Ask Paul: Why is my oil temp so hot?
Q: I don’t know if it was the hot summer or I just don’t remember my oil temperature getting this hot, but when flying the other day I noticed the oil temperature on my 1980 Piper Archer O-360 A4M (800 hrs SFOH) was running around 210°. The OAT was 33°C.
I removed the Vernatherm and tested it in hot oil with an accurate thermometer and it performs to the spec. I inspected the seat and it appears to have a very nice continuous ring around the seat, displaying good contact. I replaced the oil cooler with a new one, since the old one was the original. The baffling is in great shape, as is the engine.
I also submerged the temperature sensor in boiling water with a thermometer to verify the temperature, and the gauge is right on the money. After replacing the oil cooler, I now see the oil temp in the 200-215° range on a 33°C OAT day when operating at 75% power. During descents and in the pattern, the temp drops to 180°, which is the center of the gauge.
I have owned this plane for 20 years, and I sure don’t remember the oil temperature getting this high. Any suggestions?
BILL PEYTON
A: Hey folks, we’re back to the engine oil temperature subject again and Bill has provided us with some good details to work with. Bill mentions the OAT of 33°C, which equates to roughly 91+°F, so I think we can all agree that’s a rather hot summer day. With an OAT like this I wouldn’t be surprised to see the kind of oil temperatures you are seeing Bill, and certainly would not be concerned. We have to remember that the maximum oil temperature on any Lycoming engine is 245°F for continuous operation.
The troubleshooting you’ve done and the results you mentioned really provide me with some good information and lessen any concern that you may have a problem. I commend you for replacing your oil cooler since over many hours of operation and calendar time, they tend to be less efficient and certainly could cause a rise in oil temperature.
This brings something else to mind and that is the internal condition of oil coolers overall. If you talk with oil cooler experts like Pacific Oil Cooler Service, which has been in business for more than 50 years, they can tell you some real horror stories about some of the oil coolers they’ve worked with, which points out that we should all pay closer attention to our coolers.
Getting back to the question at hand: With the information you provided, I’d say you’re right in the ballpark and have nothing to be concerned about. Ideally, I’d like to see oil temperatures during normal cruise operation between 180° and 215°F and probably a little warmer on climb out. If you have confidence in the accuracy of your oil temperature gauge, you are not in any danger of harming your engine with the readings you’ve recorded. It’s only natural for the oil temperature to decrease on descent too, so I see nothing unusual there.
You didn’t mention anything about having a CHT gauge or what the CHT readings might be. I realize you stated the engine baffling was in good condition, but if the CHT readings were a bit higher, this may cause the oil temperature to run higher too.
Bill, from everything you’ve told me, I’d say you have nothing to be concerned about. My recommendation would be for you to continue to fly and enjoy this aircraft you’ve owned for 20 years.
Paul McBride, an expert on engines, retired after almost 40 years with Lycoming. Send your questions to: AskPaul@GeneralAviationNews.com.
