Ask Paul: Why is my oil temperature so high?

Q: During summer time when the weather is hot, when I taxi to the runway, the oil temperature in my Cessna 310 is very high. The gauges are almost at the edge of the green. During cruise, the oil temperature is a little better.

The engines are Continental IO-540s. Oil coolers are on the back of the engines. The baffles are perfect.

Can you please advise about any modifications or upgrades that might be able to solve this problem?

Haim Wilder

A: With regard to your concern over your apparent high oil temperature on your Cessna 310R powered by a pair of Continental IO-540s, I can only guess what the cause may be.

Let me just begin by saying that had you provided more specific information I may have been able to focus on a specific area or item to check first. That being said, I’ll just mention a few areas that I would normally go to if I were troubleshooting this situation. 

Not knowing the age of the aircraft or the time on the engines, I’d begin by asking you what your confidence level is in the accuracy of your instruments, both oil pressure and temperature? Aging aircraft are notorious for having inaccurate gauges.

I would suggest that you have the oil temperature gauge calibrated. If I may use some great information that my good friend Ben Visser and co-contributor at General Aviation News mentioned in previous articles, I think it may be a good starting point for you.

Here is Ben’s recommendation: “Many of the airplanes flying today have gauges that have not been checked in 20 or 30 years. In addition, many of the gauges just have a green band from around 100°F to 240°F.”

“I recommend you remove your oil temperature sensor and place it in a container of oil or water. Place the container on a hot plate with a good referenced thermometer in the liquid to check the temperature. Now heat the container up to 180°. When the temperature in the container stabilizes, check the gauge.”

“I also recommend that you paint a small mark on the face of your gauge so that you can easily see where your oil temp is relative to your 180° mark. 

Why 180˚?

Visser explains that the peak oil temperature in a normally aspirated engine is typically 50˚ higher than the temperature of the oil in the sump, the location of the sensor. He says the typical “green band” on an oil temperature gauge ranges from 120˚ to 245˚. By calibrating the gauge and operating near the 180˚ point in cruise, Visser says the oil temperature at the hottest point in the engine will exceed 212˚, boiling off any water that has accumulated in the oil. By consistently operating below the boiling point, water and acid can build up in the crankcase, leading to rust and corrosion and reduced engine life.

Ben also notes that some gauges can be operationally verified on the cheap by a local maintenance shop.

I also have a concern regarding the analog gauges. If you conduct the calibration test mentioned above, you can eliminate that concern.

Analog gauges are — in most cases, if not all — non-linear. That means that a needles width at the lower end of the gauge may not represent the same temperature at the top of the gauge. Even though the needle may be touching the top of the green arc, it may still be well within the desired oil temperature limits set forth by the engine manufacturer.

Another thought is that if your aircraft is equipped with cowl flaps, that may be an alternative to keeping your oil temperature within limits.

I apologize for being vague, but I’m confident that with some simple troubleshooting by a qualified maintenance facility, you’ll be able to correct the situation.