Loosening Stuck valves
Sticking valves are a relatively common problem on aircraft piston engines. Lycoming Service Bulletin 388 addresses the need to regularly check clearance and provides a procedure to clean carbon accumulations to prevent problems.
Stuck valves will not go away by themselves. Continued operation of the engine will only increase the risk of bent pushrods, damaged camshaft lobes, damaged camshaft followers or damaged rocker arm supports.
Valves stick more in the hot summer months than in the cold winter months. You may want to review the operating environment of the engine. Preventative action includes attention to the oil change intervals; baffle condition, and operating techniques.
A stuck valve is usually noticed when an engine is cool (first flight of the day) and freshly started. This phenomenon is often called "Morning Sickness". There may be an intermittent hesitation, or miss in engine speed. The following discussion will explain the most of the causes and what to watch for.
While at operating temperature, clearances are higher than at room temperature, allowing extra space for deposits to accumulate. Once the engine cools and the clearances shrink the deposits can start to cause a problem. At start up an early warning of trouble will be a hard miss and roughness that clears as the engine warms up, usually in a matter of seconds rather than minutes. This leads many operators to believe the engine is just cold natured, has a fouled plug or getting up in time but the reality is that the engine is giving notice that a major problem is looming. Given that a stuck valve can cause a forced landing and serious engine damage these symptoms should not be ignored. Valve clearances should be checked as soon as possible.
Valves tend to ride one side of the guide rather than having the carbon act like an encompassing bushing. This results in galling by metal to metal contact as deposits force the valve against the opposite side of the guide. This galling is what eventually causes the hard sticking that can occur in flight long after start up.
V alve sticking is almost exclusively limited to the exhaust valves. Most issues with intake valves are usually associated with improper fit or machining during repairs or loose seats usually becoming apparent soon after the cylinder is put into service. Exhaust valve issues seem to be more prevalent on helicopter engines due to their high constant loading with no variance in operating conditions. This allows prime conditions for deposits to crowd the valve that will lead to sticking.
Longer than recommended oil change intervals, high lead content of fuel in engines certified for lower octane fuels and insufficient air filtration can lead to high amounts of suspended solids that can eventually lead to stuck valves from accumulated deposits. Deposits can accumulate within the valve guide during operation as heat evaporates engine oil allowing the suspended solids to remain behind. If these deposits accumulate at a rate that is slower than they can be worn away then they usually do not become an issue. When engine oil is heavy with deposits and high operational temperatures are encountered these deposits can accumulate at a faster rate, slowly robbing the valve of clearance. High cylinder temperatures, especially with unapproved or inferior engine oils can cause oil coke to be the source of deposits as well.
Another common cause of valve sticking is the corrosion that can occur in high humidity areas as the engine sits unused for long periods of time. Corrosion can occur between the valve stem and guide binding the valve in place. Turning the propeller through can expose the problem but cylinder service will be the only way to properly fix it. Nitralloy guides are particularly predisposed to this problem since they are magnetic and will readily rust in a high moisture environment. Engines that still use these guides should not be allowed to sit for long periods without use, especially without some type of climate control such as a closed hangar.
M any contributors factors can lead to the deposits that cause stuck exhaust valves. One very important thing operators can do is change their oil often at the regular intervals specified in the operators manual thus removing suspended solids before they can accumulate in the guides. Engines that use screen filters will benefit from changing to a full flow filter to remove more particulates from the oil. Keeping cylinder temperatures in normal operating range with proper attention to air flow and baffle sealing will help by lowering guide and valve temperatures. Proper air filtration can also help as well by keeping ingested solids to a minimum.
The most common factors for sticking include:
- engine temperature
- engine design
- engine installation
- baffle condition
- operational technique
- oil grade
- frequency of oil changes
L ycoming engine design makes them suffer more frequent exhaust valve sticking than Continental engines. The hollow space in a sodium valve is half filled with sodium, which liquefies at valve temperature and sloshes back and forth in the valve as the valve moves. Sodium increases the heat conductivity from the exposed surface of the valve to the stem and so draws heat from the head.
Sodium chamber High temperatures in the exhaust valve guide oxidize oil and forms carbon deposits on the valve guide and these deposits cause the valve to stick. The most frequent reason for elevated valve temperatures is valve leakage (valve face fit). All of the combustion gas must pass around the valve face as it goes out the exhaust port. The large heat-absorbing surface of the exhaust valve face must conduct heat away from its surface. A valve that is not contacting the seat properly cannot conduct as much heat into the cylinder head as a valve with good seating. Elevated valve stem temperatures may then cause the valve to stick. The pilot will not notice a leaking valve but usually notices a stuck valve.
Lycoming valve stems operate at higher temperatures than Continental valves stems. Continental engines use solid exhaust valves whereas Lycoming engines use sodium cooled exhaust valves. The sodium in the Lycoming valve melts at 97.5°C and conducts heat from the head into the valve stem and then into the cylinder through the valve guide. The Lycoming valve stem normally operates 100°F hotter than the Continental valve stem. The higher temperatures create an environment that is more susceptible to valve sticking. Any engine, if operated at an excessive temperature, creates excessive stem and guide temperatures.
Most of the heat conducted from the head of the Lycoming exhaust valve goes out though the valve stem into the cylinder head fins. The Lycoming guide boss allows 5% of the guide to extend past the end of the boss and protrude into the exhaust port. This extended portion of the guide does not make contact with the guide boss. This reduces the ability of the guide to conduct heat from the stem into the boss. The protruding guide also absorbs heat from the flow of exhaust gas. Because of the high temperatures and combustion deposits on the exhaust valve stem, this area of the baffle inspections, and by recommending more frequent oil changes.
The valve rotator cap on Lycoming engines is prevented from coming off the end of the exhaust valve because the rocker arm face is in the way. If the valve is stuck open, the rocker face moves sufficiently far away for the cap to fall off the valve tip. When this happens not only is valve clearance excessive, but the rocker face pounds into the spring seat. The rotator cap is too big to fall down the push rod tubes. It just lays in the rocker box until you take the rocker box off. It then quietly falls unnoticed onto the hangar floor. If you notice a missing rotator cap, it is likely that the exhaust valve was stuck open in the past. Look in the rocker box or around the hangar floor and you might find it.
C ontinental engine design is more resistant to valve sticking. There are higher occurrences of intake valve sticking on Continental engines in the 0-200, 0-300 series. A stuck intake valve disrupts the breathing of the entire induction system.
Rocker Arms, Cam shaft, Followers
Each time the rocker arm tries to open a stuck valve, you risk catastrophic engine damage. The rocker arm tries to push the valve open. With a stuck valve, the valve doesn't want to move. Tremendous valve train forces develop as the camshaft lobe tries to force the valve open. When operating normally, the highest loaded surfaces in the engine are the camshaft follower and lobe, and additional loading may induce failure. Damage to these surfaces occurs because of increased loading caused by the stuck valve. A damaged camshaft lobe requires complete engine removal and disassembly.
Notice chips missing (Fig 3) from top of cam follower. When the valve fails to close the push rod becomes loose and the socket comes out of the follower. If the socket cocks sideways when the cam lobe comes around for the next valve event, the socket jams against the cam follower.
The camshaft lobe pushes the cam follower up, the cam follower pushes on the push rod, the push rod pushes on the rocker arm and the rocker arm pushes against a valve which will not open. When an engine has a stuck valve, one of five things can happen, each of which is bad:
- The push rod bends.
- The surface of the camshaft or cam follower fails. 3. The valve does open but now will not close.
- The rocker support breaks.
- (Lycoming) The valve rotator cap falls off the end of the valve stem.
What happens if the valve sticks open? You now have a massive leak in the cylinder. If the valve is an intake valve, you lose power and will need to make a forced landing. If the valve is an exhaust valve, there will not be any compression on that cylinder. If valve spring pressure is not sufficient to close the valve, the valve train unloads. As the camshaft follower rotates off the camshaft lobe to close the valve, the valve stem will not push on the rocker arm. The entire valve train, cam follower, push rod and rocker arm decouple. The end of the push rod that rests in the socket in the cam follower comes out of the socket and flings around inside the cam follower. If the push rod ball does not locate itself back into the socket when the cam lobe comes around, it may jam against the tappet housing. Crankcase damage occurs at the outside edge of the crankcase tappet bore.
The bent push rod and housing shown in Fig 4 is an example of the elasticity of the valve train in the presence of a stuck valve.
Sometimes a stuck exhaust valve bends the intake push rod or breaks the intake rocker support boss. How can this happen? If the exhaust valve sticks closed, exhaust gases will not exit from the cylinder. Gas pressure within the cylinder prevents the intake valve from opening. Either the push rod bows or the rocker support breaks. Engine damage does not always occur when the valve sticks, but the longer the engine operates in this condition, the greater the chances are that some damage will occur.
R epairing a stuck valve can be done without removing the cylinder from the engine. The procedure is described in Lycoming Service Instruction 1425 and consists of dropping the valve into the combustion chamber, reaming the guide, and then reinstalling the valve. Another method is to tie dental floss to the end of the exhaust valve and lower it down into the cylinder. Ream the guide and then pull the valve back up into the guide. If it's necessary to remove the cylinder, we recommend you inspect the condition of the camshaft lobes and the cam follower. The procedure outlined in Lycoming Service Instruction 1425 and just described can also be used on Continental engines. Lycoming's labor chart estimates the time for this procedure to be about 2.5 hrs. Of course this assumes the maintenance person is familiar with the procedure and has all tools and parts ready. Realistic times for shops unprepared would be 2 or 3 times this estimate.
The Rope Trick
I f you find you have a stuck valve, this technique may help you make it home. The technique is an old and proven method.
- Remove the rocker cover and spark plug from the cylinder of the offending valve.
- Place your finger over the plug hole and rotate the prop until you feel suction, then rotate until the piston feels like it is at the bottom of the intake stroke.
- Take a piece of rope 1/4-3/8" (preferably cotton) and feed about 2-ft of it through the spark plug hole.
- Gently rotate the prop and hopefully the rope will be position to push on the valve as the piston rises.
- If the rope did not position correctly under the valve, pull the rope out and re-feed it through the plug hole until it gets into the proper position.
Unless the valve is fused to the intake guide or it has bent, this should free the valve. If the valve remains free (lucky you) then try to squirt some M.E.K., mouse oil, or WD-40 solvent around the valve stem. Mouse oil usually works the best.
A common temporary fix is the application of chemicals that supposedly solve problems on the "top end" of engines. Most of these remedies are dependent on an included solvent that dissolves and reduces some of the foreign material that has built up on the guide or stem. Marvel Mystery Oil is one product known to help in some cases. The good news is that they can be quite effective as a temporary cure. They are effective because they dissolve the outer deposit layers in the guide boss and temporarily unstick the valve.
The bad news is they also affect the surface tension of lubricants and are guaranteed to rapidly increase the wear of the camshaft lobes (the highest pressure load in the engine). They usually do not uniformly remove the deposits and so the remaining deposits push the valve to the opposite side of the guide. This causes rapid, uneven guide wear. The dissolving slurry may even contribute to the valve sticking.