Q: I am acquainted with two IO-540 engines with 1,000-plus hours that have bent rods in the #5 cylinder and stuck valves in the #2 cylinder. One aircraft was manufactured in 2007 and the other in 2009. The oil has always been changed at 50 hours using Exxon Elite 20/50. Any particular thoughts?
RICHARD WEICHMAN, via email
A: There are many factors that may lead to bent pushrods as a result of sticking valves, so let’s look at a few of those.
First, I commend you for your oil change frequency. I assume this also includes an oil filter change as well?
It’s important to know that frequent oil and filter changes are one of the best defenses in preventing potential valve sticking, but there are others just as important, if not more so.
When I look at the information you provided, I probably have more questions than answers, but let’s proceed and see what we can come up with as a possible solution to your problem. There is no doubt the engine oils used could possibly be a factor, but my feelings are, if you are using a good aviation grade oil, such as the one you mentioned, I think we can remove that from part of the equation.
My gut feeling is that we have other issues and my first thought is with the age of the aircraft involved. My concern is the condition of the engine baffles and their proper location.
If we fail to maintain the engine baffles, they tend to get weak and do not always remain in the proper location to direct the cooling air in the correct direction for proper cylinder head cooling. If the cooling air is being lost, this may cause the temperature of the valve guides to increase. This condition, in turn, may result in coking of the oil between the exhaust valve stem and the exhaust valve guide.
Typically, if an exhaust valve is going to stick, it will occur at first engine start-up when all engine temperatures are cold. What happens during flight when the engine is operating at normal temperatures is that the oil passing between the exhaust valve stem and the guide is in a liquid form and operation is normal. However, when the engine is shut down, as the oil cools, it hardens between the valve stem and the valve guide and forms the coking material.
The sticking normally occurs when the engine is started up the next time because the clearance between the valve stem and the valve guide was reduced when the oil coked. It actually has a grip on the valve stem, so until we get some heat in the guide to loosen up that coked oil, turning it into a liquid state again, we incur our potential sticking problem.
This sticking valve may result in the engine experiencing an intermittent hesitation or miss or the engine shaking more than usual during a normal start-up. If the shaking is noticed and appears to be unlike a normal start-up, I’d suggest shutting the engine down and taking a close look under the cowl for any bent pushrods.
I’d highly recommend you review a copy of Lycoming Service Instruction 1425A, dated Jan. 19, 1988, which will explain in detail the “Suggested Maintenance Procedures to Reduce the Possibility of Valve Sticking.” Lycoming has also provided other publications covering the subject, and I’d recommend you or your maintenance facility review these and comply with any of them that may be required or recommended for your engines.
However, since you didn’t state whether these engines were factory new, rebuilt, or overhauled engines, I’d suggest you review Lycoming Service Instruction 1485A, dated July 2, 2003, which addresses the valve guide material and a recommended “Time Of Compliance.” If these engines were overhauled in the field, the facility that performed the overhaul should be able to tell you which exhaust valve guide were installed at that time.
One more important thing to mention is the envelope in which the aircraft operates. This may also be a contributing factor to the potential for sticking valves, so this is another thing you may want to review. Avoid rapid descents with reduced power, slow flight which usually means reduced cooling, and take time for a proper cool down following the flight.
Hopefully after reading my thoughts on the subject you’ll be able to resolve your valve sticking situation.