new probe connected to the harness. Mechanic fixed it. All good!

 

[Dan Thomas]

Thanks to the help from @Ryan Klems mechanic discovered a bad connection where the new probe connected to the harness. Mechanic fixed it. All good!

So the old probe was probably good? All it needed was a five-minute fix to the connection?

Troubleshooting skills are rare. Parts manufacturers like that. They sell a lot of expensive parts that way.

In a former career I was the shop foreman (and the first employee of) an air brake component remanufacturing shop. We took in cores (used components that were removed by mechanics at truck shops, or by truck owners, or by heavy earthmoving equipment folks) and completely rebuilt them and tested them. Every one. I once took a whole raft of those cores and tested them, and found that at least 80% of them worked fine. I also tested units that came back for warranty claims, and almost all of those worked just fine, too.

Troubleshooting skill rarities are not limited to aircraft mechanics. They're everywhere.