The International Cessna 170 Association

My new from Spruce oil temp gauge reads zero until some point when it starts working. Can be as little as 30 minutes and as much as more than an hour. Yesterday I flew for an hour in the morning and it started working halfway through the flight. I flew later in the day and it worked from startup. Today I flew for about 45 minutes and it never came alive.
It was changed during the pre buy because the old one didn’t work. I have the old one and tested it by putting the probe in hot water and it’s dead. I assume that’s a valid test.

Oil pressure is mid green and when the gauge comes alive the temp was 175 F yesterday. I don’t understand the principle of operation of this type of instrument. Is it a Borden tube?

Anybody seen this before?

Thanks
Rich G

Yes Rich, it’s a Bourdon tube. There’s a volatile substance in the bulb whose pressure rises with temperature. The gauge is just a pressure gauge calibrated in temperature. It’s just a wild guess on my part, but it sounds like maybe there’s kink in the copper line, which isolates the pressure in the bulb from the gauge below a certain pressure/temperature, then leaks past above a certain pressure/temperature.

My oil temp gauge does the same thing. Exactly as you described. I don't worry too much about it because I have an oil temperature probe on my engine monitor, and once it starts working the two gauges agree. I've always assumed the problem is some kind of mechanical snag in the mechanism because it's old. I don't know why a new gauge would do it, though.

The bulb is filled with ether, which boils at about 100F. So the gauge does nothing until the ether boils and pressurizes the tube, then it works according to the ideal gas law, with pressure increasing with temperature.

It’s a very fragile mechanism and may be dirty. They’re relatively cheap... buy a new one.

(Confession: I’ve take mine apart and, using a magnifying glass, found the rough-edge of the faceplate that caused it to “hang” ... and fixed it with tweezers. REinstalled and it’s worked properly ever since. When I get my instrument repair station certificate I’ll sign it off.).

Thanks for the responses. I got similar advice from my friends over on SuoerCub.org.
I will be changing it out shortly.

Rich

Be careful if you swap out an older oil temp gauge with a newer one. There were some changes a few years ago in the adapter-to-capillary bulb flange-shape. If you replace the capillary type gauge be certain to keep it mated to it’s new adapter-nut which should have the same flange shape. Putting a new capillary bulb into an older adapter-nut can lead to a leak and loss of oil.

Thanks. There is a new one in there now and it’s the one that has the problem. No leaks at all. I have the original gauge. I have a call into Keystone instrument in Lockhaven to see if they will or can repair it. I have had them do J3 Cub oil temp gauges in the past.
If that doesn’t work out and I have to buy another I will take your advice regarding the fit. Thanks for the information.

Rich

I had the same problem with a NOS oil pressure gauge. When my original oil pressure gauge quit, I found a NOS oil pressure gauge on Ebay. It was still packaged/sealed in its original brown paper wrapper with no damage to the paper. When unwrapped, it looked brand new. I tested it with my engine compression tester and the needle would hang then pop free as pressure was increased. Inspecting the gauge allowed me to see the mechanism was slightly out of alignment which caused the needle to stick. Gently re-adjusting the mechanism with tiny needle-nose pliers freed up the needle and it has worked perfectly since. The calibration was checked and adjusted using the differential pressure tester and it is accurate at all pressures. The whole process took about an hour.

Thanks for that information. I have sent my original to Keystone in Lock Haven for repair. When that comes back (if it’s repairable) I’ll do the swap and get the original back in. The one that’s in there appears to work about half the time. Either zero, or correct. Must be the actual mechanical parts in the gauge sticking but since it’s a capillary type, I’m not diving in.

Rich