since 1940

Hesitation or stumble. Fuel smell in cabin (pressurized aircraft)

 1. Broken intake valve spring. You can look right at a broken valve spring and not see the break. Take a rounded piece of metal and move the spring. The spring will separate if it is broken. A broken spring will cause intermittent power losses as the spring moves around. Fuel smell in the cockpit results when the engine backfires into the intake system. A broken valve spring will often cause the engine to run rough in the morning when the oil is cold. The engine will run fine after the oil warms up. This is because a weak valve spring can depress a lifter filled with warm light oil but a weak valve spring cannot depress a lifter filled with cold viscous oil. The lifter stays pumped up which holds the valve slightly open.

heat damaged valve spring has collapsed. 

 2. Intermittent collapse of hydraulic lifter.

 3. Stuck Valve
A stuck valve usually 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". 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 aircraft engines suffer more frequent exhaust valve sticking than Continental aircraft engines. This is not due to the quality of the pilots flying Lycoming engines versus those using Continental 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 valves that are sodium cooled have valve stems operate at higher temperatures than Continental valves stems. Continental aircraft engines use solid exhaust valves whereas most Lycoming engines use sodium cooled exhaust valves. 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 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 Beech Duke aircraft 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 aircraft engine design is more resistant to valve sticking. We see more of a tendency for the intake valve to stick on Continental engines in the Continental O200, O300 series. 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.

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

Camshaft follower (tappet) normal wear on left
C. The valve opens but won't close.
D. The rocker support breaks.
E. (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 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. 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 Service Instruction 1425 and described here can also be used on Continental engines.

 4. Lycoming O-540 L3C5D in-flight engine hesitation with negative "G" force. Install Lycoming carburetor modification kit per Lycoming Service Instruction 1398A.

 5. Continental O-200A using one-piece carburetor venturi. Install improved main fuel nozzle per Precision Airmotive Service Bulletin MSA-7 dated 10/11/94. In some installations the one piece venturi can cause engine roughness, richness, or hesitation.

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