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Thread: Won't idle when cold / auxiliary air valve / idle air control valve

  1. #1
    Van Fan davidgracevan's Avatar
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    ISC Valve

    -our van's engine has an ISC valve, when does this operate or turn on?

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    Administrator timsrv's Avatar
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    Re: ISC Valve

    There is an ISC VSV (Idle Speed Control Vacuum Switching Valve) that's controlled by the ECU. It's a solenoid operated air valve that when activated allows a small amount of air to bypass the closed throttle body valve. The result is an increase in idle speed of about 100 RPM. The ECU is programmed to complete the circuit (provide ground) to the ISC VSV when certain conditions are met. 12V positive is supplied to the ISC VSV from the AM2 ignition circuit when the EFI main relay is active.

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    Van Fan davidgracevan's Avatar
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    Re: ISC Valve

    -thanks sir Tim, my ISCV was already missing, i want to restore it back, when do the ECU turns this ISC valve on?, in what situations do this ISC turn on?

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    Re: ISC Valve

    That's a secret , well, it sort of is. Toyota never bothered to talk much about this in the service manual so I'm not sure what these predetermined conditions are. They talk about the ISC VSV in the very end of the FI section, but there's only a simple schematic and a brief description on how to test it. I wish the manual would elaborate on this more, but it doesn't. Tim

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    Van Fan davidgracevan's Avatar
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    Re: ISC Valve

    well i found this in the internet... http://www.autoshop101.com/forms/h26.pdf . based from what i read there, what i like to know is that do our ECU turns this ISC on when we shift the transmissin to Drive? coz mine RPM drops to 500-600 when shifting to drive or reverse... does it also turn on when for example theres a heavy electrical load?... what do you think sir Tim?
    Last edited by davidgracevan; 12-07-2011 at 06:20 AM.

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    Re: ISC Valve

    As far as ECUs go, ours are some of the earliest (and most primitive) ones out there. When it comes to control systems it's all about input vs output. I don't think ours are sophisticated enough to detect electrical loads (but I could be wrong). I'm not sure what year range that article is referring too, but some of the standard functions in today's ECUs weren't even thought of back when ours were built. Tim

  7. #7
    Van Fan davidgracevan's Avatar
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    Re: ISC Valve

    -ok sir Tim thanks for your thoughts. You have an A/T van right?, do yours drops rpm when the gear selector is shifted to drive or reverse?

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    Re: ISC Valve

    FWIW, my rpms drop, somewhere close to the amount you mentioned, when I put my van in gear.

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    Re: ISC Valve

    Couldn't tell you how it was originally. Only had mine for just under two years now.

  10. #10
    Van Fan davidgracevan's Avatar
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    Re: ISC Valve

    -is it originally like that sir? RPM drops when shifting gear?

  11. #11
    Van Fan davidgracevan's Avatar
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    Re: ISC Valve

    -does the ISCV compensates the RPM lose when shifting the van in gear?... guys can i hear your experiences about this ISCV?... thank you

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    Re: ISC Valve

    -can anyone help me out here?.... thanks

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    Re: ISC Valve

    wow, tim doesnt even know what it does?!?!?!

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    Re: ISC Valve

    Quote Originally Posted by gushaman View Post
    wow, tim doesnt even know what it does?!?!?!
    LOL . I only know what I read in the manual (posted above). Whatever the manual doesn't cover becomes speculation. As far as I know the kind of "van specific" information requested isn't covered in any publications I know of. If anybody else has more information please post it here. Frankly, I don't think such information is necessary to correctly troubleshoot and repair the van, but it would be helpful for the troubleshooter/mechanic to fully understand the system. For me it's much easier to understand and troubleshoot problems when such information is available. I wish there was a van specific book that explained what was going on in the engineer's heads when they designed the system. I can get into that kind of reading (not sure what's wrong with me) . Tim

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    Van Fan davidgracevan's Avatar
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    Re: ISC Valve

    -i'm just wondering sir Tim, does your van with A/T lose rpm when shifted into gear?

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    Re: ISC Valve

    Yeah, it's normal for the engine to slow down a bit when it has a load placed on it. I never really paid much attention to it so I couldn't tell you how much.......say 100-200 rpm??? The van I drive every day does not have a tachometer. Tim

  17. #17
    Van Fan davidgracevan's Avatar
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    Re: ISC Valve

    -ok sir Tim, it loses 200 rpm, if following the service manual, we should set it the idle at 750, right... shifting the gear, rpm goes down to 600-500 which i observed stresses the alternator because it is not turning at optimum speed especially during night time when it is really loaded, i just thought that it supposed to be the job of the ISCV but it turns out to be not. Some of our guys here suggested that idle should be set at 1000 so that when gear is shifted, it will at 750. i tried to set my idle at 1000, it makes the shiftshock a bit harsh... what do you think sir Tim?

  18. #18
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    Re: ISC Valve

    Set it at 750 while in neutral and forget about it. It's normal for our alternators to not charge while at idle. As long as it's not sitting at idle for extended lengths of time (with everything turned on) you will be fine. The system is designed to share the load with the battery. When it's at an idle the battery will help out with loads. When you are driving the alternator takes over and replaces energy that was taken from the battery......it's a give & take set-up. Of course this requires your battery to be in decent condition. If your battery is questionable then you may have problems. If the van needs to sit at idle for extended periods, then turn off the unnecessary loads. Tim

  19. #19
    Van Fan davidgracevan's Avatar
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    Re: ISC Valve

    -thanks sir Tim for that urgent reply. you hit it sir, during heavy traffics here, definitely i'll be sitting long at idle, another scene is the very slow moving traffic where you would just crawling and your foot stays at the brake pedal. my battery is new, checked every electrical concerns (keeping voltage drops in line acceptable) but i observed that when shifted to gear, you will notice a slight dimming of the lights, a/c blower somehow drops speed. what i do is to brake with left foot sometimes then put my right foot on the accelerator to raise the rpm back to 750 where everything is working very well. In newer vehicles with A/T, when shifted to gear, the ECU compensates the transmission load and raises the engine rpm back to the predetermined rpm, in our van that does not happen, like what you have said our ECUs a bit of a first age. i believe that the alternator still charges at idle considering before i change battery before, my old battery drops it voltage at 10 volts in the morning after parking at night, starting my engine helps it to bring back to 12 volts, but the rpm should be 750... what do you think again sir Tim?


    (P.S. i'm really gald sir Tim that we were able to share our thoughts here )

  20. #20
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    Re: ISC Valve

    Does the battery get too low to restart the van? Unless your battery is going dead it's nothing to worry about.

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