High idle
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Joined: Jun 2001
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From: BC, Canada
High idle
I've been hunting down a sticking throttle problem ever since I bought my Vette. I've since replaced the throttle body with a new BBK 52mm. The damn thing would still stick though. I sent an email to the techs at BBK and they suggested I crack open the idle set screw to keep the throttle plates from binding (there's more detail to this, but I won't go into it now). Anyway, that seemed to sovle the sticking problem but now I have a high idle. Of course I readjusted the TPS, but the IAC is not compensating for the throttle adjustment. The IAC does work, I can hear it in diagnostic mode, and if I pull the plug and start the engine, it will idle fine at about 600 RPM
It now idles at a steady 1200 RPM compared to a 800 RPM before the adjustment. I only cracked the screw a half turn, so I find it hard to believe that the IAC cannot compensate.
I've read a few other threads, and there seems to be something about 1200 RPM. It appears to be a common figure in high idle problems. Does anyone have an idea what causes it to run at this speed?
It now idles at a steady 1200 RPM compared to a 800 RPM before the adjustment. I only cracked the screw a half turn, so I find it hard to believe that the IAC cannot compensate.
I've read a few other threads, and there seems to be something about 1200 RPM. It appears to be a common figure in high idle problems. Does anyone have an idea what causes it to run at this speed?
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
Re: High idle (Goody)
Here's what I've found on a couple of Vettes I've seen this problem on.
I'm going to assume that you didn't leave any vacuum hoses off when you changed TBs.
If you're sure that you didn't adjust the torx head screw on the throttle body too much which would have made the idle so high that the IAC can't lower it, chances are the IAC cannot fully extend its pintle to close off air flow.
Sometimes the four bolts that hold the TB to the plenum can be tightened a tad too tightly and this will tweak the TB and cause the butterflies to stick. I've seen this happen lots of times; those bolts don't take much torque at all; I MEAN VERY LITTLE. I don't have my service manual for my 86 handy; maybe someone will post the ft/lbs., but I believe it's under 10.
Make sure you can hand start all the TB bolts and pull then up in a criss-cross pattern. Also make sure you are using the correct TB to plenum gasket; they do differ depending on year.
There's a procedure to reset the IAC to give the computer a new fully closed reference point.
Install a jumper wire into the underdash ALDL at the two upper right terminals
Turn ignition key to the "on" position - fan should come on and the IAC pintle will fully extend (someone standing close should be able to hear it extending)
Wait 30 seconds or so then disconnect the IAC connector at the TB with the ignition still "on".
Turn key to "off" and remove the jumper wire.
Start engine with IAC disconnected and adjust idle speed using the torx head screw at the throttle body to get the desired idle speed: 500/550 is about right. The GM 86 Service Manual shows 450, but GM issued an service bulletin advising service centers to raise it to 550.
Note: Engine may be difficult to start and keep running with the IAC disconnected so play with the throttle until idle stabilizes and engine goes closed loop.
Shut down engine, reconnect IAC connector at the throttle body, and disconnect the battery for about 30 seconds to clear and error code that may have been stored when the ECM detected the engine running with the IAC disconnected.
Start engine and drive car at moderate speeds - about 40 MPH - for several miles. This allows the computer to learn and adjust to the new setting.
If that doesn't clear it up, you should remove the IAC and using a toothbrush and spray injectory cleaner, remove all built up carbon. Using hand pressure pull on the pintle while wiggling it from side to side. The pintel will come completely out of the IAC.
When cleaning the IAC make sure no liquid runs into the body of the IAC to effect the electricals inside; hold the pintle end DOWN so the fluid runs away from the body of the IAC.
Reinstalling the pintle and spring is just the opposite or removal. Using hand pressure install the pintle and spring and press on the pintle while you wiggle it.
The only trick is to make sure the pintle is not fully extended when you reinstall it into the TB. There is a maximum mm setting (the pintle should not extend more that a certain amount), which I can't recall, but if you push the pintle back in about half-way you'll be okay.
Some guys add a drop of oil to the pintle shaft to insure free movement.
Of course you'll have to go through the resetting procedure again.
Some may say that the IAC can't be cleaned and advise you to buy a new one, but I and others have cleaned them with success.
Keep us posted.
Jake
[Modified by JAKE, 1:38 AM 3/3/2002]
[Modified by JAKE, 2:14 AM 3/3/2002]
I'm going to assume that you didn't leave any vacuum hoses off when you changed TBs.
If you're sure that you didn't adjust the torx head screw on the throttle body too much which would have made the idle so high that the IAC can't lower it, chances are the IAC cannot fully extend its pintle to close off air flow.
Sometimes the four bolts that hold the TB to the plenum can be tightened a tad too tightly and this will tweak the TB and cause the butterflies to stick. I've seen this happen lots of times; those bolts don't take much torque at all; I MEAN VERY LITTLE. I don't have my service manual for my 86 handy; maybe someone will post the ft/lbs., but I believe it's under 10.
Make sure you can hand start all the TB bolts and pull then up in a criss-cross pattern. Also make sure you are using the correct TB to plenum gasket; they do differ depending on year.
There's a procedure to reset the IAC to give the computer a new fully closed reference point.
Install a jumper wire into the underdash ALDL at the two upper right terminals
Turn ignition key to the "on" position - fan should come on and the IAC pintle will fully extend (someone standing close should be able to hear it extending)
Wait 30 seconds or so then disconnect the IAC connector at the TB with the ignition still "on".
Turn key to "off" and remove the jumper wire.
Start engine with IAC disconnected and adjust idle speed using the torx head screw at the throttle body to get the desired idle speed: 500/550 is about right. The GM 86 Service Manual shows 450, but GM issued an service bulletin advising service centers to raise it to 550.
Note: Engine may be difficult to start and keep running with the IAC disconnected so play with the throttle until idle stabilizes and engine goes closed loop.
Shut down engine, reconnect IAC connector at the throttle body, and disconnect the battery for about 30 seconds to clear and error code that may have been stored when the ECM detected the engine running with the IAC disconnected.
Start engine and drive car at moderate speeds - about 40 MPH - for several miles. This allows the computer to learn and adjust to the new setting.
If that doesn't clear it up, you should remove the IAC and using a toothbrush and spray injectory cleaner, remove all built up carbon. Using hand pressure pull on the pintle while wiggling it from side to side. The pintel will come completely out of the IAC.
When cleaning the IAC make sure no liquid runs into the body of the IAC to effect the electricals inside; hold the pintle end DOWN so the fluid runs away from the body of the IAC.
Reinstalling the pintle and spring is just the opposite or removal. Using hand pressure install the pintle and spring and press on the pintle while you wiggle it.
The only trick is to make sure the pintle is not fully extended when you reinstall it into the TB. There is a maximum mm setting (the pintle should not extend more that a certain amount), which I can't recall, but if you push the pintle back in about half-way you'll be okay.
Some guys add a drop of oil to the pintle shaft to insure free movement.
Of course you'll have to go through the resetting procedure again.
Some may say that the IAC can't be cleaned and advise you to buy a new one, but I and others have cleaned them with success.
Keep us posted.
Jake
[Modified by JAKE, 1:38 AM 3/3/2002]
[Modified by JAKE, 2:14 AM 3/3/2002]
