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Multiple Fuel Composition Sensor

This is a discussion on Multiple Fuel Composition Sensor within the Technical / Maintenance forums, part of the General Discussion category; Hi everyone, First post here... Recently got a 2002 Suburban Z71 with 5.3 liter vortech. The check engine light came ...

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    Multiple Fuel Composition Sensor


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    Hi everyone,
    First post here... Recently got a 2002 Suburban Z71 with 5.3 liter vortech. The check engine light came on and long story short, the dealershaft said it is the Multiple Fuel Composition Sensor. I figure that's maybe $100 and 1 hour to put in, but they tell me it's $800!!!!! Eight Frickin Hundred for a sensor? And that's just the part!

    I'm pretty good with maintenance and all, so I might do this one myself, but here's some questions.

    Is it possible The General designed a sensor that costs $1000 to replace?

    Do I need this thing if I never plan to use low octane fuel?

    If I go to replace it, what's involved? Couple nuts and bolts and an electrical plug? Or is this thing tied into the fuel lines?

    Any help would be appreciated.

    Charlie
    '02 Suburban Z71 5300
    '94 Formula M6 - 13.4 @ 103
    '94 Honda CB1000 (sold

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    Re: Multiple Fuel Composition Sensor

    well maybe you should ask to look at one...cuz im not finding it in the parts locater...

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    Registered User gmsuv's Avatar
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    Re: Multiple Fuel Composition Sensor

    Here's the poop from GM.
    If you want me to give you the directions on how to replace the sensor give me a reply. I'll check back tomorry!


    Fuel Composition Sensor Description
    The fuel composition sensor (FCS), or flex fuel sensor (service parts term), measures the ethanol-gasoline ratio of the fuel being used in a flexible fuel vehicle. Flexible fuel vehicles can be operated with a blend of ethanol and gasoline, up to 85 percent ethanol. In order to adjust the ignition timing and the fuel quantity to be injected, the engine management system requires information about the percentage of ethanol in the fuel.

    The FCS uses quick-connect style fuel connections, an incoming fuel connection, and an outgoing fuel connection. The two connections have different diameters, to prevent incorrect attachment of the fuel lines. All fuel passes through the fuel composition sensor before continuing on to the fuel rail. The fuel composition sensor measures two different fuel related parameters, and sends an electrical signal to the powertrain control module (PCM) to indicate ethanol percentage, and fuel temperature.

    The fuel composition sensor has a three-wire electrical harness connector. The three wires provide a ground circuit, a power source, and a signal output to the PCM. The power source is vehicle system voltage, +12 volts), and the ground circuit connects to chassis ground. The signal circuit carries both the ethanol percentage and fuel temperature within the same signal, on the same wire.

    The FCS uses a microprocessor inside the sensor to measure the ethanol percentage and fuel temp, and change an output signal accordingly. The electrical characteristic of the FCS signal is a square-wave digital signal. The signal is both variable frequency and variable pulse width. The frequency of the signal indicates the ethanol percentage, and the pulse width indicates the fuel temperature. The PCM provides an internal pull-up to five volts on the signal circuit, and the FCS pulls the 5 volts to ground in pulses. The output frequency is linear to the percentage of ethanol content in the fuel. The normal range of operating frequency is between 50 and 150 Hertz, with 50 Hertz representing 0 percent ethanol, and 150 Hertz representing 100 percent ethanol. The normal pulse width range of the digital pulses is between 1 and 5 milliseconds, with 1 millisecond representing -40C (-40F), and 5 milliseconds representing 125C (257F).

    The microprocessor inside the sensor is capable of a certain amount of self-diagnosis. An output frequency of 170 Hertz indicates either that the fuel is contaminated or contains methanol (it should not), or that an internal sensor electrical fault has been detected. Certain substances dissolved in the fuel can cause the fuel to be contaminated, raising the output frequency to be higher than the actual ethanol percentage should indicate. Examples of these substances include water, sodium chloride (salt), and methanol.

    It should be noted that it is likely that the FCS will indicate a slightly lower ethanol percentage than what is advertised at the fueling station. This is not a fault of the sensor. The reason has to do with government requirements for alcohol-based motor fuels. Government regulations require that alcohol intended for use as motor fuel be DENATURED. This means that 100 percent pure ethanol is first denatured with approximately 4 percent gasoline, before being mixed with anything else. When an ethanol gasoline mixture is advertised as E85, the 85 percent ethanol was denatured before being blended with gasoline, meaning an advertised E85 fuel contains only about 81 percent ethanol. The FCS measures the actual percentage of ethanol in the fuel.
    Last edited by gmsuv; 10-30-2006 at 12:18 PM.

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    Re: Multiple Fuel Composition Sensor

    Wow, thanks for all the info! And if it's no bother, I would love some more info on how to remove and replace. Actually I want to just look at the thing first and see if there's some corroded connections. The check engine light is intermittent...

    Is there such a thing as a helms manual? I have the one for my Formula, and that was the best $100 I ever spent on that car.

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    Re: Multiple Fuel Composition Sensor

    follow the fuel line from the fuel filter,under the truck.it has a fuel inlet and outlet and an electrical connector.check the wiring that runs close to the front driveshaft,have seen that harness rubbing on the shaft grounding the wires.if not probably bad sensor.and that price is right but ridiculous.

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    Registered User gmsuv's Avatar
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    Re: Multiple Fuel Composition Sensor

    To change the sensor:
    Disconnect the ground to the battery to remove the risk of a fire.
    Take the gas cap off to remove pressure in the tank.
    Raise the vehicle.
    Disconnect the fuel line at the filter to remove pressure in the line.
    Remove the fuel composition sensor fuel composition sensor (FCS) attachment nuts
    Disconnect the FCS inlet and outlet pipes from the FCS.
    Disconnect the FCS electrical connector
    Remove the FCS and bracket assembly from the frame rail.
    Remove the FCS bolts and remove from the bracket.

    Quick Connect Fitting(s) Service (Plastic Collar)
    Removal Procedure
    Relieve the fuel system pressure before servicing any fuel system connection.
    Wear safety glasses when using compressed air in order to prevent eye injury.
    Using compressed air, blow any dirt out of the quick-connect fitting.
    Squeeze the plastic retainer release tabs.
    Pull the connection apart.
    Last edited by gmsuv; 10-31-2006 at 07:17 AM.

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    Captain Cement Shoe 99SLT's Avatar
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    Re: Multiple Fuel Composition Sensor

    damn... $800 for a sensor. Kinda makes you want to go to a stealership after hours and pop one off of a lot car

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    Re: Multiple Fuel Composition Sensor

    Doesn't seem like it is needed if you never plan to run Ethanol?
    One Bad Bow Tie - Now for sale in the FOR SALE Section - this is a steal for a dream truck.....

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    Re: Multiple Fuel Composition Sensor

    hell i would run a higher octane...i would not spend 800 for a microprocesser...

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    Re: Multiple Fuel Composition Sensor

    I certainly don't plan to run with Ethanol, in fact I've never even seen it for sale around here (although I haven't looked either). So I'd love to not have to replace this thing. But since it's throwing some sort of code, I'm sure I'd have to reprogram the computer so that it doesn't look for the sensor's signal.

    Hopefully it's just a shorted wire that I can fix with tape and zip ties!

    Thanks for all the info guys! I really appreciate it.
    Charlie

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