My Musketeer has the Continental IO346 engine. Sometimes it wants to die when I pull the throttle back to idle. I also get conflicting advice on when to use the boost pump. Can you help?
Technical Editor:
As far as when to use the boost pump, do what the POH for your specific airplane tells you to do. Many models have different fuel system, engine, and induction combinations. Each may require a different usage. The following detailed information may help you to better understand some of the uniqueness of your engine-airframe combination. If you understand it better, you’ll be better prepared to tailor your techniques for your circumstances.
The Continental fuel injection system is very different from the Lycoming fuel injection system, as well as being different from the carbureted engines. While the IO520/IO550 systems get the most press, the IO346 needs the same kind of injection and fuel pump attention (as do the IO240, IO360, and IO470). I haven’t studied one of the IO346s in a very long time (very few were made), but I suspect that you have the same problem that often appears on the IO520/IO550 engines. See Continental Service Bulletin SID-97-3B. Here is the link info, and there is a link to all Continental service communiqués on the BAC link list. Page 11 contains the fuel flow chart for the IO346. This SID was originally published on March 24 of 1997, and was most recently revised on January 20 of 2004.
http://www.tcmlink.com/pdf2/SID97-3B.pdf
The Continental mechanical engine-driven fuel pump is a positive displacement gear-type pump (similar to the oil pump). It always displaces/moves a fixed amount of fuel for every revolution. As a result, it has to have some means of output control (as does the oil pump), or the difference between idle flow and cruise flow would be too great. This adjustment can be quite critical, and needs to be made by someone with experience on something like the Continental IO520 engine. A place that is very good with Bonanza fuel injection/mechanical fuel pump setup should be able to help.
In my somewhat limited experience, the low-idle fuel flow on the Continental injection system is often set too low to keep it running during a hot roll-out at minimum RPM (as well as on a cold start). The homebuilt planes that try to use them very often have this problem, since they seldom see expert Continental engine maintenance shops. They usually cannot be started at all without the boost pump, and often won’t keep running at cold low idle, without the boost pump being “blipped” or left on. In contrast, the Lycoming AC-style diaphragm pump moves plenty of fuel to both start the engine at cranking speeds, and to keep it running during cold low idle. Using the boost pump just speeds things up on a cold engine. The Continental mechanical fuel pump flow settings are often adjusted not only for inadequate idle flow, but as a result, they are also too lean at takeoff power, for good engine life. The GAMI folks have been cautioning Bonanza owners about this for some time now. Continental published the very extensive SID in an effort to combat this problem (and reduced cylinder life). As a quick check to see whether you are even in the ballpark, on your next take-off run, see whether your full throttle fuel flow reads between 13.3 and 14.5 gallons per hour, just after lift-off. Toward the high end is better, according to GAMI.
Here is some great additional information, with credit to Bob Steward:
The A23 and A23A use a rather unique engine and injection system. It’s different than any other Continental injection system. The POH trumps any generic checklist.
Because of the metering body on the IO-346A engine it is fuel pressure sensitive. Its really just an “adjustable leak” system rather than the very complicated air servo on the Bendix F.I. On the Bendix systems the mass flow of the incoming air is used to open the fuel delivery proportionally to “schedule” the fuel for delivery to the nozzles in the intake ports.
In the IO-346A “controlled leak” system the fuel metering is coupled directly to the throttle and moving the throttle changes opening in the valve allowing fuel to flow, and the output of the pump controls the amount of fuel sent to the injectors without regard to the mass flow of the air. This has the advantage of being cheap to produce once the calibration of the throttle and pump to the fuel flow has been determined. However, it also makes assumptions that the steady-state situation of the correct RPM for the throttle position is maintained.
This does NOT happen when the throttle is shoved in rapidly. The engine transitions from normal mixture to a momentary lean while the fuel catches the air, and then an over-rich condition until the engine revs up to use the amount of air that the throttle is allowing (and the fuel for that higher amount of air). However it also has the advantage of slightly enrichening the mixture in a climb when the engine RPM slows and the throttle is wide open.
Its an “open loop” system with no feedback on the actual air the engine is using, and the mechanical pump is designed for about 10% more fuel than is needed, which is why A23/A have a placard requiring use of fuel from the Left tank first. This is where the excess is routed.
If the throttle is moved slowly and smoothly (like we were all taught, right?), then the engine can accommodate the small relative fluctuations in the mixture as it speeds up and slows down.
Turning on the Boost pump (except to prime) raises the fuel pressure that the nozzles and metering orifices see, and this then enrichens the mixture, perhaps enough to kill the engine. The POH is very clear on the boost pump not being used except to start and in emergency situations when the mechanical pump has failed.
If the throttle is retarded smoothly and the engine dies on final then there are other problems with your 40 year old injection system and it probably needs someone that REALLY knows Continental F.I. on the IO-346A. These people are extremely rare, as the IO-346A is unique and only a few hundred were built, and most of the people that worked on them have retired or died.
I’d recommend to anyone owning one that you contact TCM (www.TCMLink.com) and buy the manuals. They will save you more than the cost of the manuals on the FIRST troubleshoot.