Ask Paul: Is my engine burning too much oil?

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Ask Paul: Is my engine burning too much oil?

Q: I have 21 hours Time Since New (TSN) on my RV-14A/IO390 EXP. I have been following the break-in instructions and consistently flying at greater than 65% power (usually closer to 75%…2500 rpm and wide open throttle at or below 5,500 feet). My CHTs and oil temperatures are great: 340-360 CHT and the oil temperature never goes over 195, even in a hard climb. I am using Aeroshell 100 mineral oil.

The issue is I am still burning a quart of oil every four to five hours. Should I be concerned?

Some “experts” are telling me I probably have glazed cylinders and others say just keep on flying it hard and the consumption will drop.  

Brian Barrett 

Van’s RV-14A. (Photo by Van’s Aircraft)

A: Brian, you’ve submitted a question that is quite common regarding engine oil consumption.

First of all, I’d like to tell you that the consumption you mentioned for your engine is very good.

However, the main thing you should be looking for at this point on an engine with only 21 hours on it is when the oil consumption stabilizes. You may still be at a point that the piston rings have not yet completely seated, but I think you’re getting close.

If it were me, I’d forget the advice from the “experts” and continue to fly it the same way that you’ve been. If you really had glazed cylinders at this point, I’d expect the oil consumption to be greater than what you’re seeing.

If you refer to your Lycoming Engine Installation and Operator’s Manual for the IO-390-Series, (P/N IOM-390-C), issued January 2017, you’ll find that a maximum oil consumption quantity is published on Page 53 of Appendix A, Table A-2.

I must admit when I discovered this in the manual, I was a bit disappointed because for some unknown reason, Lycoming decided to provide the maximum oil consumption using a figure of 0.006Lb/BHP/Hr. (Previously, Lycoming would state in the Operator’s Manuals a straight forward maximum consumption on the 200 horsepower versions of the IO-360 series of .80 quarts per hour.)

Now I don’t know about you, but this certainly didn’t do much for me as far as figuring out just what that stood for in quarts per hour. Fortunately, I have a good friend who is a former Lycoming engineer who I contacted to help me out. There is an application of the fence post method that allows you to calculate the consumption rate. He was kind enough to provide the answer rather than making me suffer through doing the calculations. I wonder how he knew that I hated math all through my school years?

The end result provided the maximum allowable oil consumption. According to the calculations, it ends up being about one quart in four to five hours.

As you can see, you are well within the acceptable consumption limits and hopefully by continuing to operate the engine as you have been, this consumption may reduce a little more. Even if the consumption remains as you’ve mentioned, it’s still okay and should not be a cause for concern. Let’s just make certain the rings have completed their seating process by seeing when the oil consumption stabilizes.

After all that being said, I could have just said: You should not be concerned and keep flying it as you have been. The other thing I’d like to mention are the operating parameters you mentioned. They are all fine and what I’d be expecting, so keep on flying.

One more thing: There is so much good information available in the Lycoming Operator’s Manuals if we could only get pilots and maintenance personnel to read them. I realize that the typical answer is “I’ve been flying for 30 years, and there isn’t anything in those publications that I don’t already know,” but that might not be true. So humble yourself, and do me a favor, and read what’s made available just in case something has changed or that you may have forgotten. Let’s put an end to the rumor that “pilots don’t read nor take direction well!”

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