High idle

[UKZR1]

I have a problem with high idle. When I pull up the engine will idle at around 2000rpm until I blip the throttle then it will drop down to normal and idle ok

I've even noticed this on a highway in a tall gear. If I lift off the engine will slow the car until around 2000rpm and then I may as well be on cruise - until I blip the throttle.

I'm off next weekend so are looking for some suggestions on what to check

The throttle cables etc don't appear to be binding or anything

[mike100]

dreaded LT5 idle hang. There are a couple of sources of vacuum leak, but the only easy way to reduce this is to lower the idle targets in the program and do your best to reduce the throttle body bypass air etc. You need a true low idle speed where the IAC isn't struggling to close off more air.

There are probably many more function in the program to do this, but they aren't really defined in any of the public chip definitions. The idle strategy doesn't really come down until the car is below 3 mph or so.

Also the latest cal for the 1990 and the later 91 chip cals address this a little as well- but they all have this inherent characteristic.

one cheat I have employed is to vent the PCV to the atmosphere and only allow about a 1/3rd of it to go back to the tube on the bottom of the plenum/TB. I also have it routing through a catch-can to help with the oil in the intake issue as well. In my case, my throttle blades leak a little too much and I couldn't achieve the target idle as easily as I liked. Once you can get it to idle at 700 rpm, the drive through, never slowing down feeling goes away.

[scottfab]

Since you've described the idle returns to normal when you blip it I'd say it is one of two problems. Start withe the simple stuff.
1. Check for sticking of the throttle plates. This is easy. Take the air bellows off and go for a drive. Once the high idle is felt don't just blip it. Pull over and reach in to the plates pushing them shut. If the idle then drops you have it.
To fix just use a bit of axle grease around the lip of the blades. This will last quite a while. If it doesn't work then you could be looking at wear and slop on the brass bushings for the plates. (I ended up buying a new TB)
2. If the blades are not the problem next go to the IAC. Pull it and carefully remove the carbon. DO NOT over clean it with any solvent. See manual for why.

[XfireZ51]

Since you've described the idle returns to normal when you blip it I'd say it is one of two problems. Start withe the simple stuff.
1. Check for sticking of the throttle plates. This is easy. Take the air bellows off and go for a drive. Once the high idle is felt don't just blip it. Pull over and reach in to the plates pushing them shut. If the idle then drops you have it.
To fix just use a bit of axle grease around the lip of the blades. This will last quite a while. If it doesn't work then you could be looking at wear and slop on the brass bushings for the plates. (I ended up buying a new TB)
2. If the blades are not the problem next go to the IAC. Pull it and carefully remove the carbon. DO NOT over clean it with any solvent. See manual for why.

I'd agree this is the first place to look. I'm assuming you haven't changed the chip and that motor has gradually started acting the way you describe?

[UKZR1]

The car isnt stock. Its a Haibeck 368 with the chip Marc did but it has been great since he built the engine (years and 10's of thousands of miles) This started last year and gradually got worse to the the point its doing it all the time and needs sorting. Its nothing to do with the IAC as I had a new one in my spare parts collection so thats been swapped and ruled out.

I also emailed Marc my original post, he also points towards the throttle plates sticking.

Looks like thats where I'll start with my trouble shooting

thanks for your help guys

[ittlfly]

Have you had the TB off the car? If so, a new gasket is probably a good idea between the TB and plenum. (ask me how I know :cry:) Also, the torque on the 5 torx bolts that hold the TB to the plenum is critical. Vacuum leaks in this area are easy to get.