Trying to help out a buddy with his '06 ZO6. (warning, this could get long.)
He bought the car last summer, and complained of slow return to idle. RPM would flare to 15-1800 after pushing in the clutch at slow crawl. Inconsistent depending on how much load was on the engine at the time. The rpm would always settle back down to normal idle within 10 seconds or so. Annoying, but not a real problem.

Fast forward, I had to put a water pump on it in January due to a leak, The throttle body was removed for easier access, and cleaned with cloth damp with cleaner, not sprayed down, just wiped.
When it was put back together the idle would hang higher, 18-2200, and either really slowly come down or stay there. You could load the engine down with the clutch and bring the RPM down to idle, where it would stay until you gave it some pedal and it would flare up again. It's more than annoying at this point.
Diagnostics for vacuum leaks found none. PCV hose replaced for the hell of it, intake gasket is like new. It went to the dealer for a throttle relearn and it was worse.
I borrowed a 87mm Trailblazer SS throttle body just to try and the car runs mint. Better than it ever did. No more hanging RPM's , no flaring, nothing. Run's perfect. No throttle relearn procedure other than let it idle up to temp.
Ok, so we decide that old stock throttle is at fault and buy a new Delco replacement.
Throw the new one on and... same issue. hanging RPM, flaring, drive-ability issues. Oh, and no DTC's through any of this. Crap!!!
Put the Trailblazer throttle on, mint, no issues.
Take the car back to the dealer with the 87mm on to show them how it is running perfect. Get them to change to the new 90mm and do the relearn procedure and before the tech gets it out of the bay its acting up. "Hmm, that's weird".

Now it's even worse, Try driving this thing when it won't go under 2500 rpm. No p0507 code, data log not showing anything odd going on with the pedal , yet it is like it is being told to open the throttle.

He got a smoking deal on a brand new 87mm throttle body, so he bought it and the car runs mint with it too.
So 2 stock 90mm throttle body make it act up, and it runs perfect with 2 different 87 truck throttles.

So, shy of the new 90mm being faulty out of the box, with the same issue as the original one, does anyone know, or can think of any reason to explain this?
What, if any, hardware or electronic differences are there between the 90 and 87.

He is perfectly happy driving it around with the 87, other than his engine covers wont fit, but it drives me nuts not knowing what is causing this. Dealer seems uninterested in digging deeper.

thanks for reading.
any insight is appreciated.

 

Makes perfect sense.

Since this is supercharged-section, I assume the car was not totally stock.

A bit too much idle airflow in the tune and that's what it'll do.

Changing to a bit smaller TB - and car's PCM not knowing that - did the the trick this time, i.e. a bit less airflow at idle and no more hanging idle.

Another option would have been to adjust the tune itself.

 

Never assume. This is a stock ZO6, stock tune.
I put it in the forced induction section seeking a higher knowledge base.

 

Correct, never assume.

I also think that since it was stock (or so you and i assume with an used car), the tech guy flashed the latest GM tune in it.

However, the way how PCM reacts is correct. Too much idle airflow. And by using smaller TB, it fixed the problem.

In some Edelbrock supercharger kits for C6 I've seen similar symptoms. Just lowering idle air fixes the hanging idle.

 

bump

 

Has anyone found the problem? I’m currently going through the same issues. Nick Williams 103 throttle body. Upgraded superchargers and car sat for a month and now it won’t idle normal.