High temps after overhaul

Q: I have a Lycoming O-320 engine that was overhauled by an IA who has overhauled lots of engines and, ever since, the oil temperature runs very high.

The oil gauge was checked for accuracy, and the oil cooler was sent out, cleaned and checked. I replaced the varitherm and the induction tubes have new couplings and clamps with no leaks.

The aircraft is on floats, so 91 octane auto fuel is used. It runs very well.

Someone has suggested that the rings maybe too tight?

Anything else we should check or just pull the top off and start by checking the ring gaps?

B.J. Riester
Auburn, New York

A: B.J., my first thought after reading your email was that you’re barking up the wrong tree.

If you had mentioned the cylinder head temperatures were running high, then I might have believed the end gap on the rings was a possible source of the trouble.

I can also say that if we were seeing a lean condition from a leaking intake coupling this would also show up with higher cylinder head temperatures rather than higher oil temperatures. This has happened in the past where the cylinders have been chromed and have a choke at the top of the barrel like the nitrided cylinders.

Since Lycoming provided straight chrome barrels only during the years of offering chrome cylinders, it never provided plain steel rings for choked barrels.

If the person who overhauled the engines just happened to install Lycoming piston rings without checking the end gap during assembly, there would be a problem. The Lycoming piston rings were for straight chrome barrels only and would fit a choked chrome barrel, but there would be insufficient end gap clearance, which would result in high cylinder head temperatures during engine operation.

I’d be curious to learn if when you replaced the oil cooler by-pass valve you complied with Lycoming Service Instruction 1316A or its latest revision?

If you just replaced the oil cooler by-pass valve without refacing the corresponding seat, then the chances of having a complete 360° seating surface for the new oil cooler by-pass valve may be the issue. It would not be unusual for these mismatched surfaces to allow some of the hot engine oil to not be forced through the engine oil cooler. The result of this would be a higher than normal oil temperature.

With regard to the type of fuel being used, I don’t believe that would have any bearing on the high oil temperature.

The only other thing that comes to mind that you might check is the air flow path to the engine oil cooler. If it is exactly like it was prior to the engine being removed for overhaul, then I guess we can count that out.

I would mention that the maximum oil temperature for your engine is 245° F continuous. On a freshly overhauled engine, you may see higher operating temperatures when compared to the engine temperatures prior to overhaul.

Since you didn’t provide any oil or cylinder head temperatures, I’m just assuming they may be exceeding the maximum allowable temperatures. I’ve tried to provide my best guess as to where you should focus your troubleshooting area.