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Lycoming fuel injection

RSA fuel injection

Bendix fuel injection

These fuel injection systems all have different names, but are nearly the same. For this article, these fuel injection systems will be referred to as RSA fuel injection. The RSA fuel injection system is commonly found on Lycoming engines and various other powerplants. RSA fuel injection systems are mass-air flow type fuel injection systems. In other words, the throttle body servo senses the amount of air moving past the throttle by use of a venturi. A diaphragm in the servo regulates the amount of fuel supplied to the flow divider (manifold, fuel spider) by comparing the air pressure from the venturi. Next, the flow divider divides the steady stream of fuel into smaller streams of fuel, one for each cylinder. The stainless steel injector lines carry fuel from the divider into the injectors located in the intake ports. A metering orifice is machined into each injector to meter the fuel into an equal flow to all cylinders. Early model injectors were single piece, and the new injectors are composed of two pieces for easy cleaning. Each cylinder has a threaded port for installation of an injector; usually the injector is installed in the intake port in most installations. Fuel for the injector system is supplied by a diaphragm fuel pump driven by the engine. The fuel injection system has no provision for fuel vapor return. A RSA fuel injection system can be operated by a boost pump continuously with no apparent change in operation with a failed engine driven fuel pump.

Engine and Airframe problems

A. Most problems are airframe fuel system faults due to suction leaks and/or restrictions up stream of the engine driven fuel pump.

1. Restricted fuel tank vents

2. Deteriorated fuel tank / cell coupling hoses

3. Restricted or seeping fuel selector valves

4. Fuel strainer / filter seeps, sometimes suction only

5. Deteriorated or restricted fuel supply hoses

6. Leaky boost / aux pump drive shaft seal, often have suction leaks running or not

7. Loose screws in fuel pump housings securing the diaphragm

8. Leaking fuel pump line fitting seals

9. Restricted finger / inlet screen in metering unit

B. Some problems are engine induction system faults. Clues are erratic idle, faster than normal idle, idle stop screw backed all the way out.

1. Leaking manifold drain (sniffle) valve

2. A leak at an induction tube elbow ("O" ring)

3. A coupling tube adapter that has worked loose in the case sump (common)

3. Crushed intake gaskets at cylinder heads

4. Leaking standby vacuum system (induction type)

C. Others problems are trouble that is often misdiagnosed.

1. Dragging / tight intake valves (usually be remedied without removing cylinders).

2. Dry valve lash clearances that are out of specification.

Engine fuel injection system problems that occur with the RSA fuel injecton system.

A. The diaphragms in the fuel servo harden and/or crack from age. Typical signs of a hardened diaphragm are:

1. Erratic or rough engine operation particularly when changing power settings

2. The engine backfiring at low power settings during taxi-in and other operations

3. Fuel seeping / leaking out of fuel metering unit

B. The mixture arm to throttle arm linkage wears due to a harmonic engine vibration.

1. The engine operates erratic at varying EGT indications.

C. Hot starts that are more difficult than normal.

1. Lack of vapor return in fuel injection system

2. Leaking center body seal in the fuel servo unit

3. Hardened fuel servo unit diaphragms

4. Improper hot start techniques:

The system is easy to over-prime and flood the engine. The operator needs to establish the amount of fuel needed to prime the engine for various ambient and engine temperatures. Priming the engine varies considerably among different airplanes even if they are of the same make and model.


The fuel system is easy to maintain and generally trouble free.

The throttle body is equiped with a fuel screen that is serviced during the aircraft's major inspections.

Few fuel system components fail between engine overhaul.

The fuel system is easy to adjust. Only the idle speed and idle mixture are adjustable. Some models contain an additional automatic mixture control for flight at high altitudes.

The system has very few fuel hoses.

The fuel injection systems are still being manufactured new, and overhaul components are available.


Black diaphragm seals should to be replaced with the red / orange type diaphragms.

A sniffle valve is usually installed in the air manifold with this type of fuel injection system. Failure to install a sniffle valve for a drain may result in a difficult to start and/or flooded engine.

A partially clogged injector can cause a lean fuel/air mixture in the cylinder.

Fuel stains around injectors on normally aspirated engines often indicate it is time to clean the injectors.

A leaking intake system on the engine can cause a roughness in idle appearing as a fuel injection problem.

Engine driven fuel pump diaphragms and throttle body diaphragms should be removed from service after 20 years.

Dealing with the hot start

The hot start is made more difficult by a lack of a vapor return in the fuel system.

1. The best solution is to install a lightweight starter capable of cranking the engine nearly twice as fast as the original Presto-lite or Delco starters.

2. Open the oil filler door (if located over top of the engine) after the engine shut down to allow hot air to escape from the cowl. Less heat will be stored in the engine and there will be less vapor to pass through the fuel injection system.

3. Install an impulse magneto in place of the original direct drive right magneto. Make sure the jumper link is removed from the RH magneto start ground terminal on back of the ignition switch.

4. If your engine is equipped with a single unit "dual magneto", make sure that ignition switch does not ground the right magneto in the start position, if it is remove the jumper link on the back of ignition switch.

5. Another option is to install a "Slick Start" booster.

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Last modified: February 24, 2007