LT1 - Idle Adjustment

[Jeff96TA]

I recently bought a 1996 T/A and am new to the LT1. The car was owned by a friend of mine before, motor was rebuilt 15k ago and has high-lift rockers on a stock cam. No other major mods but the car idles high, between 1100-1500 even when warm. When he had the motor rebuilt it had issues with idle and the computer was sent off and reprogrammed but I'm not sure if it's related.

Cutting to the chase, there is an idle adjustment screw on the TB and i'm trying to figure out how to adjust it, there is a hole on the back-side of where the screw is but for the life of me i can't see the head of it to figure out if it's hex, torx, or just phillips head.. any suggestions? Thanks..

[Rob B (shoebox)]

Idle speed is controlled by programming and is not mechanically adjustable. Connect a scanner and see what the "desired" idle speed is. It will be different at different engine temps.

[Jeff96TA]

Well there is an adjustable set screw that the throttle blade rests against.. i actually did run across a post on another LT1 board that mentioned how to adjust the throttle but it stated that you had to adjust the throttle blade, TPS, and the IAC counts simultaneously.. seems kinda complicated... guess i'm too used to my Ford..

[Injuneer]

Before you adjust anything, put a scanner on it, and see how the IAC motor is being moved by the PCM. The scanner will show you the number of steps or "counts" that the motor has opened the idle air passage. If the idle is too high (compared to the idle speed programmed in the PCM) , the PCM will try and close the IAC passage, by dropping the IAC counts. If the idle speed is lower than programmed, it will increase the counts. Obviously, it can't go below "0" to slow the idle down, and there is an upper limit of 160 counts. The PCM "learns" the correct IAC postion, so it can go to that value when the engine starts. With the scanner, you also want to check the TPS voltage. If its too high, the PCM will elevate the idle speed.

Stock programming, with a FULLY WARMED UP ENGINE is 800rpm for an M6, and 550 in drive and 650 in neutral for the A4. If your engine is idling at 1000+ RPM, its too high. If someone set it that way with a program, you can adjust the idle stop screw all you want, and it will not change the idle speed. You start to close the blades down, and the PCM opens the IAC motor to return the engine to the programmed idle speed.

If you still have the stock idle speed set in the PCM, you have a problem. Either the IAC passages in the throttle body are plugged up, the IAC motor is not working, the stop screw is out too far, the TPS voltage is above 1.0V, the coolant temp sensor is telling the PCM the engine is cold when its actually hot (PCM elevates the idle speed when the coolant is cold) or you have a large vacuum leak that is allowing air to bypass the throttle blades and the IAC system.

Did someone put an oversize aftermarket throttle body on the engine? If so, you can have problems with the stop screw being out too far. That is when you hook up the scanner, start to back off the idle stop screw, while watching RPM. When you hit the programmed idle speed, you check the IAC counts. They should be in the range of 20-40 counts with a stock cam. Maybe a bit higher with a more radical cam. Final check is the TPS voltage to make sure it is in the range of 0.20-0.90V. Some people prefer the typical stock setting of ~0.65V, and slot the screws in the TPS sensor so it can be rotated to tweek the voltage.

Start by checking the TPS voltage and IAC counts. Then you can try and back off on the idle stop screw, and see what happens.

[Rob B (shoebox)]

Well there is an adjustable set screw that the throttle blade rests against.. i actually did run across a post on another LT1 board that mentioned how to adjust the throttle but it stated that you had to adjust the throttle blade, TPS, and the IAC counts simultaneously.. seems kinda complicated... guess i'm too used to my Ford..

Yeah, I know there is a set screw, but it is not how you adjust idle speed (as explained by Injuneer).