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fuel injector service


tcm / bendix fuel injection nozzle maintenance


cleaning intervals

some shops clean the nozzles every 100 hours or at annual inspection.  others wait until a problem manifests itself.  from experience we have found that sometimes even being extra careful will dislodge or introduce foreign material that will restrict or clog a nozzle during the cleaning process.  if the aircraft is regularly serviced with clean fuel and is not operated in a dusty environment, we have found with normally aspirated engines the cleaning interval to be about 250 operating hours or every five years.  fuel stains around injectors of normally aspirated engines indicate that it is time to clean injectors earlier than normal.  turbocharged engine injector nozzles usually need to be cleaned at intervals of 500 hours or every five years; the injectors of turbocharged injectors last longer due to the use of filtered air instead of dirty atmospheric air.    

we have cleaned turbo charged engine nozzles at 500 hours intervals for the last 25 years and found it a satisfactory interval for our area�s environment.  the exception to this interval is when something happens to introduce contaminates in the turbo discharge air, such as a foreign object that damaged the impeller of the turbocharger.  any type of damage to the turbo compressor blades will probably cause foreign particles to work their way into the deck pressure lines to nozzles and restrict / clog the nozzle from the air bleed.  likewise leakage of non-filtered air from the engine air filter or air inlet system will cause the same problem.   another common problem is poor fitting alternate air valves, which is usually attributed to worn hinge, weak spring, or mis-rigged magnetic latch.  if alternate air is manually operated contamination could be a control-rigging problem or the pilot is leaving the alternate air valve open during ground operations.  if the engine air inlet system gets contaminated it can be an extensive project to get the system cleaned out.  deck line filters would be great if faa approved.  an egt probe installed on each cylinder can easily alert you to these kinds of problems if routinely monitored. 


if you have a fuel injected lycoming engine you need to be aware of ad93-05-02 and associated service bulletin 342b.   by all means obtain the diagram for bracket / clamp placement pertaining to you particular engine.  if not in full compliance then order the necessary parts and have them installed to preclude injection line failure.  support clips, line cushions, and support brackets are also necessary on a tcm engines. 


some shop insights

see that your service facility removes support clamps / clips as necessary as not to deform fuel injection lines beyond their elastic limits.  make sure they use correct / tight fitting tools as not to damage line nuts. nozzle removal and installation tools should be carefully chosen to prevent damage to the nozzle or air shroud.  six point drives are preferable but often will not clear air shroud.  some sockets end up being too short and damage line mating area.  most sockets need some lathe machine work before they are suitable.  some engines require considerable socket wall thinning and sometimes shorting.

it is a good idea to keep track of nozzle position as the previous technician may have flow checked the nozzles and installed the lower flowing ones in the cylinders that receive less air and the higher flowing ones in cylinders that receive the most air (correcting cylinder temperature-fuel relationships).   if your tcm engine has stced gami nozzles make sure their recommended cylinder assignments are followed.  a perforated parts basket with marked compartments for each cylinder nozzle is ideal.  note:  later lycoming systems use a metering insert in the top of the nozzle, it is small and very easy to loose!  make sure your technician consults the engine service manual for recommended cleaning solutions, some technicians use automotive carb cleaner, but they should only leave them in as long as necessary.  sonic cleaners are ideal but not all shops are equipped with them.   after soaking neutralize the cleaning solution by flushing with appropriate clean liquid and blow dry with clean compressed air.  make sure that no hard objects such as wire are inserted to metering bores as this could score orifice and result in increase nozzle flow or faulty spray pattern. see that each nozzle / metering orifice is held up to light and visually check for obstructions. 

the nozzles should be check for flow and spray pattern after cleaning.

one may use the engine�s fuel system for this procedure.  each nozzle is installed on the end of its fuel injection line and inserted into equal sized clear containers.  tall slender ones will give more accurate results.  the technician then operates the fuel system full rich full throttle using the fuel boost / aux. pump.   each nozzle is observed for proper spray pattern.  bottles are emptied to ensure even starting point and the bottles are filled almost full.  bottles are then set on a level surface and fuel levels observed.   all should flow equal except for tcm engines equipped with gami nozzles.  sometimes lycoming nozzles can be swapped around to get a more even fuel flow.  occasionally it doesn�t do any good because of a faulty flow divider valve.  on aircraft has a single egt probe mounted in the exhaust riser install the leanest nozzle on the cylinder equipped with the egt probe.

over-torquing of nozzles with pipe threads can actually change orifice size due to the taper of the threads.  if nozzles are straight threaded new mounting  gaskets should be installed.  certain lycoming injector nozzles require proper orientation of air bleed holes; of course, the final torque position and the proper bleed hole orientation rarely coincide.  it is very important that injector lines are properly installed and torqued to prevent in flight fuel leaks.  check and see that necessary support hardware is reinstalled.  determine if a fuel leak check has been completed.      

likewise installation of nozzle deck lines on a turbo charged installation requires more detail than meets the eye.   installing shroud seals over threaded and boss section of nozzles can shave off little slivers of seal material that will float around in the deck system and give you lots of unnecessary trouble shooting experience.  use of the proper little �bullets� will preclude this problem.  heavy wall heat shrink tubing will work if your technician does not have access to the tools.  make sure deck system is completely clean before installing and new seals are used!   

if you operate a tcm injected engine, consider checking the fuel injection system pressures at this time in accordance with tcm sid 98-3 and adjusting as required.  of course it requires special equipment!

submitted 8/25/00 by roger a. stern  a&p / ia  


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