When slowing to a stop my idle will hang at 1k until I'm completely stopped, then it will drop to 750. What is causing this?

Any help is appreciated

 

close your throttle blade by turning the set screw 1/4-1/2 turn. THat should fix it. If not then check for vacuum leaks and inspect the iac. That is the first step..

 

What should I look for when inspecting the iac? What might be wrong with it?

 

Black build up known as carbon. This can effect IAC movement. If there is a heavy amount then you might want to clean the entire passage. The motor can just wear out. Your car most likely just needs the throttle blades closed a hair. Lets assume the throttle blades, IAC, TPS, throttle cable, vacuum lines are all OK. Then you might want to try and adjust the IAC settings in the PCM. That shouldn't be needed though and please don't drill a hole in the TB.

BTW, nice car!

 

Is it possible for the blade to go past full closed and slightly open in the opposite direction? I ask this because I know my set screw is all the way down tight

 

Unless the set screw is not touching the tab on the throttle shaft, I would just do as I suggested. Not quite sure if the blade can go backwards. Don't think so but can check later today..

 

No they will NOT go backwards, or even go perpendicular unless someone has been overly-enthusiastic about their TB porting.
The blade isn't round and doesn't exactly fit the TB bore unless it's slightly angled.

 

my tuner programmed that effect into my tune. He calls it cruise control. It works great to handle a rough idling camshaft and 3600 stall. If all else is okay, you may be able to have it tuned to correct this.

 

The haning sounds like your throttle cracker or follower. Either reduce the decay rate, or reduce the amount of air they add

 

I have an '06 MN6 Corvette and, when I come to a complete stop, my idle screams to 2k and stays there. If I tap the gas it will oh so slowly come down to about 1200 and stay there. I had kooks headers and a cam installed and it has been tuned with HP Tuners software. It has been checked all over for vacuum leaks; the IAC valve is fine [the car only as 3900 miles on it]; the throttle body is good; and we tried drilling a hole in the butterfly hoping that would fix the problem but it made it worse. Now, get this, if I turn the a/c on, I have no screaming idle whatsoever. The only problem I have with the a/c on is when I press the clutch in to downshift, it will fluctuate from 600 rpms to 1200 rpms. When I stop all is well unless the a/c is off. If it is off, it screams back up to 2k. It has thoroughly driven me and my tuner nuts. For some reason, the throttle jumps from 7% to 25% on its own and we have tried everything known to man to stop it. No dice. It is going back to the tuner on Monday to try something else. Wish me luck. Good luck with your issue and please post when you find the problem.

 

I mistakenly went down the drilled blade route. Vette's don't have an IAC valve, it's a throttle position calculation or opening adder to the throttle position. At idle without IAC the throttle position should be 0%, with IAC contribution thottle opening will be roughly 7%. Your fluctuation is the engine gasping for choking on air. Plug the hole, you are only compounding the problem. The A/C puts an additional load on the engine, hence the difference in idle. Sounds like idle the airflow tables should be changed due to the difference in flow caused by the cam and headers. Then there are the throttle cracker tables. It's best to start monitoring from a cold start till full operating temp. Good luck

 

I never drove the car with the drilled hole in the throttle. When we couldn't get it to idle, we replaced the throttle body with a new one. All the airflow tables have been changed several times. My tuner has tried absolutely everything he can think of. He is very good at what he does and has been tuning especially GM cars for years but this one has got him stumped. He kept the car for 2 weeks and couldn't get it right. The car runs much better with the a/c on but we know this is just a bandaid until he gets this figured out. He is hoping to talk to someone who has experienced this but we haven't found anyone yet with this problem. It's like the pcm and the tuning software just don't get along. My car is the first '06 he has done and I think he said the '06 pcm is very different than the '05 so I guess he has to figure out its idiosyncracies.

 

What size cam? May be getting air from a leak, could have a leak in the exhaust trowing off the O2.

 

I can't remember the exact specs but they are something like 230/236 512 112. It has been checked thoroughly for leaks. My tuner thought it was a vacuum leak and during the time he had the car, he checked everything. I took it to 2 different people here to make sure and there are no leaks to be found. It is a totally different car with the a/c on though. I have a little surging but not the 2k revving thing. IF the a/c is off, the only way to keep the rpms down when stopped is to ride the clutch which I don't like doing so I just leave the a/c on as low as I possibly can. If I rev the engine with the a/c off, the rpms will fall to 2k and stay there. Turn the a/c on and they go down to 1000 which is what the idle is set at.


Robin

 

Did you try to close your throttle blade set screw "two full turns"?

 

I am not sure if that has been done or even tried. I am taking it back to the tuner on Monday morning and I will ask him about that. I haven't done any of the work myself. My tuner/installer has done everything. After the cam install, we elected to put the FAST and LPE intakes on hoping that would solve the problem but there was no change. When I took the car to the tuner when he kept it 2 weeks, I did not have to press the gas to go. I could get the car up to say 60 mph and take my foot off the gas totally and it would continue going 60. It would stay like that for a couple miles before it would "realize" that I had taken my foot off. Then, it would finally begin to slow down. That has been resolved. Thanks.


Robin

 

Cam is not so radical, the car should be able to learn with maybe a stumble or some hunting. Possibly crossed O2's, but it sounds more like airflow. I remember what part of the learning curve I was on when I drilled my throttle. Drilling is common for F bodies. Try logging air flow from startup.
We may have hijacked a thread.
Rick