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Thread: Fuel Sending Unit ? - 77 Sierra MC-460

  1. #1

    Fuel Sending Unit ? - 77 Sierra MC-460

    When I start cold (Ottawa, ON is very cold) the engine will not idle below about 1300-1400 RPM without stalling. We have a Tannis Heater and an engine cover on the plane and never attempt a start unless the engine oil temp on the UBG is at least 40 degrees or better. Usually fires up just fine and runs great once the temps are in the 90's (by the time you get to do the run up, I will not take off unless the oil temp is 100 or better, just a personal guess). During the time between 40 and 90 if you pull the power below 1300 RPM the engine will quit. Almost seems like it is not getting fuel even with the mixture full rich. I also tried yesterday with the mixture leaned a bit to prevent the plugs from being fouled and to help warm the engine a bit faster but same problem. Open the throttle a bit further and it restarts fine. Any other IO-360 drivers had this problem? My mechanic says that this is not normal for a fuel injected engine and suspects it may be related to the Fuel Sending unit or something with the throttle body.

    Bruce

    C-GMTT


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  2. #2

    Fuel Sending Unit ? - 77 Sierra MC-460

    Bruce, what is "very cold"? My belief is that the engine should idle OK on
    a cold start down to maybe just above freezing. It is common to need to
    keep it above 1,000 RPM for a minute or two, even at the 0 to 10 degrees
    Celsius range, just like any other kind of engine. Below freezing, it may
    take a longer warm-up before it will run at idle. It is better to keep it
    between 1,000 and 1,500 RPM on cold start-up, to get enough oil splash on
    the high-mounted Lycoming camshaft.



    Any engine with carburetion or non-electronic fuel injection has to deal
    with fuel vaporization before the fuel can burn. In low temperatures, it is
    difficult for the fuel to vaporize; a lot of it remains in liquid or large
    droplet form, and just passes through as soot or unburned fuel. Carburetion
    is the worst about this, but even the Bendix fuel injection relies on a hot
    intake valve and port area to achieve evaporation. This factor is why a
    cold engine requires priming (carburetor) or boosting (fuel injection). It
    requires a much richer mixture to keep it running when cold. Low RPM makes
    the problem much worse. Higher RPM on a cold engine creates more "vacuum"
    in the induction system, due to the more efficient "pumping action" by the
    engine. The lower induction pressure helps the fuel vaporize. When both
    engine and air are cold, and idling at low speed, the vacuum is reduced, so
    the mixture has to be richer to achieve enough vaporized fuel for
    combustion. These same circumstances are why auto and industrial engines
    used to have a carburetor choke, in days gone by.



    By now you may have deduced your solution, if you really need to have one.
    You can probably improve your cold-engine idle RPM simply by setting the
    mixture cog-wheel a bit richer. The adjustment is on the side of the
    injector servo; your mechanic should be very familiar with it. Keep in mind
    that if you do this, the mixture will be richer than normal when taxiing
    back in with a hot engine. The engine will "lope" at hot low idle, if you
    fail to lean it during ground ops when fully warmed up; and your RPM rise
    during shut-down will be much greater (maybe 200 RPM or more).



    You can experiment with this just to confirm the diagnosis. If you do
    decide to leave it richer, you'll need to adjust it back to normal when
    warmer weather arrives. You will also need to lean during ground ops with a
    hot engine, or you will foul plugs. Frankly, if I were you, I would leave
    it at the normal setting. The engine is trying to tell you that it is not
    yet warm enough in the cylinder heads and induction system for normal
    operation. It is best to just let it warm up completely before attempting
    all forms of normal operation. But as suggested, you can set it a bit
    richer temporarily, just to confirm the diagnosis (or you can leave it that
    way).



    Assuming you have the TANIS oil sump heater pad, but not the cylinder head
    heaters, the cylinder heads and induction system don't get any significant
    heat from the sump heater when it is really cold out. The only way to heat
    the whole system is to run a duct from a small box heater, up into the
    cowling outlet on the bottom. The heater can be located safely away from
    the plane, so oil can't drip on it, etc. Lakewood made a perfect unit that
    was about six inches square, with the controls on the upper rear, but I have
    not found them in some time. It was easy to shape a duct adapter to the
    heater, tape it on using the metal-foil HVAC tape, and run an air duct into
    the exit area beneath the cowl. Coupled with a cowling blanket and cowl
    inlet plugs, you could set these on low heat and they worked perfectly to
    keep the entire engine compartment warm; magnetos, oil cooler, induction
    tubes, etc. This is far better for starts in very cold weather, versus
    single-point heating. This is particularly true of the 200 HP IO360,
    because the induction system does not run through the oil sump, as it does
    on most other models. The cooler induction provides more power from a
    cooler inlet air charge, but it needs more warm-up when cold.



    My personal preference, when the current consumption isn't an issue, is to
    hook up the heater when you park the plane in the hangar. Open the oil door
    and turn on the fan only, to flush out the high heat load. Then before
    leaving the hangar, close the oil door, put in the cowl plus and lay on the
    blanket, and turn the heater on low heat. This will keep the engine from
    ever cooling below the dew point, so condensation cannot form in the engine.
    It will also keep everything from going below a comfortable temperature, so
    your next start is effortless. If it is extremely cold, you can leave the
    heater on the higher heat setting, at the cost of added kilowatt charges if
    you pay the electric bill. With the cowl blanket and cowling enclosure, it
    doesn't take much heat to keep things comfortable. This is particularly
    true when the heater doesn't have to warm everything up; it just has to keep
    them from cooling down all the way. That makes a big difference. I once
    purchased a sump heat pad; but after more research, I went the heater route
    instead (and sold the pad).









    _____

    From: musketeermail@yahoogroups.com [mailto:musketeermail@yahoogroups.com]
    On Behalf Of Dwyer, Bruce
    Sent: Monday, January 23, 2006 11:34 AM
    To: musketeermail@yahoogroups.com; bac-mail@beechaeroclub.org
    Subject: [musketeermail] Fuel Sending Unit ? - 77 Sierra MC-460



    * When I start cold (Ottawa, ON is very cold) the engine will not idle
    below about 1300-1400 RPM without stalling. We have a Tannis Heater and an
    engine cover on the plane and never attempt a start unless the engine oil
    temp on the UBG is at least 40 degrees or better. Usually fires up just fine
    and runs great once the temps are in the 90's (by the time you get to do the
    run up, I will not take off unless the oil temp is 100 or better, just a
    personal guess). During the time between 40 and 90 if you pull the power
    below 1300 RPM the engine will quit. Almost seems like it is not getting
    fuel even with the mixture full rich. I also tried yesterday with the
    mixture leaned a bit to prevent the plugs from being fouled and to help warm
    the engine a bit faster but same problem. Open the throttle a bit further
    and it restarts fine. Any other IO-360 drivers had this problem? My mechanic
    says that this is not normal for a fuel injected engine and suspects it may
    be related to the Fuel Sending unit or something with the throttle body.

    Bruce

    C-GMTT





    [Non-text portions of this message have been removed]



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