In the last segment, we reviewed the basic design and operation of the Continental continuous-flow fuel injection system. Now let’s take some time to review proper preventive maintenance of these rarely maintained systems.
Fortunately for those of us in the field, Continental Motors does an outstanding job of making their maintenance manuals, service instructions, bulletins, and other documentation readily available. And, all of this information is available at no cost with the click of a mouse on the company’s website.
The bible of fuel injection system maintenance for Continental engines is SID 97-3F, which covers all aspects of proper inspection and adjustment of these systems. This is a very well-written manual, but exceptionally underused. If you fly behind an IO-550, 520, or other fuel-injected Continental engine, you should have a printed copy of this document readily accessible with the rest of your maintenance documents and logbooks.
Most owners and mechanics take an “if it isn’t broke, don’t fix it” approach to these systems, but that’s a mistake for two reasons: reliability and engine longevity.
Aircraft fuel systems serve dual purposes: providing fuel for the combustion process and helping to cool the engine. If certain parts of the system are not maintained, they can fail during flight. Cleaning the fuel injectors is a good example. Most people think that fuel injector cleaning is focused on the nozzle itself. However, while clear flow through the nozzle is critical, that’s not the part of the fuel injector most subject to failure. Fuel injectors are designed to atomize the fuel into a mist ideal for the combustion process. In order to do this, they require an air intake built into the injector itself. There is a tiny screen to block contaminants from entering with the air. Over time, this screen collects dirt and other contaminants that can reduce fuel distribution. Left unchecked, it may eventually reach a critical point where the injector may not properly function. There is only one injector per cylinder, so you do not want to wait until the injector fails in flight.
As noted above, the second function of the fuel system is to aid in engine cooling during high power operations. This is why the fuel setup and adjustment process detailed in SID 97-3F is so important. There are two major adjustments to the system that affect the fuel flow: the low-end (unmetered) pressure and the high-end (metered) pressure. The low-end pressure affects the fuel pressure and flow available from the fuel pump during idle operations. But, it’s the high-end pressure that is so critical to engine cooling.
This high-end, metered pressure is the fuel available on the fuel distribution side of the system. This is the fuel pressure as it is delivered to the cylinders at full throttle. The output of the fuel pump can change seasonally, due to regular wear or with any fuel component repair/replacement. If the fuel flow is too low during high-power operations it can lead to cylinder overheating and detonation. Both can be very damaging to the engine. If the fuel flow is too high, the mixture can be so rich as to reduce the power available from the engine. You could easily be experiencing lower performance due to over-rich operations and not even know it. But your aircraft’s performance will be reduced, along with your safety margins.
Finally, some engines are equipped with altitude-compensating fuel pumps that automatically adjust the mixture as the aircraft climbs. These systems also require checking to ensure that they are properly adjusting the mixture according to the specs.
Continental Motors is currently delivering a free, live webinar series on engine maintenance and fuel systems during April and May. The schedule and links to the live events (and recordings of past webinars) are available on SocialFlight.com and through the SocialFlight apps. It’s a fascinating and informative series that I highly recommend.
Fuel injection systems are designed to be fairly bulletproof with few moving parts. The result is outstanding reliability, but it also can lead to complacency, and that’s not a good way to maintain your engine. Proper fuel system setup requires special gauges, fittings, and hoses that many mechanics do not have readily available. This is one of the reasons that so much of this maintenance is neglected. However, “if it isn’t broke, don’t fix it” is not the proper way to treat your aircraft (and the people inside). So, make the effort to find a mechanic with the tools and experience to do the job right and make it a part of your regular maintenance routine. It will pay dividends in safety and the long-term health of your engine.
Next time, we’ll cover what happens when something really goes wrong with the fuel injection system and how to proceed with getting it properly fixed. Until then, happy flying!
Interested in aircraft maintenance? View the archives of Jeff Simon’s Aircraft Maintenance series.
Jeff Simon is an A&P mechanic, pilot, and aircraft owner. He has spent the last 14 years promoting owner-assisted aircraft maintenance as a columnist for several major aviation publications and through his how-to DVD series: The Educated Owner. Jeff is also the creator of SocialFlight, the free Apple/Android app and website that maps over 20,000 aviation events, airport restaurants, webinars, and educational videos, including many how-to videos for the subjects of these articles.