Erratic Idle

[Night_train01]

I have developed a very erratic idle on my grand national. When the car first starts it ides around 850 rpm, it will do this for a few minutes then all of the sudden jump straight to over 1500 rpm eventually topping out around 2200 rpm. I checked the TPS and at idle it is .40, the IAC with the engine off is 140, with the engine running it is 0. I thought maybe a vacuum leak but I sprayed all connections with carb cleaner and did not see a difference. I am not sure why to try next. Suggestions?

 

[RED LS1]

I know there's an IAC reset procedure on GN/TType but I haven't done it. That said, I think it's suppose to be between 10-20ish at idle. You may want to travel that path. At least take it off and clean it out

 

[Night_train01]

I read up on the IAC procedure, with the engine running and me adjusting the butterfly in the throttlebody the highest I can get the IAC to read on the scanmaster is 06, I know the flap is open pretty wide and the rpms were approx. 2500
This is a brand new IAC and a brand new throttlebody. I may just buy another IAC and see if mine is defective. I am reading that the IAC at idle should be closer to 30

 

[Night_train01]

I found this during my search for high idle. "setting a new IAC" I am not sure what is meant by the first line, Jumper ALDL


Jumper ALDL connector pins A and B. (top right two pins)
Turn the ignition on but do not start the engine. The ECM sets the IAC count to zero.
Wait 30 seconds so the IAC has more than enough time to seat the pin. Leave ignition on.With ignition still on, disconnect the IAC electrical connector.
Turn the ignition off, remove the ALDL jumper, and reconnect the IAC electrical connector.This establishes the seated or zero reference point.
The next time the engine is started, the IAC will go to a count of 150, then count down (extend) as the engine warms.
I have this on how to adjust the steering column rod, let us know all works well.

 

[RED LS1]

I read up on the IAC procedure, with the engine running and me adjusting the butterfly in the throttlebody the highest I can get the IAC to read on the scanmaster is 06, I know the flap is open pretty wide and the rpms were approx. 2500
This is a brand new IAC and a brand new throttlebody. I may just buy another IAC and see if mine is defective. I am reading that the IAC at idle should be closer to 30


It should be like 10-25. Make sure you're turning the engine off and resetting the key forward. It should start at 140 and gradually decrease to the setting. I had to do it a few times and finally got down to around 20 at idle. Than you have thadd

 

[mikestertwo]

Did this problem start when you put the new throttle body on?

 

[Night_train01]

Yes it started after I installed the new throttle body and new IAC.

 

[mikestertwo]

Yes it started after I installed the new throttle body and new IAC.

Check the throttle plate in the throttle body to make sure it is aligned properly. I have seen aftermarket throttle bodies with the plate grossly misaligned and do not close properly. They feel like they are closed but when you inspect them there is a gap between the plate and the throttle body leaking air. Are you using the stock IAC manifold or an aftermarket? Does the new IAC have the correct shaped pintle on the end of it?

 

[Night_train01]

The throttle plate looks to close properly, I installed one of the angle IAC mounts

 

[bison]

Sounds like throttle blade is hanging up or the iac is damaged or hanging up somehow If IAC is at 0 and cant control idle

 

[terry richards]

Sounds like throttle blade is hanging up or the iac is damaged or hanging up somehow If IAC is at 0 and cant control idle

I have developed a very erratic idle on my grand national. When the car first starts it ides around 850 rpm, it will do this for a few minutes then all of the sudden jump straight to over 1500 rpm eventually topping out around 2200 rpm. I checked the TPS and at idle it is .40, the IAC with the engine off is 140, with the engine running it is 0. I thought maybe a vacuum leak but I sprayed all connections with carb cleaner and did not see a difference. I am not sure why to try next. Suggestions?

I have developed a very erratic idle on my grand national. When the car first starts it ides around 850 rpm, it will do this for a few minutes then all of the sudden jump straight to over 1500 rpm eventually topping out around 2200 rpm. I checked the TPS and at idle it is .40, the IAC with the engine off is 140, with the engine running it is 0. I thought maybe a vacuum leak but I sprayed all connections with carb cleaner and did not see a difference. I am not sure why to try next. Suggestions?

Had same proublem,It was the c14 tps 5 volt reference wire was corroded Popped wire out with paper clip(black plastic hole next to c14 pin) rock steady now

 

[Night_train01]

This morning I removed the new Accufab throttle body to get a better look at the plate, it appears to seat properly in the housing, I do not see any gaps when the plate is fully closed. I assume the IAC controls the two air passages one on each side of the plate. The only other ports on this throttle body is the two water lines, I have plugged both of them. I also blocked the vacuum holes in the top of the throttle body with a plate, I run all my vacuum lines to the GN1 1" plate between the dog house and the intake manifold. Next I will removed the 45 degree IAC block to see if anything is blocked.

 

[Night_train01]

Update, I replaced the IAC, set it with the A+B jumper process and the car runs perfectly now. The scanmaster shows IAC between 17-40 as I drive. My guess is the new one I put in over the winter was defective......anyway thanks for all the suggestions.

 

[Buckshot G/N]

Yes it sounds like your problem started over the winter with a defective IAC. By not driving it until better weather you were exposed. There is nothing like a happy T/B owner when there car is running right. ..

 

[Chuck Leeper]

Check the screws in the throttle plate.