Sorry only just got back in from Cardiff.
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Yes it's really called an ISC.
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To check this electrically, with the plug removed, you really need to check the resistance between the middle pin on the ISC body and each of the two outer pins.
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This should be around 19
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- 23 ohms.
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I wouldn't worry too much about the voltage
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But having re-read your original post, I missed the photo of the pipe you removed.
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It's a while since I had an NA, but looking at an old photo of my NA engine bay it looks as though that is the pipe to the VSV or the idle up valve I was talking about.
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I suggest you first try disconnecting the plug to this to see if that sorts out your fluctuating idle speed.
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If it does, that suggests something is forcing it to open and close, or the valve is faulty.
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But if your idle has only been fluctuating since you remove your aircon rad, it's possible that this is the issue, and there may be a sensor somewhere that is now confused resulting in the VSV opening and closing> That's just a guess as I've never tried removing AC, I like my home comforts far too much.
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If however you suspect the VSV is faulty, there is one easy check you can make.
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With the plug disconnected try blowing into the outboard pipe on the valve
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(the one directly below the electrical socket) and check that no air is coming out of the other pipe.
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IE the valve is fully closed when power is off.
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If air is still leaking out of the second pipe, the valve is probably faulty.
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If it's not the aircon rad removal and the VSV is operating correctly, I'm starting to run out of ideas.
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But from what you've said, the VSV is definitely worth checking.
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Paul