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    Thread: Here is a comprehensive article on the ISV (Idle Stabilizing Valve)

    1. 10-24-2006 10:27 PM #1
      I put this together from Vortex threads on this subject. Edited and reworked for better reading - unable to credit the exact sources, but thanks to them anyway. Hope it helps some of you:
      Idle Stabilizer Valve – MK2 8v and 16v
      Some information gathered from postings on the Vortex forums that will hopefully aid in the understanding of the workings of the Idle Stabilizer Valve (ISV).
      VW have used 3 types of ISV for the Mk2’s.
      The first version was a small valve (it either opened or closed) added to the CIS basic engine management system. It was used to raise the idle back to approximately 950 when a load was put on the engine. CIS models with air conditioners used 2 valves. The auxiliary air regulator (AAR) controlled cold fast idle, they can all be removed but you will have to press the throttle open to compensate for cold starts etc.
      The second version was an adjustable valve controlled by the ECU; it did the work of the AAR and was capable of controlling the idle speed more accurately with varying engine loads providing the basic throttle settings are correct. It has a limited swing and can only compensate to a point, it can also be removed but you'll have to press the throttle like the first version.
      The third version, which is the one we are primarily discussing, uses a bigger valve and a more complex ECU that controls the idle 100%, and cannot be removed without modifications.
      All versions are really bypass valves; they allow air to go around the throttle valve, and therefore they cannot affect the mixture or the performance. The basic purpose of this device is to regulate idle speed more accurately by bypassing the closed throttle body. It also serves as a kick-up valve for the air conditioner by compensating for compressor load even when cruising.
      The ISV assembly is a brass colored tube about 5 inches long by 1 ½ inches in diameter. It mounts on a bracket directly over the middle of the valve cover, with small hoses that bypass the throttle valve. There is a 2-wire connection at on the driver’s side end (Digifant system) and a 3-wire connection if it is CIS-E or CIS-E Motronic. They both connect to the computer.
      Interestingly, the Digifant idle stabilizer is effective enough to allow a manual transmission car to creep along in first gear at idle speed without stalling.
      It isn't so much a valve as it is a quaint little servomotor that acts as a slaved sub-throttle controlled by the smog computer. It has a small valve on one end and is centered by a coil spring. The valve is operated by a basic DC motor shaft with point contact brushes. Total opening area of the valve is about ½ sq inch, and the spring holds it about 80% closed when unpowered.
      For the CIS-E, the center connector goes right to +12V. The other two pins are pulse switched to ground at the computers discretion. Duty cycle of the pulse opens or closes the valve as necessary.
      Symptoms of a faulty ISV or one that is getting progressively or intermittently faulty are:
      1. Idles low or rough when cold (improving after warm-up).
      2. An idle "jump" for a second or two (internal problem in ISV).
      3. Erratic idling (surging) and stalling, especially when the air conditioner compressor cuts in by not compensating for the extra load. The symptoms would likely appear slowly over a period of time, but getting progressively worse.
      4. A real tell-tale sign of a faulty Digifant ISV is to let your foot off the throttle pedal while running (particularly higher revs). If the rev needle drops immediately to the 500-600 range before stabilizing at 800 or so, there is a failure occurring. Sometimes the idle will drop so low that the car stalls. A correctly working ISV should drop to about 1000 or 1100 rpm when letting your foot off the gas pedal and then the needle should slowly ease down to normal idling speed (about 800).
      As a general rule of thumb, if your idle oscillates over a wide rpm range (600-1400) the ISVis suspect; if it oscillates over a narrow range (800-1000), the oxygen sensor may be bad.
      A quick test would be to see if the ISV is working at all: Turn the ignition on but do not crank. Pop the hood and you should be able to hear the ISV humming slightly. Touch it and you should feel a vibration. If so, we at least have a working unit, but not necessarily a fully functioning one. Sometimes the ISV internal servo motor can become so internally clogged that it won’t work or vibrate at all.
      Remove it by taking off a couple of hoses and sliding it out of the rubber ring. Spray ‘electronic parts cleaner’ into the two holes while holding it upright so it can run out. Alternatively, you can use WD-40 and throttle or carb. cleaner as long as it does not have “chloro” in it. Use rubber gloves and work outdoors. These chemicals are extremely nasty. Avoid using brake cleaner as it destroys plastic and rubber. Look inside for carbon build up and hit it directly, spray a lot of fluid in and let it run out. Do this repeatedly and shake the ISV once in a while. Try to get the metal inside as clean as possible. Spray again and repeat until it sparkles inside. Once it is entirely clean spray some starter fluid through it and let it dry before re-installing.
      For the CIS-E ISV: Disconnect the connector from the valve, with engine off, measure resistance (ohms) from the center pin to each side, both should be about 12.5 ohms. If either one reads high or open, replace it. Reason: The internals of the valve form a bi-directional DC servo-motor, the windings of which are connected through a commutator, which has a nasty habit of arcing until the connection is gone. Pins 1 to 3 should show 25 ohms (both windings at once). If you see an open connection here, you have a bad idle stabilizer.


    2. Member MK2SnowPilot's Avatar
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      10-25-2006 10:36 AM #2
      Nice write up - someone add this to the faq!

    3. Member Funkatollah Insaney's Avatar
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      10-25-2006 10:43 AM #3
      X2 for the FAQ!!!!

    4. 10-25-2006 10:50 AM #4
      good info [IMG]http://*****************.com/smile/emthup.gif[/IMG]

    5. 10-25-2006 10:59 AM #5
      Useful post. [IMG]http://*****************.com/smile/emthup.gif[/IMG]

      FAQ bound.

    6. Member mant's Avatar
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      10-25-2006 10:59 AM #6
      excellent work. x3 for faq [IMG]http://*****************.com/smile/emthup.gif[/IMG]

    7. Member OstTrefftWest's Avatar
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      10-25-2006 11:17 AM #7
      Bravo. Definitely belongs in the FAQ. Nice work!

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      10-25-2006 11:38 AM #8
      bump to my watched so i remember this

    9. 10-25-2006 10:56 PM #9
      I will get Hamster to FAQ it

    10. Member fixwithahammer's Avatar
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      12-04-2006 02:09 AM #10
      I made a profile just to say that, that is the most help full thing that I have seen on hear -woot woot! Vw

    11. Member G-rocco's Avatar
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      12-30-2006 04:53 PM #11
      Quote, originally posted by SimonH »
      I4. A real tell-tale sign of a faulty Digifant ISV is to let your foot off the throttle pedal while running (particularly higher revs). If the rev needle drops immediately to the 500-600 range before stabilizing at 800 or so, there is a failure occurring. Sometimes the idle will drop so low that the car stalls. A correctly working ISV should drop to about 1000 or 1100 rpm when letting your foot off the gas pedal and then the needle should slowly ease down to normal idling speed (about 800).A quick test would be to see if the ISV is working at all: Turn the ignition on but do not crank. Pop the hood and you should be able to hear the ISV humming slightly. Touch it and you should feel a vibration. If so, we at least have a working unit, but not necessarily a fully functioning one. Sometimes the ISV internal servo motor can become so internally clogged that it won’t work or vibrate at all.

      This is exactly what my 91 Jetta is doing. I was going to clean it, but thought I'd check it first. It hums, but does not vibrate. Will cleaning it help in this case?
      I would have just cleaned it anyway, but I only have brake cleaner, and apparently that's too harsh.
      And a huge [IMG]http://*****************.com/smile/emthup.gif[/IMG] for pulling this info together

    12. 12-31-2006 02:46 PM #12
      just want to add to that great write up by saying i replaced my isv twice bought from main dealers at 275euro each with no difference turned out the wires goin to isv plug were twisted and had gone bare and would only short when revs were increased and engine moved! took me a while to find it to say the least cause it hummed when car was not running!

    13. Member gdub's Avatar
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      12-31-2006 04:02 PM #13

      Nice post, I think most of us have replaced this bad boy at least once...it will definetly help out the guys just getting into the MK2. [IMG]http://*****************.com/smile/emthup.gif[/IMG]

    14. 12-31-2006 10:38 PM #14
      Quote »
      Interestingly, the Digifant idle stabilizer is effective enough to allow a manual transmission car to creep along in first gear at idle speed without stalling.

      (Digi)
      The car will do this even w/ the ISV disconnected.
      Also the article should mention for digi cars there should be continuity between the terminals on the ISV. Dont disconnect the ISV when the car is on, apparently this can damage the ECU.
      To test the wiring/signal to the ISV...
      Disconnect the ISV. get the car hot, rev the motor from idle to 3k rpm, 3-4 times. Testing the ISV plug it should fluctuate between 390-450mA. If you disconnect the CTS it should read steady 430mA.

    15. Member Ryan_GTI's Avatar
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      01-18-2008 02:51 PM #15
      Back from the dead.
      Will PB Blaster work to clean the ISV? I see it says WD-40 will work, and I assume PB would as well, but I figured I'd ask just to make sure.
      BTW, awesome post

    16. 01-18-2008 03:23 PM #16
      Remove it by taking off a couple of hoses and sliding it out of the rubber ring. Spray ‘electronic parts cleaner’ into the two holes while holding it upright so it can run out. Alternatively, you can use WD-40 and throttle or carb. cleaner as long as it does not have “chloro” in it. Use rubber gloves and work outdoors. These chemicals are extremely nasty. Avoid using brake cleaner as it destroys plastic and rubber. Look inside for carbon build up and hit it directly, spray a lot of fluid in and let it run out. Do this repeatedly and shake the ISV once in a while. Try to get the metal inside as clean as possible. Spray again and repeat until it sparkles inside. Once it is entirely clean spray some starter fluid through it and let it dry before re-installing
      I have an 8v CIS car, where is this located on my engine?

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      02-26-2009 05:49 AM #17
      For cis-e, is there a way to adjust the idle mixture without a dwell meter?
      With the idle screw tightened all the way in and the isv unplugged (80% closed), I'm seeing an idle of around 1400 rpm. I imagine this may be normal to allow for easy starting before the ecu dials down the isv? Or is my isv faulty and sticking open?
      Is there a rough estimate of what the base idle speed should be with the isv completely removed (or with it simply disconnected and going beyond the 1400 rpm), so that when it's in place, it will be having around a 30% duty cycle?
      I really suck at smog.

    18. 02-26-2009 07:51 AM #18
      so my 1.8 8V idles real high (3K) until i unplug the isv....then it drops down to 1k or so.
      would this indicate faulty isv?

    19. Geriatric Member VDub2625's Avatar
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      02-26-2009 08:42 AM #19
      Quote, originally posted by ziddey »
      For cis-e, is there a way to adjust the idle mixture without a dwell meter?
      With the idle screw tightened all the way in and the isv unplugged (80% closed), I'm seeing an idle of around 1400 rpm. I imagine this may be normal to allow for easy starting before the ecu dials down the isv? Or is my isv faulty and sticking open?
      Is there a rough estimate of what the base idle speed should be with the isv completely removed (or with it simply disconnected and going beyond the 1400 rpm), so that when it's in place, it will be having around a 30% duty cycle?

      Use an ampmeter hooked in line with the differential pressure regulator and you'll set the idle according to various DIYs around here and on the web (search for "Know more then the tuners." on Google).

    20. Member perickomx's Avatar
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      02-26-2009 01:55 PM #20
      i can understand a lot of things..now!!
      thanks!!

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      02-26-2009 02:50 PM #21
      hey thanks for the post. I've been trying variations of that but haven't turned anything up yet. got anything more specific? are you talking about jp's cis-e writeup?
      I really suck at smog.

    22. Geriatric Member VDub2625's Avatar
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      02-26-2009 03:22 PM #22

    23. Member
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      02-26-2009 09:33 PM #23
      ahhh, thanks for the link! I remember seeing this before. That's definitive then. my isv is broked. cheers
      I really suck at smog.

    24. Member monoaural's Avatar
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      02-26-2009 09:47 PM #24
      Quote, originally posted by deeb »
      so my 1.8 8V idles real high (3K) until i unplug the isv....then it drops down to 1k or so.
      would this indicate faulty isv?

      Try swapping the harnesses on the CTS. that was my problem. New blue CTS, runs perfect.
      -Jon

    25. Geriatric Member VDub2625's Avatar
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      02-26-2009 10:00 PM #25
      Quote, originally posted by monoaural »
      Try swapping the harnesses on the CTS. that was my problem. New blue CTS, runs perfect.

      Yeah, swap the blue and black plugs. The blue CTS is for the ECU, the black one is for the gauge. But since they are the exact same sensor electrically, if you switch them and the car runs better, then that was the problem. You can also check to see if the gauge works to test the blue sender

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