since 1940

Stuck Valve in Lycoming or Continental Aircraft Engine

A stuck valve typically occurs when the engine is first started and is often identified by an intermittent hesitation, or miss, in engine speed. We call this "morning sickness". In flight symptoms reported by pilots include slight roughness to severe roughness and loss of engine power. A sticky valve can be the intake or the exhaust and can stick open or closed so the symptoms can vary greatly. 

Flying the airplane with a stuck valve increases the risk of engine damage. The repair criteria should be to un-stick the valve as quickly as possible. The engine's operating environment and design causes valves to stick. Many factors influence this environment, including: engine temperature, engine design, engine installation, baffle condition, operational technique, oil, fuel, and frequency of oil changes. Even ambient temperatures have a great influence on valve sticking. Valves stick more in the hot summer months than in the cold winter months. Lycoming engines suffer more frequent exhaust valve sticking than Continental aircraft engines. 

Continental exhaust valve, seat, and guide

This is not due to the quality of the pilots flying Lycoming aircraft engines versus those using Continental aircraft engines. The difference in tendency has to do with how design differences between Lycoming engines and Continental engines influence the valve operating environment. High temperatures in the exhaust valve guide oxidizes 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. 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 sodium cooled valve stems operate at higher temperatures than Continental valves stems. Continental engines use solid exhaust valves whereas most Lycoming engines use sodium cooled exhaust valves. 

Lycoming exhaust valve broken at valve head. Inside of valve is hollow up to the point where the black mark ends. The black shows how heat is conducted up the stem. On a solid-stem Continental valve most of the stem is not dark because it is not as hot. Lycoming engines that use sodium cooled valves use a rocker arm that pushes against the valve rotator cap whereas on Lycoming engines using solid stem valves the rocker arm pushes against the valve stem.

The sodium in the Lycoming valve melts at 97.5oC 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 100oF 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 guide bellmouths or gets bigger. This increases the clearance between the guide and the stem and allows combustion products and heat to travel up the valve stem. These combustion products create lead deposits and acids which increase the corrosive environment.

Lycoming aircraft engine valves also stick because of corrosion buildup on the valve stem. Corrosion increases the dimensions of the part, thereby reducing the valve stem-to-guide clearance. The high stem temperatures, combined with a design which allows more combustion products into the guide bore, create a corrosive environment which is seldom seen on Continental engines. Lycoming TIO-541 engines installed in the Beechcraft Duke use an oil-cooled exhaust guide. Cooling oil circulates in a groove between the exhaust guide and the guide boss. If this groove cokes up with oxidized oil and becomes blocked, the exhaust guide and valve overheat and stick. If you have a stuck exhaust guide on this engine, be sure to check the oil passage by blowing compressed air through the oil fitting in the cylinder head. Proper valve guide cooling, valve rotators, proper metering of oil through close valve-to-guide clearances are engineering elements which reduce the tendency for valve sticking.

Environmental influences that create valve sticking are: high temperatures, dirty oil, high lead content fuels, hot engine shut-downs, and poor engine baffling. The repair shops that overhaul or repair your engine cannot change the engineering and have little control over the environment in which your engine operates. From the standpoint of cylinder repairs, all that can be done is to use the correct parts, dimensionally match the parts carefully, and control the surface finish of the guide. The aircraft repair shop can influence the environment by checking engine health, precise timing, careful baffle inspections, and by recommending more frequent oil changes.

Continental engine design is more resistant to valve sticking. We see more of a tendency for the intake valve to stick on Continental aircraft engines in the O200 , O300 series. There are several reasons for valve sticking in the O200, O-300 cylinder.

Continental o-200 (C-85, O-200,O-300, GO-300) cylinder showing exhaust port. Note short cylinder fins are far smaller than other engine models. 

1. These cylinders have small cooling fins on the exhaust port; much less than any other Continental or Lycoming engine. Cooling, keeping the exhaust valve and guide properly cooled becomes more critical. Besides the usual items such as engine baffling in proper working order, the exhaust valve seat should be ground to maximize heat transfer from the valve into the seat.
2. The design of the exhaust port has changed over the years and some of the newer port designs cause the exhaust gas to be pushed up into the guide causing a buildup of carbon deposits on the port side of the guide. This is easily confirmed by examining the guide ID for carbon build-up on the port side. Carbon build-up on the guide pushes the valve stem into the guide. If the guide is the original aluminum bronze style the guide quickly wears out. If the guide is a harder ni-resist style then the guide doesn't wear and the valve sticks. The Ni-resist was used to prevent premature guide wear but it does not get to the root of the problem. The problem is carbon build-up on the port side of the guide. Going back to the original port design as existed on the A50222 cylinder placed the end of the exhaust guide away from the direct flow of exhaust flowing out the port. 
3. Use of Alcor TCP help rid the cylinder and oil of lead deposits and this may help prevent valve sticking.

A stuck intake valve disrupts the breathing of the entire induction system. The power loss results in a forced landing. Do not use Marvel Mystery Oil or other solvents to un-stick a valve. Solvents may un-stick the valve in time but not immediately. Eventually the valve may un-stick, but not before your camshaft lobes have been damaged.

Solvent treatments dissolve the outer deposit layers in the guide boss and temporarily un-stick the valve. The remaining deposits push the valve over to the opposite side of the guide and cause rapid, uneven guide wear. The valve stem may stick or it may cause rapid guide wear where the stem is forced against the guide material opposite of the deposit buildup. Each time the rocker arm tries to open that 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. 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:

A. The push rod bends.

Lycoming bent push rod and housing

B. The surface of the camshaft or cam follower fails.

Lycoming camshaft follower (tappet) showing normal wear on the right and severe damage caused by sticking valve on the right.

C. The valve does open but now will not close.

D. The rocker support breaks.

E. (Lycoming) The valve rotator cap falls off the end of the valve stem.

Lycoming exhaust valve face showing damage caused by flying the airplane with a leaking exhaust valve.

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 de-couple. 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. 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.

Aircraft 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. 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. Repairing 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. You may want to review the operating environment of the engine. Pay particular attention to the oil change intervals, baffle condition, and operating techniques. The procedure outlined in Lycoming aircraft engine Service Instruction 1425 and described here can also be used on Continental aircraft engines.

Video - How to inspect the camshaft follower

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