Direct injection of fuel into cylinders offers better fuel distribution and easy cold starting, without the threat of carburetor icing. This article will show you the parts of a typical Lycoming fuel injection system and the most common trouble spots to check if your engine starts running rough.

Fuel-injected engines have been common in automobiles for years, and are gaining popularity in General Aviation aircraft. 

Fuel injection systems have several advantages over carbureted systems. With fuel injection, each cylinder gets an almost identical amount of fuel. This helps each cylinder put out an equal amount of power. This in turn makes the engine run smoother and more efficiently. 

In contrast, carbureted systems are prone to have cylinders which run slightly rich or lean in comparison to the rest because of the differing lengths of the intake pipes.

Fuel-injected engines are much easier to start when the engine is cold because each cylinder gets primed with an identical amount of fuel. 

Fuel injection systems also are free from the threat of carburetor icing. 

Fuel injection systems do have a couple of disadvantages when compared to carbureted systems. Fuel-injected engines can be difficult to start when hot. After shutdown in the hot summer months, they typically require a “flooded” start with the mixture full lean, and throttle full forward as the engine is cranked. This process can be frustrating for folks unfamiliar with the quirks of fuel-injected engines. 

The fuel injection system is also very intolerant of even the slightest piece of dirt or debris in the lines or injectors. 

Carbureted systems are generally easy to start when the engine is hot. They also, by design, tolerate impurities a little better than fuel injection systems do. 

Aircraft owners who fly behind fuel-injected engines will likely enjoy many years of reliable and efficient operation. Wise owners should still want to know what’s under the cowl, in order to make troubleshooting problems with their injection system quick and easy. 




Major parts of a fuel injection system

The main parts of a typical fuel injection system are an engine-driven fuel pump, a fuel/air control unit (fuel servo), a fuel distributor (flow divider) with its associated fuel lines and the fuel nozzles themselves. Most airplanes also have an electric fuel boost pump, which provides fuel pressure for starting and as an emergency backup. 

The engine-driven fuel pump is designed to provide constant fuel pressure to the inlet of the fuel servo.



Fuel servo

The fuel servo is a fuel injection system’s fuel- and air-metering unit.

The airflow to the intake pipes of the engine cylinders is controlled through the throttle body and butterfly valve in the servo. The pilot’s throttle movements directly control the amount of air entering the engine. This butterfly valve is similar to the butterfly valve in a carburetor. The throttle body is made with a venturi inside; again similar to those in a carburetor. 

However, the venturi in a fuel servo is only there to provide air pressure settings to an inside chamber in the fuel control section of the servo, not to provide nozzle suction for fuel discharge as it does in a carburetor. 

Fuel flow is controlled by the fuel servo’s ball valve, located in the fuel regulator portion of the servo. The ball valve is regulated by a series of diaphragms and springs. The diaphragms are used to allow the opposing pressures of incoming (impact) versus venturi air and metered versus unmetered fuel pressure to constantly regulate the amount of fuel sent out to the nozzles. 

As shown in photo H (right), the front housing of the fuel servo’s automatic mixture control (AMC) provides the opening for impact air pressure. The shape of the housing creates the venturi for the throttle body. 

Impact air pressure is ducted through impact tubes from an opening in the front of the throttle body (ahead of the venturi) to an enclosed chamber on one side of a diaphragm. Air from the low-pressure venturi section of the throttle body is ducted to a chamber on the opposite side of the diaphragm. 

As airflow through the throttle body is increased or decreased by the pilot’s throttle control, the air pressure in the venturi itself increases or decreases inversely. As airflow increases, venturi pressure drops. As airflow decreases, venturi pressure rises. The pressure difference between the impact air (which stays constant except for atmospheric changes) and the venturi air causes the diaphragm between the two chambers to move slightly whenever there is a change in air pressure on one side or the other. This difference in pressure between impact air pressure and venturi pressure in a fuel servo is known as “air metering force.”

The fuel servo ball valve in the fuel regulator is attached to the diaphragm in such a way that it moves toward a more open or closed position as the diaphragm moves in response to air metering force. Note that the venturi air pressure is the main controlling factor for the amount the servo valve is open at any given time. 



Flow of fuel

Fuel flows from the engine-driven fuel pump through a metering jet in the fuel servo. The metering jet opening is controlled by the pilot’s manual mixture control. This fuel is considered “metered” fuel pressure. It is piped to a chamber in the fuel regulator inside the fuel servo. A separate line of unmetered fuel pressure is piped off before the fuel reaches the metering jet, and sent to another chamber in the fuel regulator. This unmetered fuel pressure chamber is separated from the metered fuel pressure chamber by a diaphragm. 

As changing venturi pressure causes movement in the servo valve, it also causes movement between the metered and unmetered fuel chambers. because the servo valve works in conjunction with both diaphragms. 

A reduction in venturi pressure (increased throttle and butterfly valve opening) causes a slight movement of the servo valve toward a more open position until the metered fuel pressure is increased to the point that the servo valve stops continuing to open and stays set at its new, more open position. Increased venturi pressure (decreased throttle and butterfly valve opening) results in a movement of the servo valve toward a more closed position until the decreased metered fuel pressure causes the valve to stop moving and it stays set at a slightly more closed position. 

This process governs the amount of fuel that is sent to the nozzles throughout all throttle settings.


Automatic mixture control

The AMC helps keep the fuel-air mixture ratio constant by adjusting the pressure differential between impact air pressure and venturi air pressure. It provides a variable orifice between impact air pressure and venturi air pressure—thus modifying the same “air metering force” referenced above. The AMC doesn’t replace the pilot’s manual mixture control; it works in conjunction with it.


Flow divider

From the fuel regulator section of the fuel servo, fuel is routed to the flow divider. The flow divider, which some mechanics call a “spider” because of its shape, is mounted on top of the engine. It provides a central point for fuel distribution to each fuel line and nozzle. The flow divider has a spring-loaded diaphragm which opens with fuel pressure from the fuel servo and closes when fuel flow ceases. This setup provides a positive cutoff of all cylinders simultaneously at shutdown. (See photos 01 and 02, page 28.)


Fuel lines and nozzles

The fuel lines connecting the flow divider to the nozzles are hard lines made of stainless steel.

The last unit in the flow of fuel to each cylinder is the fuel nozzle itself. The fuel nozzles are made of brass and are very simple in their construction. The nozzle is essentially a hollow small tube with a calibrated opening on the outlet and a couple of restrictions that reduce the diameter of the tube internally. Each nozzle is calibrated to provide maximum fuel flow necessary at full throttle settings on the discharge end. The nozzles have a receptacle for the fuel line on the opposite end. There are no internal moving parts in the nozzles themselves.

Some nozzles are the two-piece type, and have a removable center section. These pieces should be kept together as a set any time the nozzles are removed. 

The nozzle is also where the fuel is mixed with air to atomize the fuel to make it combustible. Normally-aspirated engines have air bleed screens on the outside of the nozzle, while turbocharged planes have a sealed connection that vents the nozzle air chamber to the turbocharged “top deck pressure” (turbocharger compressor outlet pressure). (See photos 03 and 04 on page 28.)

On both normally-aspirated and turbocharged configurations, the intake manifold pressure is slightly lower than the pressure in the air bleed chamber of the nozzle, so air is continually drawn through the air bleed into the manifold. (See photo 05, page 28.)


Fuel injection system maintenance and troubleshooting

Most of the time, fuel injection systems operate trouble-free. When a problem occurs in the fuel injection system, it is often intermittent and sometimes can be difficult to pinpoint at first. 

Rough-running engines are usually fairly straightforward to diagnose. Usually a defect in the ignition system, such as a fouled spark plug or incorrect magneto timing is to blame, but occasionally trouble in the fuel system is the culprit. If the ignition system has been ruled out, it’s time to examine how the engine is getting fuel.

Most mechanics start at the nozzles and work their way backward until the source of the trouble is found. 


Clogged fuel nozzles

When a problem occurs in a fuel injection system, it usually is caused by small pieces of dirt or debris that partially clog a line or injector. If one or more of the nozzles becomes restricted, fuel pressure will increase because the servo keeps sending out the same amount of fuel.

The fuel flow meter in the cockpit displays fuel flow in gallons per hour; but this number is derived from a fuel pressure reading at the flow divider. An increase in fuel flow may be seen on the gage if one or more nozzles are clogged, even though throttle settings remain unchanged. Higher pressure at the divider caused by a clogged nozzle shows up as higher flow rates on the fuel flow meter. An increased fuel flow indication along with a rough-running engine is an indication that one or more nozzles may be partially or fully plugged. 

The reason for the roughness is simple; the cylinder with a clogged injector is only getting enough fuel to run intermittently. 

This can be verified if the aircraft has EGT probes on each cylinder. On the cylinder(s) with partially clogged nozzles, the exhaust gases will be hotter than other cylinders; evidence that the cylinder is running too lean.

 A simple way to check for restrictions (flow test) each nozzle and line is to remove all the nozzles from the cylinders. The fuel lines should be unclamped as needed to give enough slack so that they aren’t bent or damaged in the process. After removing the nozzles, reconnect each of them to the correct fuel supply line. 

Place each nozzle in a small clear cup or jar that is labeled for the corresponding cylinder. Have someone in the cockpit turn on the master switch and fuel boost pump, with the mixture rich. Slowly advance the throttle from idle to full and back again while someone else observes the output of the nozzles. Each one should have roughly the same flow. 

Next, remove the jars without spilling any of the fuel. Compare the fuel level of the cups. A partially clogged line or nozzle should have a cup with a lower fuel level than the others. (See photos 06, 07 and 08 on page 30.)

Lycoming Service Instruction 1275C gives instructions on nozzle cleaning. The nozzle should be cleaned with acetone or MEK and blown out with compressed air. No picks or sharp tools can be used in the discharge hole or it will be deformed. 

If a particular nozzle or line has a chronic clogging issue and becomes clogged quickly even after cleaning, it may be best to replace both the line and nozzle. Even though a line or nozzle has been cleaned, microscopic particles or debris often remain and become dislodged with subsequent use, clogging the nozzle once again. 

Caution should be used when removing or installing fuel nozzles. The nozzle is screwed into the intake plenum of each cylinder. The plenum is located outside of the cylinder combustion chamber, in the intake manifold preceding the intake valve. 

The end of the nozzle that threads into the cylinder has fine-tapered pipe threads. The intake plenum is aluminum and the receiving threads in it are also aluminum. It is very easy to accidentally cross thread or overtighten a nozzle. The aluminum threads in the cylinder are easily damaged if this happens. (See photo 09, page 30.)

Generally, nozzles should be threaded in finger tight, then torqued to 40 to 60 inch-pounds maximum. If the threads do get badly damaged in the cylinder head it can be an expensive repair; the cylinder may have to be removed. Also, overtightening the union nut on the incoming fuel line can easily strip the relatively soft brass threads on the nozzle, or damage the nozzle inlet. 

Dirty nozzle air bleed screen

A dirty air bleed screen on a nozzle causes a higher than normal fuel flow out of the affected nozzle. The manifold suction that is always constant at the discharge end of the nozzle doesn’t have an air bleed to reduce it slightly. The fuel servo sends out the same amount of fuel, but with one nozzle pulling through more than its fair share, the rest of the nozzles run too lean. 

This can cause a rough idle, lower than normal fuel flow indication and a higher than normal rpm rise as the mixture is cut off. For reference, the normal rpm rise at cut off is usually 25 to 50 rpm. (See photo 10 on page 32.)


Fuel lines and clamps

Fuel lines are prone to cracking if exposed to too much vibration, so they are typically clamped at several points along their length to minimize any shaking or flexing. 

The clamps catch a lot of heat and the rubber cushion in them dries out and shrinks over time, allowing the fuel lines to shake a little inside the loose clamps. Lycoming has an AD that requires repetitive inspections of the clamps and fuel lines for tightness and security, and replacement of defective clamps. (See photo 11, page 32.)

The lines have union nuts with threads that are easily stripped if the nut is over-torqued. They should be finger tight plus approximately 1/6th to 1/12th turn (one-half to one flat) more when using a wrench for tightening. New replacement fuel lines come as straight units that must be bent and formed to match the old line being replaced. 

Fuel servo center seal

A leaky center seal on the main fuel servo causes the whole system to run overly rich; so much so that the engine is hard to cut off with the mixture control. 

To check for a blown center seal that is allowing fuel to get over into the air chambers of the servo, disconnect the fuel hose between the fuel servo and flow divider. It is easiest to reach at the flow divider. Install a plug tightly in the line to seal it. Remove enough of the intake ducting so that the impact tubes can be observed and turn on the boost pump with full rich mixture and full throttle settings. If fuel comes out the impact tubes, the center seal is leaking and the servo will need to be sent out for repair. Blue fuel stains around the impact tubes also indicate a leaking center seal. 

Fuel inlet screen

If blue stains are observed on and around the servo, the cause is a leaking seal and there is no need go further (and pull the fuel inlet screen) because the whole servo will need to be removed for repair.

However, if a fuel servo is operating erratically, but no obvious leakage is observed, the fuel inlet screen is the next place to check. A clogged screen will cause the system to run too lean.

This screen should also be removed and cleaned periodically as part of routine maintenance. The screen should be cleaned with a solvent such as acetone and blown out with compressed air. (See photos 12 and 13 on page 34.)

If the screen is removed to troubleshoot erratic fuel servo operation, it should be tapped open side down on a clean towel before cleaning so any contaminants can be inspected.

Lower intake system manifold drain valve

Finally, if the previous steps have not helped locate the source of trouble, it is worth examining the lower intake system manifold drain. The drain is made of brass and has a one-way check valve to allow excess fuel and oil drain out of the intake manifold without allowing any air to come into the intake manifold. If the check valve malfunctions, it can cause the engine to run erratically.

Pilots and owners who operate a fuel-injected engine may already know the advantages of this type of system, but still need to be able to identify the pieces, what they do and how they fit together. This article should give you a good working understanding of the many parts of a Lycoming fuel injection system.

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting preventive maintenance tasks.

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to editor [AT] piperflyer [DOT] com.


Lycoming Service Instruction No. 1275C