Home | What’s the story on sticking valves, or stuck valves? Is that what is meant by morning sickness? Is it usually an exhaust valve? What about the wobble test?


What’s the story on sticking valves, or stuck valves? Is that what is meant by morning sickness? Is it usually an exhaust valve? What about the wobble test?

What’s the story on sticking valves, or stuck valves? Is that what is meant by morning sickness? Is it usually an exhaust valve? What about the wobble test?

Technical Editor:

Bob Steward and I had some email discussion on this topic, with the result that I am putting the outcome of the exchange up as a new posting. I have to emphasize that we do not have solid statistical data from a valid sample, to make these truly authoritative statements, so make use of the information as you see fit. Just consider it to be another opinion for your consideration.

1. The recent link to an overview of sticking Lycoming exhaust valves may well contain correct information as far as what contributes to stuck Lycoming valves. There are some aspects that bear elaboration.
The document kept stressing the parallel (“straight”) valve engines for a reason. The angle valve engines, though they typically operate at a higher specific output per cylinder, have more cooling fin area for the exhaust guide. The angle valve engines have not experienced the same level of valve sticking problems as the parallel valve engines. As Bob described it, one measure of likely cooling effectiveness is the ratio of fin area to horsepower output in the cylinder. A related factor is whether the valve guides are close together or spread apart. It is harder to get enough fin area between the straight valve guides. Also note that Bob recently reported his finding of a correlation between excessive fin casting flash in this area (blocking airflow), and valve sticking.
It is very misleading to hold up the Continental engines as an example of good valve train design and long valve life. The IO520 in particular (the most common of their engines) is notorious for cylinders rarely making it to TBO. Most don’t even make it to 1,000 hours without work; in the higher-powered versions, they often don’t make even half of that number. The chief cause tends to be broadly cited as “low compression”; so much so that Continental recently emphasized a new leak-down test regimen that results in better numbers. They even issued a “ruling” that slight exhaust valve leakage is acceptable. I have my own opinion of why they did both things. The Continental IO520 uses 8.5 to 1 compression; the angle- valve IO360s and IO540s are typically 8.7 to 1. Pull the prop through a couple of strokes on the Lyc and Cont engine, and see what a drastic difference there is in static compression, in favor of the Lyc.
The primary factor I have seen for low compression on the Continental is a pitted and burned exhaust valve seating surface, and sticking exhaust valves. It also seems to take very little coking in the Continental guide to make the valve start sticking. Removing a minuscule amount of coke from the stem and guide frees the valve up again; for some reason, there is very little tolerance. If you ask ten Lyc straight-valve owners and ten Continental IO520 owners whether they have had any valve or cylinder trouble in the past 1,000 hours, my money is that you’ll get at least five Continental “yes” answers for every Lyc owner’s “yes”.
In the Lycoming, exhaust valve sticking seems to be primarily the result of wear-related clearances that allow more room for oil to form coke in the hot guide. That’s why the “wobble test” is a good warning indicator. The Lyc sodium-stem exhaust valve helps prevent burning and pitting on the valve seating area, but it does put more heat into the guide. The guide wear accelerates rapidly with a rise in sustained operating temperature. There’s nothing much you can do to reduce the Continental valve troubles; they occur on engines run hard and easy, both lean and rich of peak. I believe that you can reduce the chances of Lycoming valve trouble (keep reading).

2. The recommended limits for operational settings, especially CHT temperature guidelines, are likely too high to prevent exhaust guide coking in the Lyc straight-valve engines, under many circumstances. The engines should not be “babied” as far as power settings, but the temperatures need to be watched. Two things you can do to guard against excessive guide wear are to keep the CHT to 360 degrees or less, and to buy and install an instrument like the JPI 700 (so you can tell whether you are succeeding on all four cylinders). It also greatly helps if you keep the oil clean. I recently put up an extensive post on that topic, so I won’t elaborate on it again here.

3. It will help considerably, if you do whatever it takes to keep the CHT to 360 degrees or less. Examples of options are: keeping the mixture richer in climb (not the same as always keeping it “full rich”); flattening the climb; keeping the oil cooling system in perfect shape (including the Vernatherm valve); keeping the baffle seals in perfect shape, with all leakage holes sealed with high-temp RTV; and if all else fails, reducing power as circumstances permit. Ten knots difference in climb speed can make a ten to twenty degree difference in CHT. Unless you must clear terrain or other high obstacles, or if ATC issues a crossing or time limit to an altitude, what difference does it make if you take a little longer to get to altitude? I wouldn’t worry too much about transient excursions that exceed 360 degrees, such as on the takeoff roll and initial climb. I believe that it takes sustained higher temperatures, as can occur in an extended climb or in cruise, to form coke in the valve guide (and to wear the guide).
If you have the typical oil cooler duct down the side of the cowling, look at the area of the duct box where the air must make the turn into the fins of the oil cooler. Adding some radius fillets there with the blue Super-Fil lightweight aircraft filler, covered with a thin layer of fiberglass and good (something like Vinylester, not boat/car polyester) resin, can get a lot more turbulence-free air through the full face area of the oil cooler. So can making sure all the pebbles and grass blades are removed from the fin area.

4. FWIW, in cruise, peak EGT is seldom the best place to operate on the mixture and temperature curve. The only thing it gives you is peak temperatures; it does not give best economy, and it does not give best power. Best economy comes at 50 degrees or so lean of peak. This also tends to give the lowest operating temperatures and fewer combustion deposits in the oil. Unfortunately, carbureted engines can seldom run smoothly enough to get that lean, and the loss of power (thus speed) can be too noticeable unless you cruise below 5,000 feet or so. If you can’t (or don’t want to) run lean of peak, you should probably stay at least 75 degrees rich of peak; 100 to 125 degrees is better. If you are properly instrumented as recommended, this gets a whole lot easier. Just set it at 100 degrees rich for the cylinder that peaks first, then richen further if needed to keep the CHT at 350 or below on the hottest cylinder, for the selected power setting. If you fly 100 hours per year and save one gallon per hour by leaning to peak EGT, or worse yet, 30-50 rich of peak, how many years’ worth of fuel savings will it take to pay for one premature cylinder failure? However, if you cannot run lean of peak, and must lean for range or an internally cleaner engine, peak EGT is far better than 30-50 rich. You can’t hurt the non-turbocharged engine with too high an EGT, if the CHTs are staying on target. See the FAQs that address leaning operations for more on this subject.

My thanks to Bob S for his input to this, that raised my comfort level in posting these opinions. Go see Bob to get your “wobble test”!

Thank you for adding to the resources available for your Fellow BAC Members.