Question for Paul McBride, the General Aviation News engines expert: During flight, the oil temperature goes full scale up and down — yellow, redline, then a normal green reading.
I checked the connection at the sender and it was tight. There has been no action on the wiring to the sender. The cannon plugs appear good.
The engine, which has 2,300 hours, is on a 2018 Piper Archer.
Alan Wilson
Paul’s Answer: Alan, it sounds like we need to use the old Biblical method here, which is “seek and ye shall find.”
This kind of problem is sometimes a challenge to find, but let’s give it a shot.
My first suggestion in troubleshooting this situation would be to completely eliminate the aircraft wiring. Ideally, if you can get a known good instrument and using a different wire coming from the sender and bring it into the cockpit, this may be one of the easiest ways to confirm whether it’s the sender or a wiring or gauge problem.
If you don’t experience the issue on the ground, then you can still bring the test wiring and gauge into the cockpit by securing the wire coming from the engine compartment to the cockpit with some 100 mile per hour tape on the side of the aircraft.
Learning the results of this test will dictate the next step.
Assuming the condition remains the same as before, then we might suspect the sender unit itself. Replacing the sender with a known good, or a new one, and repeating the test still using the external wire and gauge should provide us with additional information.
If the problem still exists, then I’m going to suggest that you check the actual temperature of the engine oil in the sump by using a laboratory glass bulb thermometer.
Fly the aircraft for a period of time to get the engine oil temperature at what you normally would see prior to the problem you’re having now.
Once back on the ground, remove the oil dipstick and very carefully slide the thermometer down the oil dipstick tube until you feel it hit bottom. Then just pull it back about a half inch and hold it for approximately 30 seconds. Very carefully remove the thermometer and check the reading. Needless to say, if you aren’t careful, you’ll learn how to remove an engine oil sump so you can clean out the mercury.
I’d be surprised if you found the oil is as actually hot as the current malfunctioning gauge is telling you with its yo-yo readings.
Hopefully, you’ll find during the first test with the remote wire and gauge that the problem is with the wiring.
Many years ago, while I was at Lycoming Engines and assigned to Piper Aircraft in Lock Haven, Pennsylvania, they had a new Aztec that during the first test flight, every time the nose was pointed down, the oil temperature would go to the top of the redline. Bringing the aircraft back to straight and level flight would also bring the engine oil temperature back to the green area on the gauge.
After several hours of troubleshooting, by both me and the Piper technicians, the cause of the problem was discovered to be where the oil temperature gauge wire came through the firewall. Somehow, the wire had chaffed and every time the aircraft nose was pointed down, the wire would ground out, causing the cockpit gauge to go crazy.
This proved that sometimes it’s the simple things that cause you the most trouble.
If the oil temperature was actually at the red line, the oil pressure would be near zip. I doubt that’s the case!
I recently purchased a new sender from Aircraft Spruce. It caused the gauge to read Less than 30C. Problem was the flange at the end of the sparkplug shorting to the wires where they are soldered to the sender plug washer. Shink tubing that should have gone all the way to the plug washer to provide insulation was nearly 1/4″ higher up on the wires.