Minimum idle: vacuum leak?
 

This feels like a stupid question, but here goes:

With the idle bypass valve screwed all the way tight and the idle air control valve set to 0% duty in test mode, I can't get idle below about 850 rpm. Shouldn't it stall under these conditions? The idle is smooth, I just thought the engine shouldn't be getting any air in this condition.

I'm running a 99-00 VICS manifold. I've been over the thing half a dozen times checking for some hidden vacuum port I might have missed. I've sprayed with brake fluid all over, and I can't find a leak. The idle valve closes and seals at 0% duty. The throttle butterfly seats nicely. What am I missing here?

My idle has been high since I installed the turbo ish too... I brought the idle valve as low as it would go and the car was still idling at 1k rpm.

Subbed.

Nuts. The brake fluid I used is the non-flammable formulation. Back to testing...

*Edit* brake cleaner, not brake fluid. Jesus. More coffee needed...

Are you sure the idle valve is closed at 0%? On the early NA valves, it most certainly is not.

Edit: and I assume you mean brake cleaner, not brake fluid. If you are using brake fluid... stop

No, I'm not positive. That's what I'm trying to learn. I pulled the idle valve while the car was off. The valve rests in a completely closed and sealed position. I also unplugged the IAC while it was set to 0% duty, and there was no change in idle. The idle test function adjusts the idle all the way down to about 850 rpm with the bypass screw shut. Map kpa is about 32 ish at 850 rpm. This all feels fine, I just thought the engine shouldn't be getting air in this config.

haha, that paints a picture. My bad. I was using non-flammable brake cleaner. I did some more testing this morning with ultra flammable instant gum cutter, and still haven't found anything.

I guess I would be inclined to confirm the monotonicity of the idle control valve response curve. You say you are using a VICs mani, but not which valve you are using.

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The manifold, TB and IAC are all off of a 99. I'm controlling the IAC at 511Hz. I don't think it's an IAC control issue. I can control it down to 850 rpm by reducing the duty down to 18% or lower with the idle valve test tool. At that point, unplugging the IAC does not reduce the idle any further. I also pulled the IAC to confirm that nothing prevents it from closing completely.

I'm just trying to get clarification that the car shouldn't be able to run with the throttle plate shut, the idle valve closed and the idle screw shut.

Gotcha,

Well, the amount of air required for 850 RPM and ~31kpa sounds to me like a bigger number than what should be getting into the manifold under these conditions.

https://cimg9.ibsrv.net/gimg/www.mia...e56b2eef67.jpg
I physically blocked the TB completely, and the engine died immediately. I guess there's air getting past either the bleed valve or the IAC. Either way, things are stable, and it isn't going to be a boost leak.

There should be a small amount of bypass at the throttle valve I think. Check the throttle stop.

yeah, the throttle never seals completely.

you should tune the idle bypass screw (which sources air behind your blocker, btw) to idle at a min of 600RPM. you've cut off that supply of air and why it's stalling.

Yeah, I get that. I can see the inlet mouth of the idle bypass screw just before the throttle plate. My concern was that I couldn't get the idle below 850 no matter what I did, so I was concerned it was a vacuum leak. I think curly probably hit it: I never adjusted the stopper screw for the throttle plate. As long as it's not a vacuum leak, and it idles well, I don't think it makes a ton of difference.

If you're truly concerned about a vacuum leak, you'll want to look into a smoke tester.