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  1. #1

    Default Fuel Bowl Solenoid Valve

    This is for my 1985 Mustang GT:

    Down by the battery is a charcoal canister that stores gasoline vapors from the gas tank and carburetor. There is "purge valve" on the top of this canister that has a large 3/8" hose that connects from the purge valve to the carburetor fuel bowl. In this hose, there is an electrical connector that my research has described as a "Fuel Bowl Solenoid Valve." it looks like this:

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    Since the wires on the solenoid that is on the car now have been cut, I can't figure out WHERE this plugs in? The diagram above shows that the red wire connects to the ignition system, but I don't see where. And is the white wire a ground wire? Where does it connect?

    Thanks!

  2. #2

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    The bowl vent solenoid(s) can connect to any switched 12V source, and the white wire gets bolted to
    the nearest convenient ground. On an '85 GT, the solenoids were originally fed by a red/yellow hash
    wire, that was part of the engine harness that connects through a grey 8-pin connector at the bottom
    of the driver side fender apron, near the ignition module.

    The solenoids must close when energized, or the engine can run rich because the fuel bowl will have
    an incorrect pressure reference.

    '85 GT would have had two vent solenoids, one for each fuel bowl, and they would have connected
    to two separate canisters, one on the frame rail in the engine compartment, the other up under the
    passenger side fender. The '85 GT used two purge valves that were originally separate from the
    canisters, but the earlier style canister with the purge valve on top is functionally equivalent.
    Cheers,
    Jeff Cook

    '85 GT Hatch, 5-speed T-Top, Eibachs, Konis, & ARE 5-Spokes ... '85 GT Vert, CFI/AOD, all factory...
    '79 Fairmont StaWag, 5.0, 62K original miles ... '04 Azure Blue 40th Anny Mach 1, 37K original miles...
    2012 F150 S-Crew 4x4 5.0 "Blue Coyote"... 65 coupe, 289 auto, Pony interior ... '67 coupe 6-cyl 4-speed ...
    '68 Vert, Mexican block 307 4-speed... '71 Datsun 510 ...
    And a 1-of-328 Deep Blue Pearl 2003 Marauder 4.6 DOHC, J-Mod, 4.10s and Lidio tune

  3. #3

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    Quote Originally Posted by JACook
    The solenoids must close when energized, or the engine can run rich because the fuel bowl will have
    an incorrect pressure reference.

    '85 GT would have had two vent solenoids, one for each fuel bowl, and they would have connected
    to two separate canisters, one on the frame rail in the engine compartment, the other up under the
    passenger side fender. The '85 GT used two purge valves that were originally separate from the
    canisters, but the earlier style canister with the purge valve on top is functionally equivalent.
    Thanks, JACook. Why did Ford use two of everything?

    Is there any reason why I couldn't/shouldn't be able to combine the two vapor hoses from each fuel bowl into one hose and use only one solenoid and one purge valve, and only one canister?

  4. #4

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    You don't really need two of everything, but I would keep the two bowl vent solenoids, and tee them
    together downstream. Each bowl should receive it's own pressure reference from within the air cleaner,
    without being influenced by the other.
    Cheers,
    Jeff Cook

    '85 GT Hatch, 5-speed T-Top, Eibachs, Konis, & ARE 5-Spokes ... '85 GT Vert, CFI/AOD, all factory...
    '79 Fairmont StaWag, 5.0, 62K original miles ... '04 Azure Blue 40th Anny Mach 1, 37K original miles...
    2012 F150 S-Crew 4x4 5.0 "Blue Coyote"... 65 coupe, 289 auto, Pony interior ... '67 coupe 6-cyl 4-speed ...
    '68 Vert, Mexican block 307 4-speed... '71 Datsun 510 ...
    And a 1-of-328 Deep Blue Pearl 2003 Marauder 4.6 DOHC, J-Mod, 4.10s and Lidio tune

  5. #5

    Default

    Quote Originally Posted by JACook
    You don't really need two of everything, but I would keep the two bowl vent solenoids, and tee them
    together downstream. Each bowl should receive it's own pressure reference from within the air cleaner,
    without being influenced by the other.
    So you are saying I need to have a separate vapor hose for each fuel bowl, a bowl vent solenoid in each hose, THEN I can tee the hoses together into one vapor canister?

    But if I am understanding correctly, why do I need a dedicated bowl vent solenoid for each fuel bowl, if the only thing these solenoids do is close off the fuel vapor hoses when the engine is turned on? Wouldn't they open and close at the same time? (I am not trying to contradict you, I am just trying to understand this system and why Ford would use two of these; there must be a reason I am missing.)

    Also, in the second picture of my post, there is also a round valve placed inline. From my research, this is a "thermal vent valve" that is a thermostatic-controlled valve that opens when above 90 degrees to vent the fuel bowl to the vapor canister and closes when the temperature is less than that to contain the fuel vapors inside the carburetor. I suppose this will help with gas evaporation and keep the fuel bowls from going dry as fast?

    But then I noticed that some vehicles and certain years used both the "fuel bowl solenoid valve" AND the "thermal vent valve" and others just used one or the other. From the description below, you would think BOTH are needed:

    http://www.autozone.com/autozone/rep...00c1528006db3a

    The fuel bowl thermal vent valve is inserted in the carburetor-to-canister vent line. The fuel bowl thermal vent valve works in conjunction with the fuel bowl solenoid vent valve to control the opening and closing of the hose between the carburetor fuel bowl vent and the carbon canister. The fuel bowl thermal vent valve is simply an open/close valve that depends only on temperature to open or close the flow path. It does not require any vacuum or electrical input to function. The valve is closed at temperatures below 90°F (32°C) and open at all temperatures above 120°F (49°C). At temperatures between these limits, the valve may be open or closed because the valve operates by an internal disc of thermostatic material whose exact temperature switch point cannot be predicted...

    ...The fuel bowl solenoid vent valve is a normally open valve located in the carburetor-to-canister vent line. The solenoid vent valve closes off the fuel bowl vent line when the engine is running and returns to the normally open condition when the ignition switch is turned off.
    The 1982 - 1984 Mustang GT used both, but (I think) the 1985 Mustang GT used only the "fuel bowl solenoid valve." Without the thermal vent valve, that would mean the fuel bowls would vent ALL THE TIME. Do you know why the 1985 models didn't use the "thermal vent valve" at all?
    Last edited by RickFury; 08-13-2014 at 08:07 PM.

  6. #6

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    Some good questions here.

    Quote Originally Posted by RickFury View Post
    So you are saying I need to have a separate vapor hose for each fuel bowl, a bowl vent solenoid in each hose, THEN I can tee the hoses together into one vapor canister?
    Yes.

    Quote Originally Posted by RickFury View Post
    But if I am understanding correctly, why do I need a dedicated bowl vent solenoid for each fuel bowl, if the only thing these solenoids do is close off the fuel vapor hoses when the engine is turned on? Wouldn't they open and close at the same time?
    So let's back this up a bit. Carburetors operate off pressure differential between the air in the fuel bowls, and the
    low pressure area generated by the venturii. It's that first half we're concerned about here. The pressure acting
    on the fuel bowls has a significant effect on fuel mixture, which is one reason why carburetors have those bowl
    vent tubes inside the perimeter of the air filter. The air filter will always have some resistance to flow, and if the
    bowls are referenced to ambient air, the carburetor will tend to run like it would if the choke is partially closed.

    This is why we have bowl vent solenoids that close off when the ignition is on (older carburetors had mechanical
    means to open and close the vents). Now, you may have noticed that most generic Holley carbs have eliminated
    the external bowl vents altogether. You could do the same, by capping off the external vent tubes. The downside
    to doing this is you get fuel vapor accumulation in the air cleaner, that causes hot start issues.

    OK, so that's why we need bowl vent solenoids, but why do we need two of them?

    There is another component to the bowl pressure reference, caused by airflow past the vent tubes. This effect is
    accounted for in the calibration. Because airflow is seldom equal between primary and secondary bores, the bowls
    do not (and should not) see the same pressure reference. But if you just tee the two bowl vents together into the
    same vent solenoid, they will.

    Quote Originally Posted by RickFury View Post
    Also, in the second picture of my post, there is also a round valve placed inline. From my research, this is a "thermal vent valve" that is a thermostatic-controlled valve that opens when above 90 degrees to vent the fuel bowl to the vapor canister and closes when the temperature is less than that to contain the fuel vapors inside the carburetor. I suppose this will help with gas evaporation and keep the fuel bowls from going dry as fast?
    The thermal vent valve in the vent lines is there to prevent vapor from the fuel tank from being pushed out through
    the carburetor rather than the canisters when the car is sitting. For example, when your car's been sitting overnight
    in the driveway, and then sunlight warms the rear of the car. The vapors will follow the path of least resistance, and
    since the carburetor is colder, the vapor pressure is lower, and presents much less resistance than the canisters do.
    The thermal vent valve keeps that from happening.

    I'm not sure why these were discontinued on the '85s, other than I do know the '85s have a small orifice in the tank
    vent line that might could accomplish the same thing. I don't know if the '84s had an orifice.

    One more thing - Best practice is to make sure your bowl vent lines route smoothly downhill, without any low spots
    where vapor can pool and condense. That's not difficult to do, but oddly enough I sometimes see the hoses running
    down against the intake manifold, then back up over the valve covers. Best way is to just run them around the front
    between the cylinder head and alternator.
    Cheers,
    Jeff Cook

    '85 GT Hatch, 5-speed T-Top, Eibachs, Konis, & ARE 5-Spokes ... '85 GT Vert, CFI/AOD, all factory...
    '79 Fairmont StaWag, 5.0, 62K original miles ... '04 Azure Blue 40th Anny Mach 1, 37K original miles...
    2012 F150 S-Crew 4x4 5.0 "Blue Coyote"... 65 coupe, 289 auto, Pony interior ... '67 coupe 6-cyl 4-speed ...
    '68 Vert, Mexican block 307 4-speed... '71 Datsun 510 ...
    And a 1-of-328 Deep Blue Pearl 2003 Marauder 4.6 DOHC, J-Mod, 4.10s and Lidio tune

  7. #7
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    Default

    Not trying to threadjack here...

    Quote Originally Posted by JACook View Post
    There is another component to the bowl pressure reference, caused by airflow past the vent tubes. This effect is
    accounted for in the calibration. Because airflow is seldom equal between primary and secondary bores, the bowls
    do not (and should not) see the same pressure reference. But if you just tee the two bowl vents together into the
    same vent solenoid, they will.
    Jeff, what would the effect be of using only one solenoid for both bowls? Rich or lean condition depending on airflow at any given moment?
    '85 GT

  8. #8

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    Not to thread-jack either, but JA Cook, you are one heck of a good man! You dispense much needed high quality tech assistance on these carbs. I think many people here find your input invaluable and just wanted to say thanks!

    Back to the show!
    2011 Mustang GT Track Day car
    1985 Mustang Dominator GT

    Previous cars:
    1979 Mustang 5.0 upgraded to a 351W (1987 - 1991)
    1985 Black Mustang GT Dominator T-Top (bought from Bluesfannoz In 1992 sold in 1993 RIP)
    1990 Mustang GT Titanium hardtop w/black leather (1993 - 1994)
    2004 Mustang GT w/a few bolt ons 255 RWHP (2004 - 2006)
    1985 Mustang GT Medium Canyon Red Factory Sunroof 32,000 miles - Sold to the right person (2013 - 2016)

  9. #9

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    Quote Originally Posted by qikgts View Post
    what would the effect be of using only one solenoid for both bowls? Rich or lean condition depending on airflow at any given moment?
    I'm not sure how you could predict the result, because you're combining the pressure reference for two
    different fuel bowls, under varying conditions. I do believe the differences are going to be pretty minor,
    but I still think it's best to keep the bowl vents separated until after the solenoids.

    An interesting bit of history - Holley used to equip some 4160 carburetors with a balance tube between
    the primary and secondary bowls, but that practice was discontinued in the mid '70s. This tube was in
    addition to the fuel supply transfer tube, and it's job was to keep fuel seepage past the secondary float
    valve from causing the fuel level in the bowl from creeping up too high if the secondaries weren't being
    used. The tube was first implemented back in the days when the steel float valves were all they had.

    The 4100 'shoebox' carbs had a channel at the top of the body casting that was for the same purpose.
    Cheers,
    Jeff Cook

    '85 GT Hatch, 5-speed T-Top, Eibachs, Konis, & ARE 5-Spokes ... '85 GT Vert, CFI/AOD, all factory...
    '79 Fairmont StaWag, 5.0, 62K original miles ... '04 Azure Blue 40th Anny Mach 1, 37K original miles...
    2012 F150 S-Crew 4x4 5.0 "Blue Coyote"... 65 coupe, 289 auto, Pony interior ... '67 coupe 6-cyl 4-speed ...
    '68 Vert, Mexican block 307 4-speed... '71 Datsun 510 ...
    And a 1-of-328 Deep Blue Pearl 2003 Marauder 4.6 DOHC, J-Mod, 4.10s and Lidio tune

  10. #10
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    FWIW, I've been running both of the bowl vents tied together into one solenoid for some time now and you're likely right that effects of combining them may be minor. Seems to be "good enough" as I don't notice any issues but of course I have nothing to compare with. I forget exactly "why" I found that the solenoid was bad though I don't recall it being directly related to any driveability issues. I think I may have been just cleaning up around the vacuum line spaghetti near the battery/passenger side of the engine and kind of ran into it.

    My 4180C idles smooth, doesn't surge under cruise and under WOT pulls without any flat spots but I did spend plenty of time to get it that way, most of which was not the fault of the carb.
    Last edited by qikgts; 08-17-2014 at 12:08 PM.
    '85 GT

  11. #11

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    Hi, anyone with a pure stock 85GT could tell me where exactly the ground wires for the purge solenoids attach ? and do they both attach on the same bolt ?


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