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Thread: fuel boost pump

  1. #1

    fuel boost pump

    Hi all:

    I have a A23-24(fuel injected)mouse, and last week I noticed that the
    fuel pressure would not increase with the electric boost fuel pump on
    before engine start.
    I therefore replaced the pump, and even though the mechanic says there
    is flow thorough the new pump, the fuel gauge will barely move. It
    usually shows 5 PSI in flight with the engine driven pump.
    May be I changed the boost pump unnecessarily?
    Any comments appreciated

    Walter
    N3670Q

  2. #2
    Walter,

    I have a digital fuel flow/pressure gage (also an A23-24), and if anything, the fuel pressure with the boost pump for priming may go as high as 0.2 psi. Typical in-flight pressure is around 2.3 - 2.5 psi.

  3. #3
    Since I don't know how/where your sensors are installed, I can only speak from conjecture. The most common reason for this concern is that the sensor is installed in the fuel divider, or is T'd into the factory line from the divider.

    The factory fuel flow gauge is in fact a pressure gauge, marked in GPH instead of PSI. The engineering assumption is that if the pressure is at a certain planned level, a defined amount of fuel must be passing through the divider and into the engine. That is true only when all is working as advertised. But if an injector becomes clogged, the pressure in the divider will actually rise, since the metered fuel is 'seeing' more resistance; so the gauge will show a higher fuel flow, at a time when real flow is actually reduced.

    When operation is normal, pressure really is quite low in the divider. Only metered fuel makes it to the divider. Since it immediately moves from the divider to the injectors, there is no resistance to create pressure. This is particularly true at idle, when flows are so low.

    If your gauge sender is actually plumbed into the system ahead of the injector servo, you should be seeing much higher pressure readings. The engine-driven pump on a FI engine normally produces around 15-17 PSI. The electric boost pump usually has a setting a couple of PSI higher. Since the carbureted engines typically run on 3-4 PSI, it is important to get the correct pump for a given installation.

    I am a bit surprised that a technician would just take your word for it and replace a very expensive boost pump, without checking out the complaint, so this conjecture may be incorrect. But it is highly unlikely that both the engine-driven pump and the boost pump have both failed at the same time; and the fuel injection could not function to properly meter fuel, with such low fuel pressure.

  4. #4
    The A23-24 did not come with a fuel flow gage like the Sierra. It has a fuel pressure gage, and the normal range is 0-12 psi. Normal fuel pressure has always been in the low end, and I've never seen it over 5 psi, and that reading was due to a clogged injector nozzle, and as was mentioned. Normal pressure readings should be lower. The pressure is read right at the flow divider, and there is a 1/4 hose directly from the divider to the gage, via a bulkhead connector on the rear baffle.

    I had the boost pump overhauled last year after it developed a slight leak, but there was never a change in pressure before or after the overhaul. It's almost never used, except to prime a cold engine. Unlike a carburated engine, it is off for takeoff and landing, and other than emergencies, not used. Dukes (the manufacturer) recommends an overhaul every 10 years, so it's probably just as well that you had it serviced.

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