Idle adjustment screw question
04-06-2010, 06:25 PM
#1
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Idle adjustment screw question
I was just wondering how the idle adjustment screw physically works to allow more or less air intake at idle. From what I recall, turning clockwise and lowering the screw in towards the throttle body restricts air flow (lowers idle RPM) and likewise turning counter-clockwise allows more air to enter (raises idle RPM). Is the screw affecting the resting throttle plate position? I thought there was a separate adjustment for that?
Also, on a related note, if the idle adjustment screw is tightened all the way down and B&C terminals are jumpered to disable the ICV, should this completely cut off all airflow? I'm wondering because if this is the case, a car with absolutely no vacuum leaks should die correct? Is this a way to confirm or prove the absence of vacuum leaks or should the car continue to idle either way?
Also, on a related note, if the idle adjustment screw is tightened all the way down and B&C terminals are jumpered to disable the ICV, should this completely cut off all airflow? I'm wondering because if this is the case, a car with absolutely no vacuum leaks should die correct? Is this a way to confirm or prove the absence of vacuum leaks or should the car continue to idle either way?
04-07-2010, 08:50 AM
#2
Drifting
The screw is basically a throttle plate bypass. Turning the screw out enlarges the size of the bypass port.
Not sure about your 2nd question. Never tried it, but even at full closed the port may not be completely blocked.
Before you get too deep into setting your idle rpm, make sure your throttle plate is closing completely at idle. Rotate the throttle cam slightly. If you hear a 'click' at the start of rotation, the plate is closing completely. If not, you need to adjust the TPS position.
Not sure about your 2nd question. Never tried it, but even at full closed the port may not be completely blocked.
Before you get too deep into setting your idle rpm, make sure your throttle plate is closing completely at idle. Rotate the throttle cam slightly. If you hear a 'click' at the start of rotation, the plate is closing completely. If not, you need to adjust the TPS position.
04-07-2010, 11:14 AM
#3
Rennlist Member
I just rebuilt my throttle body... and event with out the idle stop screw touching, there are still some "leaks" around the throttle plate (when holding it up to the light.)
With the idle stop screw set, the plate is open a little more.
For that idle adjustment screw, The Rennbay instructions said, from fully closed, 1/2 turn open was a good starting point. However, I found that mine was set to 2.125 turns open (and has run fine - and as far as I know, is how it was set at the factory).
With the idle stop screw set, the plate is open a little more.
For that idle adjustment screw, The Rennbay instructions said, from fully closed, 1/2 turn open was a good starting point. However, I found that mine was set to 2.125 turns open (and has run fine - and as far as I know, is how it was set at the factory).
04-07-2010, 01:49 PM
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So from what I understand now, the screw is used to block or open a port that allows air to bypass the throttle plate. Also the plate, even when fully closed, probably doesn't make an air tight seal. So between the plate not sealing and the screw perhaps not being able to fully block the port, it seems like the car may still draw enough air to idle. Thanks guys!
