Improved Fuel Injector Nozzles

Reports from operators of fuel-injected engines and from Lycoming service representatives provide some insights into the subject of clogged fuel-injector nozzles. Providing our readers with information from these sources may help some of them recognize and treat a similar problem in their own aircraft.

A letter from the owner of a twin-engine aircraft with Lycoming IO- 540 engines indicated that after 900 hours of operation, the engines had performed flawlessly except for minor problems involving the fuel-injection system. This particular aircraft was equipped with an exhaust gas temperature (EGT) probe at each cylinder, so the cylinder causing a problem could be pinpointed by using the EGT analyzer. Here is a description of how this problem was observed by this owner.

From time to time, there would be erratic combustion on one cylinder which would either raise the exhaust gas temperature as shown on the analyzer (an indication of a lean mixture on that one cylinder), or in some rare cases, the individual cylinder would become inoperative. Thorough cleaning of the nozzle and line had little effect, but simply replacing the nozzle and line caused the cylinder to operate normally and brought exhaust temperatures back into line; EGT was again responsive to mixture control.

The basic problem boils down to almost microscopic pieces of brass, rubber or other forms of dirt that get into the fuel nozzle. These bits and pieces are extremely hard to dislodge, and they may severely restrict fuel flow to the individual cylinder. Cleaning the line and nozzle does not always remove the dirt and correct the problem, although on the surface it would appear that it should.

When fuel flow is only partially blocked by dirt in the injector nozzle, the exhaust gas temperature will rise and not respond to mixture control until idle cut-off is reached. The reason for this is that the blockage now becomes the primary restriction and is independent of mixture-control position.

In those aircraft that do not have an EGT probe on each cylinder, erratic combustion or engine surging, which may be an indication of clogged or dirty fuel-injector nozzles or dirty fuel, may possibly be checked by noting the fuel-flow gauge. Those fuel flow gauges that actually measure pressure will have an indicator calibrated to show gallons or pounds of flow per hour. With this type of instrument, a clogged nozzle will cause a pressure increase and, therefore, an unusually high fuel-flow indication. Direct flow meters do not react in this manner.

To pinpoint the individual nozzle or nozzles that are clogged, it will be necessary for a mechanic to flow-check all lines into containers of equal size. The stream from each nozzle should be smooth and steady, with no fluctuation. The amount of fuel from each nozzle should be equal when the containers are viewed after the flow-check is complete. A clogged nozzle or nozzles may be identified by a smaller amount of fuel in its container after the flow-check period.

As indicated earlier, fuel-injector nozzles have traditionally been difficult to clean. Under no circumstances should the fuel-injector nozzle be probed with a sharp instrument. The proper method of cleaning described in Lycoming Service Instruction No.1275C includes washing the nozzle thoroughly with acetone and blowing it out with compressed air. Also, as indicated earlier, there have been times when cleaning did not return the fuel flow to normal, and the only recourse was replacement of the nozzle and line to achieve satisfactory engine operation.

The difficult job of cleaning fuel-injector nozzles has been made easier with the introduction of the “two-piece, air-bleed injector nozzle.” These nozzles are installed in production engines and are available as replacements for the nozzles that were used previously. They are physically and functionally interchangeable with the corresponding old-style nozzle.

The minor fuel flow problems cited by the aircraft owner in the first few paragraphs of this article were experienced with old-style fuelinjector nozzles. The new two-piece injector nozzles introduced by Lycoming Service Instruction No. 1414B have an advantage; they can be taken apart for easier cleaning. This feature should make troubleshooting and repair of dirt-related fuel-flow restrictions much easier. We should emphasize that Cleanliness is Extremely Important when installing, cleaning or working with fuel-injector nozzles as they can very easily be contaminated with small amounts of dirt. See Lycoming Service Instruction No. 1414B for two-piece nozzle installation instructions, and Lycoming Service Instruction No. 1275C for cleaning and test information.