If anyone can give some guidance on this, it would be most appreciated!

1999 Corvette coupe

Problem: Fast Idle

The problem has been occurring off and on for a couple of years now, but has been occurring almost all of the time since last summer. Other than fast idle, engine seems to perform normally. Exhaust tips show black soot (as opposed to gray), so may be running rich.

Symptoms:

On a cold start, seems to start and idle normally.

During cold weather, problem less pronounced. Comments below more related to warmer weather conditions.

For the first 5 - 15 minutes of driving, idle can be as high as 2000 rpm. After coming to a stop, it can take 10 - 20 seconds for the idle to drop to normal 700 rpm.

After driving for 30 minutes or so, idle can become normal, but if the car is parked for awhile, when started, the idle goes back to 1800 to 2000 rpm. It gradually drops closer to normal idle the longer it is driven. Lately, I have heard a slight pop as the idle drops from 1800 or so back to 700.

Idle problem seems sensitive to altitude and temperature changes, but not consistently so.

I switched from ported mass air sensor back to stock mass air sensor. At first this seemed to help, but problem gradually returned. I also removed Hypertech programming from car with the same result. At first seemed to help, but problem returned. I disconnected battery for several hours, in an attempt to get computer to reset itself. This helped, but problem returned. Basically, every time I change something in an attempt to correct the problem, the computer seems to adapt and all is well at first, but the problem returns. The same thing happened when new factory updates were downloaded to the computer.

Could this be Smog Control System related? A bad sensor? A throttle problem? A few years ago, the stock intake manifold was replaced with an LS6 intake manifold (same as on current Z06)

 

have you cleaned the throttle body?? maybe its sticking

 

May sound silly but check your front 02 sensors, mine did this for the longest time and I tried everything to correct it until I had a computer hooked up to it and found one of the front 02's was bad, changed it and I have not had the problems since.

 

Thank you! This sounds like a good candidate for a solution. I hate to show my ignorance, but where are the front O2 sensors located?

 

Sounds like a classic example of a vacuum leak. You said you have an LS6 manifold installed....only difference from stock. I'd start investigating closely for a hot engine vacuum leak. Good Luck.