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  1. #1
    FEP Senior Member
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    Default Air injection system, how does it work.

    Simple question right.

    If I install an aftermarket efi system on my 85GT, will the air injection system cause the oxygen sensors see free air in the exhaust and richen up the mixture unnecessarily?

    When does the air pump system inject air through the heads vs. through the cat pipe?
    85 Saleen Mustang(s)

  2. #2
    FEP Power Member qikgts's Avatar
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    Default

    Attachment is from the '86 Mustang EVTM.

    Then, within this thread there is very specific info about the '85 system. Hopefully this will get you going down the right track although its a bit convoluted...

    http://vb.foureyedpride.com/showthre...ns-83-vs-84-85
    Attached Images Attached Images  
    '85 GT

  3. #3
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    That’s a tough read.

    When cold, thermactor air should be sent to the heads. Once hot, it goes to the cat pipe.
    85 Saleen Mustang(s)

  4. #4

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    Quote Originally Posted by BaconB8 View Post
    When cold, thermactor air should be sent to the heads. Once hot, it goes to the cat pipe.
    That is partially correct.
    When the engine is not running in closed-loop, such as when it's cold, air is sent to the heads.
    Once the system goes closed-loop, air will usually be sent downstream, but I have observed that it switches briefly to
    upstream after the throttle closes, and the engine has gone to idle speed.

    You are correct that the O2 sensors must not see upstream air once it is in closed-loop mode, or they will think the engine
    is running too lean.
    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
    FEP Power Member fgross2006's Avatar
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    Quote Originally Posted by JACook View Post
    That is partially correct.
    When the engine is not running in closed-loop, such as when it's cold, air is sent to the heads.
    Once the system goes closed-loop, air will usually be sent downstream, but I have observed that it switches briefly to
    upstream after the throttle closes, and the engine has gone to idle speed.

    You are correct that the O2 sensors must not see upstream air once it is in closed-loop mode, or they will think the engine
    is running too lean.
    Jeff can i ask you a question about the TAD.
    Earlier this summer my car was running poorly. Pulled codes and the only code said Thermactor not in baypass. i had replaced both TAB and TAD as well as all the solenoids with NOS a few years back. Had been running well until this summer. Luckily I saved the original parts and put the old TAB back in and the code cleared and the car started running great.

    Heres where I get confused. I tested both TAB's with a vacuum pump and both hold vacuum and both lift the valve when vacuum is applied. But the NOS part i had been using will not bypass in either state, vacuum or no vacuum. Id it holds vacuum and I can visually see the valve lift when I apply vacuum, why is allowing air to pass through and not dump out the bottom (bypass)? With or without vacuum.

    Just curious if I can open it up and fix it. its good the original still works and solved the issue but im thinking how much longer will this 35 year old part continue to function.

  6. #6

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    Does your system use the combo diverter/bypass valve? Or two separate valves like was used on my '85?

    Generally, the Bypass valve would default to bypass when no vacuum is applied to the diaphragm.
    This is because the AIR system does not function at WOT.

    The Diverter function in the CFI system defaults to downstream. So for your ECU to see unexpected air in the
    exhaust stream, you would have two things not as they should be.

    The bypass valve can occasionally have debris holding the valve off the seat, and this will allow some air to go
    upstream. In the downstream position, the upstream air valve is kept closed only by spring pressure. If the
    valve moves freely, and the diaphragm holds vacuum, I would inspect/clean the valve disk and seats. Do make
    sure the diaphragm does not leak at all.
    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
    FEP Power Member fgross2006's Avatar
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    Default

    Quote Originally Posted by JACook View Post
    Does your system use the combo diverter/bypass valve? Or two separate valves like was used on my '85?

    Generally, the Bypass valve would default to bypass when no vacuum is applied to the diaphragm.
    This is because the AIR system does not function at WOT.

    The Diverter function in the CFI system defaults to downstream. So for your ECU to see unexpected air in the
    exhaust stream, you would have two things not as they should be.

    The bypass valve can occasionally have debris holding the valve off the seat, and this will allow some air to go
    upstream. In the downstream position, the upstream air valve is kept closed only by spring pressure. If the
    valve moves freely, and the diaphragm holds vacuum, I would inspect/clean the valve disk and seats. Do make
    sure the diaphragm does not leak at all.
    mine has 2 separate valves. the TAB (first one) does not dump air when no vacuum is applied. And when I apply vacuum it holds it. No bleed down. I can see the internal valve lift and drop as i pump vacuum and release so I cant figure out why it allows air to pass through whether there vacuum or not.

    Can I open this part up by cutting the bottom seal with a razor? I have nothing to loose if I damage it. Its currently not functioning so if I cant fix it I'll have to toss it anyway.

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