Fast idle

[rchurch5945]

I have a 96 Tahoe with the 5.7 with 175,000 miles. The accelerator pedal started sticking at idle and I would have to quickly stomp it to unstick it. It would only happen at idle. It also began idling at 1,300 - 1,500 rpm, and drop down to 700 - 800, then back to the high idle. It would idle up and down sporadically but mostly stay at the high idle.

I pulled the throttle body and it was pretty gunked up so I cleaned it and replaced the Idle Air Controller that is mounted on the side of the throttle body. I put it back together and now it idles at a steady 2,300 rpm. I pulled the throttle body back off and double checked that the manifold gasket was good. The good news is the accelerator doesn't stick anymore.

I found a very small (approx. 1/16") vacuum hose that has one end connected to a piece on the intake to the right of the throttle body and I can't find anywhere that the other end connects. It is a small vacuum leak but holding my thumb over the end doesn't change the way the engine runs. It doesn't miss or backfire. It actually runs better now but I can't get the idle down below 2,300. Help!

 

[Doubeleive]

put up a picture of this vacuum line and where the one end is attached, some one will know I haven't messed with the obs in a long time so no idea off hand myself without a visual, as far the the idle you may just need to drive it a ways and let it relearn, try disconnecting the battery for about 10-15 minutes also so it can all reset then just drive it and let it try and do it's thing.

 

[drakon543]

as much as i messed with mine it never had to relearn idle much past 30 seconds and even then it never went that high. thr only time it idled that high was when i forgot the brake booster vacuum line.

 

[liquify33]

If you’re super duper sure it isn’t vacuum, could the accelerator cable be bound?

 

[96-2D-Hoe]

My truck only increased 100-200 RPM with a vacuum hose broken. If there's nothing wrong with the accelerator cable you might want to put the old idle air controller back in and see if you have a bad one. If you post a pic of the vacuum hose I'll see where it goes.

 

[rchurch5945]

Thanks everyone. I'll post a pic of the vacuum hose tomorrow. I'll also try these suggestions. I'm thinking I can just disconnect the accelerator cable and see if the idle drops. That should let me know if it is bound. If that's not it I'll clean and re-install the old idle control and see if that makes a difference.

I've let it run 10 minutes a few times to see if it would reset itself but it didn't. I'll keep y'all posted.

 

[rchurch5945]

Thanks everyone. I'll post a pic of the vacuum hose tomorrow. I'll also try these suggestions. I'm thinking I can just disconnect the accelerator cable and see if the idle drops. That should let me know if it is bound. If that's not it I'll clean and re-install the old idle control and see if that makes a difference.

I've let it run 10 minutes a few times to see if it would reset itself but it didn't. I'll keep y'all posted.

 

[rchurch5945]

Thanks everyone. I'll post a pic of the vacuum hose tomorrow. I'll also try these suggestions. I'm thinking I can just disconnect the accelerator cable and see if the idle drops. That should let me know if it is bound. If that's not it I'll clean and re-install the old idle control and see if that makes a difference.

I've let it run 10 minutes a few times to see if it would reset itself but it didn't. I'll keep y'all posted.

 

[rchurch5945]

Thanks everyone. I'll post a pic of the vacuum hose tomorrow. I'll also try these suggestions. I'm thinking I can just disconnect the accelerator cable and see if the idle drops. That should let me know if it is bound. If that's not it I'll clean and re-install the old idle control and see if that makes a difference.

I've let it run 10 minutes a few times to see if it would reset itself but it didn't. I'll keep y'all posted.

 

[rchurch5945]

Thanks everyone. I'll post a pic of the vacuum hose tomorrow. I'll also try these suggestions. I'm thinking I can just disconnect the accelerator cable and see if the idle drops. That should let me know if it is bound. If that's not it I'll clean and re-install the old idle control and see if that makes a difference.

I've let it run 10 minutes a few times to see if it would reset itself but it didn't. I'll keep y'all posted.

 

[Doubeleive]

double check the iac you cleaned and see if it has any grime around the edges where it moves and make sure it is mounted properly

 

[rchurch5945]

First, I apologize for the multiple posts - my computer was running slow and I didn't think it was posting my last post.

Thanks for everyone's replies. I took the new IAC off and re-installed the old one. The Idle is back down to 1,300 -1,500 rpm. I cleaned it before re-installing it. So, I am back to where I started. I'm thinking it is the vacuum leak, so I took a picture of the engine. One red arrow points to the piece on the manifold that the vacuum hose is attached to (which is on the driver's side of the engine) and the other red arrow points to the vacuum hose. The hose runs directly behind the throttle body towards the passenger side of the engine. Any ideas where the hose should be connected to? The open end is behind the throttle body and there is no other piece of hose and I cannot find anywhere it might connect to. Thanks!

 

[Doubeleive]

vacuum check valve?

 

[drakon543]

this is where the small hose is connected on my 97.

 

[drakon543]

hope that helps some if you need anymore reference pictures hit me up and ill try to remember to post something for you. im pretty bad with remembering to post replys if i dont see an alert notification so pm or tag me. aside from my main ac lines everything is still hooked up on my truck dirty but hooked up.

 

[east302]

First, I apologize for the multiple posts - my computer was running slow and I didn't think it was posting my last post.

Thanks for everyone's replies. I took the new IAC off and re-installed the old one. The Idle is back down to 1,300 -1,500 rpm. I cleaned it before re-installing it. So, I am back to where I started. I'm thinking it is the vacuum leak, so I took a picture of the engine. One red arrow points to the piece on the manifold that the vacuum hose is attached to (which is on the driver's side of the engine) and the other red arrow points to the vacuum hose. The hose runs directly behind the throttle body towards the passenger side of the engine. Any ideas where the hose should be connected to? The open end is behind the throttle body and there is no other piece of hose and I cannot find anywhere it might connect to. Thanks!

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Did you disconnect the throttle cable to rule out an issue there?

If you have a scanner, see what the throttle position sensor is reporting at idle.

The hard tube goes to the water valve solenoid on the passenger firewall. It’s above the heater hoses. The line from the PCV fitting connects at top and there is a second line that goes to the water valve on the heater hoses.

The line to the PCV is in red, the line to water valve is marked in yellow below:

When the a/c panel is set to the “max” cold detent, the solenoid energizes and vacuum is applied to the water valve. This closes off coolant flow to the heater core to maximize a/c performance.


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