The Cessna 120-140 Association Forum

I just had an annual and my A&P is concerned with my oil temp gauge. This gauge’s first mark is 100 F and since the temp outside has be hovering around 50F I have found the needle doesn’t move off that mark. I have a c90 engine and I swear in the summer I have seen the needle move to 120F. When I first bought the airplane in WI in May 2019 when it was cold, flying it around I noticed the gauge needle never moved and so I was concerned. I asked the seller to check it out and he did and found it was working. So I assumed that the oil temperature during cold weather months ever got above 100F. Does this make sense that ambient temps 60 and below would keep the engine oil temp 100 or below? And if not then is the gauge or the sensor? How does a A&P test it?

They do fail and temps not coming up is a sure sign. ...best way to check is to immerse the probe in "has-just-come-to-a-boil" boiling water -- gauge should show close to 212F. ...a bit inconvenient to do on the ramp, of course, but a bit simpler in a hangar*. Since it's not a super big expense and relatively simple to replace gauge assembly, it may be time to do so(?). Mac

*"When performing the test be aware that the probe should be submerged in the boiling water, but not laying at the bottom of the container. The electric hot plate beneath the cooking pan, or a burner flame beneath the pan, will likely make the metal at the bottom of the cooking pan hotter than the boiling point of water. If the probe was resting on the bottom of the pan it may sense the hotter temperature of the metal pan, and then the test would not be accurate."

You can also get a good idea by pointing an IR temp gun at the housing just after a flight.
But I bet lunch that your temp gauge is bad.

An old electric coffee pot or hot cup of some kind will also work. Just make sure you know the temp of the water in the container you're using and compare it with the temp on your oil temp gage using the method described in this string. These gages don't last forever. I changed mine about 10 years ago and it's starting to fail. They're not expensive and easy to change.

Thank you. I’m gonna replace the gauge.

Suggest you test the new gauge before installing it, while it's still easy to do on the kitchen stove. I bought a new gauge earlier this year, and it only read 162 deg in boiling water, which is not just bad but bad in the worst direction. Sent it back to A/C Spruce, and the replacement read 206 (boiling point at my elevation is about 210). Figured I can live with that, especially knowing the error is there.

Yeah, don't throw parts at the problem without verifying they're bad first. It's simple to get a tin can with water and put the probe in it and boil it with a propane torch.

Read the Neal Wright article about mixing and matching parts if you do end up replacing the gauge.